Paleontological discoveries in the Chorrillo Formation (upper Campanian-lower Maastrichtian, Upper Cretaceous), Santa Cruz Province, Patagonia, Argentina

Abstract

The first fossil remains of vertebrates, invertebrates, plants and palynomorphs of the Chorrillo Formation (Austral Basin), about 30km to the SW of the town of El Calafate (Province of Santa Cruz), are described. Fossils include the elasmarian (basal Iguanodontia) Isasicursor santacrucensis gen. et sp. nov., the large titanosaur Nullotitan glaciaris gen. et sp. nov., both large and small Megaraptoridae indet., and fragments of sauropod and theropod eggshells. The list of vertebrates is also composed by the Neognathae Kookne yeutensis gen. et sp. nov., two isolated caudal vertebrae of Mammalia indet., and isolated teeth of a large mosasaur. Remains of fishes, anurans, turtles, and snakes are represented by fragmentary material of low taxonomical value, with the exception of remains belonging to Calyptocephalellidae. On the other hand, a remarkable diversity of terrestrial and freshwater gastropods has been documented, as well as fossil woods and palinological assemblages. The Chorrillo Formation continues south, in the Las Chinas River valley, southern Chile, where it is called Dorotea Formation. Both units share in their lower two thirds abundant materials of titanosaurs, whose remains cease to appear in the upper third, registering only elasmarians (Chorrillo Formation) and hadrosaurs (Dorotea Formation). Above both units there are levels with remains of invertebrates and marine reptiles. It is striking that the dinosaurs of the lower two thirds of the Chorrillo and Dorotea formations are represented by large basal titanosaurs and Megaraptoridae coelurosaurs, being the Saltasaurinae and Aeolosaurinae sauropods and Abelisauridae theropods totally absent. In contrast, these taxa are dominant components in sedimentary units of central and northern Patagonia (e.g., Allen, Los Alamitos, La Colonia formations). Such differences could reflect, in part, a greater antiquity (i.e., late Campanian-early Maastrichtian) for the Chorrillo fossils, or, more probably, different environmental conditions. Thus, knowledge of the biota of the southern tip of Patagonia is expanded, particularly those temporarily close to the K-Pg boundary.

Full text

Rev. Mus. Argentino Cienc. Nat., n.s.
21(2): 217-293, 2019
ISSN 1514-5158 (impresa)
ISSN 1853-0400 (en línea)
Paleontological discoveries in the Chorrillo Formation (upper
Campanian-lower Maastrichtian, Upper Cretaceous), Santa Cruz
Province, Patagonia, Argentina
Fernando. E. NOVAS1,2, Federico. L. AGNOLIN1,2,3, Sebastián ROZADILLA1,2, Alexis M.
ARANCIAGA-ROLANDO1,2, Federico BRISSON-EGLI1,2, Matias J. MOTTA1,2, Mauricio
CERRONI1,2, Martín D. EZCURRA2,5, Agustín G. MARTINELLI2,5, Julia S. D´ANGELO1,2, Gerardo
ALVAREZ-HERRERA1, Adriel R. GENTIL1,2, Sergio BOGAN3, Nicolás R. CHIMENTO1,2, Jordi
A. GARCÍA-MARSÀ1,2, Gastón LO COCO1,2, Sergio E. MIQUEL2,4, Fátima F. BRITO4, Ezequiel I.
VERA2,6, 7, Valeria S. PEREZ LOINAZE2,6 , Mariela S. FERNÁNDEZ8 & Leonardo SALGADO2,9
1 Laboratorio de Anatomía Comparada y Evolución de los Vertebrados. Museo Argentino de Ciencias Naturales
“Bernardino Rivadavia”, Avenida Ángel Gallardo 470, Buenos Aires C1405DJR, Argentina - fernovas@yahoo.
com.ar. 2 Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina. 3 Fundación de Historia Natural
“Felix de Azara”, Universidad Maimonides, Hidalgo 775, C1405BDB Buenos Aires, Argentina. 4 Laboratorio de
Malacología terrestre. División Invertebrados Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”,
Avenida Ángel Gallardo 470, Buenos Aires C1405DJR, Argentina. 5 Sección Paleontología de Vertebrados.
Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Avenida Ángel Gallardo 470, Buenos
Aires C1405DJR, Argentina. 6 División Paleobotánica. Museo Argentino de Ciencias Naturales “Bernardino
Rivadavia”, Avenida Ángel Gallardo 470, Buenos Aires C1405DJR, Argentina. 7 Área de Paleontología.
Departamento de Geología, Universidad de Buenos Aires, Pabellón 2, Ciudad Universitaria (C1428EGA) Buenos
Aires, Argentina. 8 Instituto de Investigaciones en Biodiversidad y Medioambiente (CONICET-INIBIOMA),
Quintral 1250, 8400 San Carlos de Bariloche, Río Negro, Argentina. 9 Instituto de Investigación en Paleobiología
y Geología de la Universidad Nacional de Río Negro, General Roca, Río Negro, Argentina
Abstract: The first fossil remains of vertebrates, invertebrates, plants and palynomorphs of the Chorrillo
Formation (Austral Basin), about 30km to the SW of the town of El Calafate (Province of Santa Cruz), are des-
cribed. Fossils include the elasmarian (basal Iguanodontia) Isasicursor santacrucensis gen. et sp. nov., the large
titanosaur Nullotitan glaciaris gen. et sp. nov., both large and small Megaraptoridae indet., and fragments of
sauropod and theropod eggshells. The list of vertebrates is also composed by the Neognathae Kookne yeutensis
gen. et sp. nov., two isolated caudal vertebrae of Mammalia indet., and isolated teeth of a large mosasaur. Remains
of fishes, anurans, turtles, and snakes are represented by fragmentary material of low taxonomical value, with
the exception of remains belonging to Calyptocephalellidae. On the other hand, a remarkable diversity of terres-
trial and freshwater gastropods has been documented, as well as fossil woods and palinological assemblages. The
Chorrillo Formation continues south, in the Las Chinas River valley, southern Chile, where it is called Dorotea
Formation. Both units share in their lower two thirds abundant materials of titanosaurs, whose remains cea-
se to appear in the upper third, registering only elasmarians (Chorrillo Formation) and hadrosaurs (Dorotea
Formation). Above both units there are levels with remains of invertebrates and marine reptiles. It is striking
that the dinosaurs of the lower two thirds of the Chorrillo and Dorotea formations are represented by large
basal titanosaurs and Megaraptoridae coelurosaurs, being the Saltasaurinae and Aeolosaurinae sauropods and
Abelisauridae theropods totally absent. In contrast, these taxa are dominant components in sedimentary units of
central and northern Patagonia (e.g., Allen, Los Alamitos, La Colonia formations). Such differences could reflect,
in part, a greater antiquity (i.e., late Campanian-early Maastrichtian) for the Chorrillo fossils, or, more probably,
different environmental conditions. Thus, knowledge of the biota of the southern tip of Patagonia is expanded,
particularly those temporarily close to the K-Pg boundary.
Key words: Chorrillo Formation, Southern Patagonia, Late Cretaceous, fossils
Resumen: Hallazgos Paleontológicos en la Formación Chorrillo (Campaniano-Maastrichtiano,
Cretácico Superior), Provincia de Santa Cruz, Patagonia, Argentina. Se describen los primeros restos
fósiles de vertebrados, invertebrados, plantas y palinomorfos de la Formación Chorrillo (Cuenca Austral), aflo-
rante unos 30km al SW de la localidad de El Calafate (Provincia de Santa Cruz). Los fósiles de dinosaurios no
avianos incluyen el elasmariano (Iguanodontia basal) Isasicursor santacrucensis gen. et sp. nov., el titanosaurio

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
218
INTRODUCTION
The Chorrillo Formation (Upper Cretaceous,
Campanian-Maastrichtian; Arbe, 2002; Nullo et
al., 2006) extensively crops out to the south of
Centinela River, Santa Cruz Province, southern
Patagonia, Argentina (Figure 1). The Chorrillo
Formation extends NE to SW as a narrow
band with a maximum E-W width of 2 kms.
The Chorrillo Formation continues southward
on the Chilean side, being partially equivalent
to the Dorotea Formation (Upper Cretaceous,
Campanian-Maastrichtian; Macellari et al.,
1989; Vogt et al., 2014; González Abarca, 2015;
Manriquez et al., 2019).
Feruglio (in Fossa Mancini et al., 1938) was
the first to call “Estratos de Chorrillo” (i.e.,
“Chorrillo Beds”) to these rocks cropping out
to the south of the Argentino Lake, indicating
the presence of fossil logs and dinosaur bones
(Feruglio, 1944-45). Aside from these citations
concerning the presence of unspecified dinosaur
remains, the first to discover a partial sauropod
skeleton was Francisco Nullo in 1980, then geo-
logist of Argentine Geological Survey, while ex-
ploring the top of the hills of the Alta Vista farm
(“estancia”). Nullo informed on the discovery
to the renowned paleontologist José Bonaparte,
who collected a partial cervical vertebra of such
titanosaur specimen (see description below).
Bonaparte illustrated the titanosaur bones -still
yielding on the ground- in a popular book about
South American dinosaurs (Bonaparte, 1996).
From the same locality, Bonaparte also collected
(but did not describe) some isolated theropod re-
mains, including a partial ulna and a shed tooth,
forming part of the Paleontological Collection of
the Museo Argentino de Ciencias Naturales, in
Buenos Aires.
Recent explorations carried out in La Anita
and Alta Vista farms, approximately 30 km SW
from El Calafate city, allowed relocation of the
titanosaur specimen discovered by Nullo, but
also resulted in the discovery of novel vertebrate
remains, which are described below. The explora-
tions were carried out on from January 13th-17th
and March 14th-19th, 2019.
The complex topography of the region made
the access somewhat difficult, even for 4x4 ve-
hicles, thus long and exhausting walks were re-
quired to reach the fossil sites. Notwithstanding
such circumstantial inconveniences, a rich collec-
tion of fossils made up by large titanosaur bone
fragments, theropod remains, medium-sized
ornithopods, tiny vertebrate bones, gastropods,
and plant remains, starts to be amassed from
this southern region of the Argentine Patagonia.
The fossil remains studied below, the first to be
formally described from the Chorrillo Formation,
shed valuable information on a wide variety of
Late Cretaceous organisms in the southern cone,
complementing the fossil record that has been
obtained from equivalent stratigraphic levels in
the geographically close Las Chinas River Valley,
in southern Chile (Leppe et. al., 2014; González
Abarca, 2015; Manriquez et al., 2019).
GEOLOGICAL SETTING
We follow the stratigraphic interpretations
gigante Nullotitan glaciaris gen. et sp. nov., Megaraptoridae indet. de tamaños pequeño y grande, y fragmentos
de cáscaras de huevo pertenecientes a saurópodos y terópodos. La lista de vertebrados se compone también del
Neognathae Kookne yeutensis gen. et sp. nov., dos vértebras caudales aisladas de Mammalia indet., y dientes aisla-
dos de un mosasaurio de gran tamaño. Han sido colectados también restos de peces, anuros, tortugas y serpientes
los cuales están representados por material fragmentario de escaso valor taxonómico, con la excepción de restos
pertenecientes a Calyptocephalellidae. Por otra parte, ha sido documentada una notable diversidad de gasteró-
podos terrestres y dulceacuícolas, así como leños fósiles y asociaciones palinológicas. La Formación Chorrillo se
continúa hacia el sur, en el valle del río Las Chinas, Chile, en donde es denominada Formación Dorotea. Ambas
unidades comparten en sus dos tercios inferiores abundante material de titanosaurios, cuyos restos dejan de apa-
recer en el tercio superior, registrándose solo elasmarianos (Fm Chorrillo) y hadrosaurios (Fm Dorotea). Por en-
cima de ambas unidades existen niveles con restos de invertebrados y reptiles marinos. Llama la atención que los
dinosaurios de los dos tercios inferiores de las formaciones Chorrillo y Dorotea estén representados por titanosau-
rios basales de gran tamaño y celurosaurios Megaraptoridae, estando ausentes los Saltasaurinae, Aeolosaurinae
y Abelisauridae, los cuales son componentes dominantes en las unidades sedimentarias del Maastrichtiano de la
provincia de Chubut y del norte patagónico (p.ej., formaciones Allen, Los Alamitos y La Colonia, por ejemplo).
Estas diferencias podrían reflejar, en parte, una mayor antigüedad (i.e., Campaniano tardío -Maastrichtiano tem-
prano) para los fósiles de Chorrillo. Se amplía así el conocimiento de las biotas del extremo sur de Patagonia, en
particular de aquellas temporalmente cercanas al límite K-Pg.
Palabras clave: Formación Chorrillo, Patagonia austral, Cretácico Tardío, fósiles.
_____________

219
Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
expressed by Nullo et al. (2006) in recognizing
in the study area the following stratigraphic
succession (from bottom to top): Alta Vista, La
Anita, La Irene, Chorrillo, and Calafate forma-
tions. Both Alta Vista and La Anita bedrocks
form the high cliffs of the homonymous farms.
The dinosaur-bearing beds of the Chorrillo
Formation form continuous outcrops on the high
plateaus on the top of these hills (Figure 1). In
Argentina these beds extend to the international
border with Chile, close to the “Hito Baguales
2” (Figure 1; Nullo et al., 2006). The top of the
Chorrillo Formation laterally inter-fingers and
is overlaid by the marine Calafate Formation
(Marensi et al. 2004; Odino Barreto et al., 2018).
The rock succession comprehended by La Anita,
La Irene and Chorrillo formations conforms an
upper Campanian-early Maastrichtian regressi-
ve episode, started with the deltaic deposits of
the La Anita Formation, and followed upwards
by braided and meandering fluvial deposits of
the La Irene and Chorrillo formations (Arbe &
Hechem, 1984; Macellari et al., 1989; Nullo et al.,
2006; Moyano Paz et al., 2018; Tettamanti et al.,
2018).
Tettamanti et al., (2018) synonymized Cerro
Fortaleza, La Anita, La Irene, and Chorrillo for-
mations under the name of “Upper Cretaceous
Continental Deposits”. We concur with these
authors in interpreting these lithologically simi-
lar beds as part of a same diachronic episode of
Campanian through Maastrichtian continental
sedimentation. However, we prefer to keep the
original lithostratigraphic names for these units,
waiting for more paleovertebradological infor-
mation from both Chorrillo and Cerro Fortaleza
beds, eventually offering more confident age de-
terminations for these beds.
Regarding the age of the Chorrillo Formation,
it has to be no older than the underlying Early
Campanian Alta Vista beds (the age of which
is based on abundant marine invertebrates;
Blasco et al., 1980; Nullo et al., 2006) and the
Maastrichtian La Irene Formation (the age of
which is based on palynological assemblages;
Povilauskas et al., 2008). Moreover, the Chorrillo
Formation inter-fingers to the SE with the
shallow marine deposits of the Cerro Cazador
Formation (Macellari, 1988), which yielded the
associated presence of Eubaculites and Maorites,
interpreted as early Maastrichtian in age (Nullo
et al., 2006). The Chorrillo Formation is overlaid
by the marine Calafate Formation, considered to
be late Maastrichtian based on dinoflagelate cysts
association (Marenssi et al., 2004; Guler et al.,
2005; Nullo et al., 2006). In sum, the dinosaur-
bearing, continental deposits of the Chorrillo
Formation exposed at the top of the high pla-
teaus of the La Anita and Alta Vista ranges, may
be early Maastrichtian in age. Evidence from the
equivalent Dorotea beds supports, however, that
the lower thirds of the Chorrillo Formation be
late Campanian, at least (see below).
The Argentine sequence formed by both the
continental Chorrillo and the marine Calafate
formations resembles the sedimentary unit that
in southern Chile is named as Dorotea Formation
(F. Nullo, pers. comm.). As Cecioni (1957) and
Katz (1963) have noted, rock units cropping out
on both sides of the international border are
inseparable from a formational point of view.
This observation also applies for Chorrillo plus
Calafate (on the Argentine side) and Dorotea (in
Chile), which exhibit almost the same lithological
characteristics and fossil content. The Dorotea
Formation includes basal and middle sections
equivalent to Chorrillo, and an upper section res-
embling, both lithologically and in fossil content,
to the Calafate Formation.
The stratigraphic succession of the Chorrillo
Formation is made up by intercalated levels of
greenish and reddish sandstones, intercalated by
some conglomeratic banks. Important is to say
that the following description is tentative, re-
quiring for more detailed stratigraphic and sedi-
mentological surveys. Immediately above the La
Anita beds, the base of the Chorrillo Formation
is characterized by green mudstones with frag-
mentary plant remains. Following upwards there
is a bank of conglomerates and coarse sandsto-
nes containing abundant fossil wood and badly
preserved leaves. These stratigraphic levels with
abundant plant remains are replaced upwards
by the already mentioned greenish and reddish
sandstone levels, with an approximate thickness
of 250m (Nullo et al., 2006), containing dinosaur
and other vertebrate remains. Such succession is
capped by a meter thick sandstone bank, green
in color, bearing abundant specimens of tiny bi-
valves (approximately 2-3 cm long), including
ostreids, mytilids and pectinids (see below for
taxonomic identification). The sequence of gree-
nish and reddish sandstones is cyclic and mono-
tonous, thus being difficult to recognize discrete
members.
We informally separate the Chorrillo beds
into three “levels”, but this arbitrary subdivision
pends on future sedimentological studies. The ti-
tanosaur bones originally discovered by F. Nullo
come from the lower third of the formation, but

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
220
remains of titanosaurs of similar size also come
from the middle portion of the unit. A thick
bank of conglomerates exposes on the base of
the upper third of the Chorrillo Formation, and
above it repeats again the alternate sequence of
greenish and reddish sandstones bearing dino-
saur remains. The upper levels of the Chorrillo
Formation yielded abundant remains of diffe-
rent individuals of a single species of a basal
iguanodontian, mixed with several shed teeth of
a large, indeterminate mosasaur. The sequence
ends with a conglomeratic level (approximately
one-meter-thick), overlaid by a bank (roughly
one meter thick) made up by slender and elon-
gate bivalves (Gryphaeostrea cf. G. callophyla
Ihering, 1903 and Cubitostrea cf. C. ameghinoi
Ihering, 1902, indeterminate Mytilidae and
Pectinidae; D. Pérez, B.Santelli, and M.Álvarez,
pers. comm.). This couple of banks (conglomerate
plus bivalves) represents the base of the Calafate
Formation.
For southern Chile, Manriquez et al., (2019)
described the stratigraphic sequence of the
Dorotea Formation and its fossil content. At the
base of the formation, they identified leaf impres-
sions of a variety of taxa; from the middle section
(exposed in a fossil spot they name “Saurópodo”)
they described (in addition to plants) titanosaur,
ornithischian, and bird bones, turtle and frog
fragments, and mammal and theropod teeth. The
paper by Manriquez et al. (2019) does not specify
the taxonomic referral of the fossils collected, and
no reference of the completeness of the collected
materials is offered. However, such fossil assem-
blage from the “Saurópodo Member” looks simi-
lar to the lower and mid-sections of the Chorrillo
Formation. Interesting is to say that the levels
of “Saurópodo Member” are comprehended by
radiometric datings below (74.9my) and above
(71.7my), thus strongly suggesting these depo-
sits correspond to the Campanian-Maastrichtian
boundary (Manriquez et al., 2019).
In sum, general aspect of Chorrillo plus
Calafate beds resembles the sequence of Dorotea
beds, suggesting these units represent a same
sedimentary sequence. However, precise corre-
lation between these beds requires further stra-
tigraphy survey of the entire region. Besides,
future exploration and study also need to be
done to elucidate if the Chorrillo Formation is
equivalent or not with the dinosaur-bearing,
continental deposits of the Cerro Fortaleza
Formation (radimetrically dated as Campanian;
Sickman et al., 2018), exposed north of Argentino
Lake, on both margins of the La Leona River.
MATERIALS AND METHODS
Study area
The fossil remains come from different
spots and stratigraphic levels, within an area
of approximately 2000 m2 (Figure 1). The fossil
spots are the following:
-Nullo site (locality 1): Yielded six titano-
saur bone accumulations, which do not neces-
sarily correspond with discrete individuals.
Stratigraphically it corresponds to the lower
third of the Chorrillo Formation.
-Titanosaur tibia site (locality 2): This place
provided a single, well preserved titanosaur ti-
bia, titanosaur eggshells, sauropod teeth, the-
ropod eggshells, avian coracoid, snake vertebra,
and fish teeth. This site yielded the woods descri-
bed above. Stratigraphically it corresponds to the
lower third of the Chorrillo Formation.
-Megaraptorid site (locality 3): This place pro-
vided fragments of a single large, indeterminate
Megaraptoridae, represented by 2 vertebrae,
proximal pubis and rib fragments. Invertebrate
bioturbations were found in close proximity.
Stratigraphically this locality corresponds with
the middle portion of the Chorrillo Formation.
-Puma cave site (locality 4): 50 specimens
of terrestrial molluscs have been collected from
mid-levels of the Chorrillo beds, in close proxi-
mity to mammalian, turtle, snake, ornithopod
and theropod bone remains, and sauropod teeth.
Six articulated caudal vertebrae of a large tita-
nosaur come from 10 m far from this fossil spot.
All these specimens were produced by the same
stratigraphic level, corresponding to the middle
portion of the Chorrillo Formation.
-Ornithopod site (locality 5): Association of
different sized specimens corresponding to a
single, new species of an elasmarian ornithopod,
represented by vertebrae, phalanges, a, and me-
tatarsals. Found in close association with seven
mosasaur teeth. Stratigraphically it corresponds
to the upper third of the Chorrillo Formation,
roughly 20 meters below the beds with marine
molluscs (interpreted as the base of the Calafate
Formation).
Collected material
The collected materials belong to the collec-
tion of the Museo Padre Molina, Río Gallegos,
Santa Cruz province. Some specimens, coming
from the same site and stratigraphic levels, were
collected in 1981 by J.F. Bonaparte and are housed
at the Vertebrate Paleontology Collection, Museo
Argentino de Ciencias Naturales “Bernardino

221
Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
Rivadavia” (MACN), Buenos Aires.
Specimen preparation
Vertebrates were extracted and prepared as
is the usual technique in vertebrate paleontology.
Most bones here described do not need mecha-
nical preparation because they were found free
of sediment in most cases. The bones that were
embedded in sedimentary matrix were prepared
with mechanical hammers at the Laboratorio
de Anatomía Comparada y Evolución de los
Vertebrados preparation lab.
The fossil woods were thin-sectioned in
transverse (TS), longitudinal tangential (TLS)
and longitudinal radial (RLS) sections, and stu-
died using light Microscopy (Olympus BX51). At
least 25 measures were taken, and data is given
as the mean and standard deviation, and range
(in parenthesis). For the generic classification of
the studied woods we followed the key for fossil
genera proposed by Philippe & Bamford (2008).
Descriptive terminology used here follows the
IAWA list of microscopic features for softwood
identification (Baas et al., 2004), with the addi-
tion of terms defined in Philippe and Bamford
(2008), seriation and contiguity indices propo-
sed by Pujana et al. (2016). Pit counting me-
thod follows suggestions made by Philippe et al.
(2013). The palynological samples were treated
following standard techniques for extraction and
Figure 1. Map showing studied area. Numbers 1 through 5 indicate the localities from which fossil specimens
were collected.

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
222
concentration of palynomorphs (Volkheimer &
Melendi, 1976). Observations were made with an
Olympus BX-51 microscope while photographs
were taken with an Olympus DP25 digital came-
ra. Coordinates of the illustrated specimens are
given as England Finder coordinates.
Institutional abbreviations
MPM, Paleontological Collection, Museo
Regional Provincial “Padre Molina”, Río Gallegos,
Santa Cruz province, Argentina; MACN-Pv,
Paleontological Vertebrate Collection, RN, Río
Negro Collection, Museo Argentino de Ciencias
Naturales “Bernardino Rivadavia”, Buenos
Aires, Argentina.
SYSTEMATIC PALEONTOLOGY
Vertebrata Cuvier, 1812
Teleostei Müller, 1844
Genus and species indeterminate
Referred material: MPM 21516, three crush-
ing tooth crowns (locality 4) (Figure 2E-G).
Description: Three isolated crushing teeth
crowns have been collected. The largest one
is 13mm in diameter. They are subcircular in
contour in occlusal view, with a gently convex
occlusal surface, and a concave ventral surface.
The tooth bases show concentric growth lines on
the ventral side.
Comments: Crushing teeth similar to those
described here resemble pharyngeal teeth of
semionotiforms, sparids, sciaenoids, and bas-
al percoids, among others (e.g., Cione, 1987).
Similar teeth were previously reported from
the Maastrichtian Loncoche (González Riga,
1999; Previtera & González-Riga, 2008), Los
Alamitos (Cione, 1987), and Allen (Martinelli &
Forasiepi, 2004) formations. The semionotiform
genus Lepidotes, was repeatedly reported from
Argentina, and South America (Arratia & Cione,
1996). These semionotiforms bear crushing vom-
erine teeth, with short roots and exceptionally
broad and rounded crowns, thus differing from
the low crowns with concave roots here described
(see Jain, 1985). Information at hand forbids re-
ferral of crushing teeth from the Chorrillo beds
to any particular teleostean clade.
Amiiformes Hay, 1929
Amiidae Bonaparte, 1838
cf. Vidalamiinae Grande & Bemis, 1998
Genus and species indeterminate
Referred material: MPM 21517, isolated tooth
(locality 2) (Figure 2 H).
Description: This is a lanceolate and acute
tooth, with vitreous enamel surface. Under the
enamel layer there is a relatively thick layer of
dentine constituted by vascular canals. At its
base, the tooth is not crenulated, indicating the
absence of plicidentine. Tooth crown possesses
elongated, narrow peduncles which are constrict-
ed below the apex. The apex is formed by a short,
translucent, and flattened acrodin cap with both
acute mesial and distal carinae.
Comments: The combination of lanceolate
teeth lacking plicidentine at its base (presence of
plicidentine is typical of lepisosteiforms; Cione,
1987; Grande, 2010) and a translucent cap with
mesial and distal carinae is diagnostic of vida-
lamiine amiiform fishes (Grande & Bemis, 1998;
Martinelli et al., 2013). In spite of such similari-
ties, the isolated nature of MPM 21517 precludes
a more accurate determination.
The fossil record of Mesozoic amiiforms in the
southern Hemisphere is strongly biased (Grande
& Bemis, 1998; Martinelli et al., 2013; Brito et
al., 2017). From the southern cone, only three lo-
calities have yielded amiid remains (Bogan et al.,
2010, 2013), of which only two (i.e., El Anfiteatro
and Cerro Tortuga localities, Río Negro province,
Argentina; Bogan et al., 2010) belong to the Late
Cretaceous. Because of their fragmentary nature,
these remains from Río Negro have indetermi-
nate affinities below family level. Present report,
if correctly identified, constitutes an important
addition to the meager record of the clade for the
continent, and may represents the southernmost
record for Amiiformes.
Anura Fischer von Waldheim, 1813
Genus and species indeterminate
Referred material: MPM 21518, isolated tibio-
fibula (locality 4) (Figure 2D).
Description: The distal halves of a tibia and a
fibula have been collected. These bones are sepa-
rated by a narrow and relatively shallow longi-
tudinal groove. The distal end of each bone is
subcircular in cross-section. Based on preserved
fragments, the tibiofibula may have been mea-
sured 25 mm when complete.
Comments: The presence of an elongate and
fused tibiofibula is a feature diagnostic of
anurans (Jenkins & Shubin, 1998). However,
the morphology of the distal half of tibiofibula
lacks diagnostic features to offer a more precise
allocation within the clade. Present specimen, to-

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Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
gether with remains described below, represents
the southernmost records for a fossil frog from
the Late Mesozoic.
Calyptocephalellidae Reig, 1960
Genus and species indeterminate
Referred material: MPM 21519, distal end of
right humerus with eroded distal margin (local-
ity 4) (Figure 2A-C).
Description: The distal end of the humerus (1.5
mm in maximum transverse width), probably
belongs to an adult individual, as suggested by
degree ossification and well-finished structures.
The distal articular ball, as well as both epicon-
dyles, are distally eroded. The humeral shaft is
subtriangular in cross-section, with a relatively
sharp crest running along its medial margin,
which probably ended in a well-developed deltoid
crest as it occurs in living anurans. The distal
end of humerus bears a ball-shaped and ventrally
projected articular condyle, bordered by similar-
sized, flange-like, and transversely expanded ul-
nar and radial epicondyles. The ulnar epicondyle
shows a flattened anterior surface that is sepa-
rated from the articular ball by a shallow groove.
The radial epicondyle is broader than the ulnar
epicondyle and its anterior surface is concave
Figure 2. Fishes, anurans, and turtles. (A-C), Indeterminate Calyptocephalellidae, MPM 21519, distal end of
right humerus with eroded distal margin in anterior (A), posterior (B), and medial (C) views. (D) Indeterminate
anuran, MPM 21518, incomplete tibiofibula. (E-G) Indeterminate Teleostei, MPM 21516, tooth crown in lat-
eral (E), occlusal (F), ventral (G) views. (H) cf. Vidalamiinae indet., MPM 21517, isolated tooth in side view.
(I-K) Indeterminate Chelidae, carapace fragments. (I) Plastral fragment. (J) Incomplete costal plate. (K) Middle
peripheral plate. Abbreviations: ab, articular ball; ue, ulnar epicondyle; vf, ventral fossa. Scale bar: 5 mm.

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
224
and crescent-shaped in anterior view. The ven-
tral fossa is not clearly outlined.
Comments: MPM 21519 exhibits the following
unique combination of features, only observed
in the extant genus Calyptocephalella (Báez,
1987; Otero et al., 2014): 1) distal articular ball
large and placed at the center of the distal end;
2) prominent, subequal-sized and flange-like ra-
dial and ulnar epicondyles, resulting in a roughly
symmetrical distal end; 3) humeral shaft distally
unexpanded; 4) poorly defined ventral fossa. The
incomplete nature of MPM 21519 does not allow
referral the specimen beyond the family level.
The Mesozoic record of anurans in South
America is patchy. Basal anurans of the clade
Pipoidea have been recorded from the Early
Cretaceous of Brazil (Carvalho et al., 2019)
and from mid-to Late Cretaceous localities of
Brazil and Argentina (see Báez, 2000). The re-
cord of neobatrachians is also restrictive, be-
ing currently represented by nearly complete
specimens from the Early and Late Cretaceous
of Brazil (e.g., Báez & Perí, 1989; Báez et al.,
2009, 2012), and disarticulated specimens from
Campanian-Maastrichtian localities in Chubut
and Río Negro provinces of Argentina (i.e., Báez,
1987; Martinelli & Forasiepi, 2004; Muzzopappa
& Baez, 2009; Agnolin, 2012).
Most neobatrachians are represented by
highly fragmentary remains assigned to the non-
monophyletic clade “Leptodactylidae”, and most
of them have been assigned (or related with) the
living Helmeted toad Calyptocephalella (Báez,
1987; de la Fuente et al., 2007; Agnolin, 2012),
the only extant member of Calyptocephalellidae.
This genus is currently restricted to a single
species, Calyptocephalella gayi, being ende-
mic from the temperate region of south-central
Chile (Otero et al., 2014). However, during the
Mesozoic and Cenozoic, calyptocephalellids
were geographically widespread, with possible
reports from Late Cretaceous of India, Africa
and Madagascar (see Agnolin, 2012). In South
America, Calyptocephalellids are recorded from
Late Cretaceous to Miocene beds, in several loca-
lities along the Patagonia of Argentina and Chile
(Agnolin, 2012; Otero et al., 2014). Because of
its great antiquity, calyptocephalellids were con-
sidered as being part of the “ancient assembla-
ge” or “Andean-Antarctic” batrachofaunas that
inhabited the southern end of South America
during the Mesozoic, up to Miocene times
(Vuilleumier, 1968; Cei, 1980; see Agnolin, 2012).
Testudines Linnaeus, 1758
Chelidae Gray, 1825
Genus and species indeterminate
Referred material: Chelid materials include
MPM 21520 (consisting in two peripheral and one
costal plates; locality 2), MPM 21521 (consisting
of an anterior peripheral, one mid-peripheral,
three posterior peripherals, fragment of plastral
bridge, three incomplete costal plates, and distal
end of radius; locality 4) (Figure 2 I-K). Based on
locality and size differences, specimens belong to
at least two different individuals.
Description: In both localities, available speci-
mens were found in an area of few square me-
ters. However, the different degree of preserva-
tion and different size of the specimens indicate
the presence of different individuals. The plates
resemble each other in being relatively thin and
in having a similar ornamentation, suggesting
that all belong to the same taxon. Largest avail-
able plates suggest a maximum carapace length
of 40cm, approximately. Peripheral plates are
relatively small and dorsoventrally thin. Mid-
peripherals show relatively small and shallow
costal fossae. There is no suture for the contact
with costal plates. Plate decoration consists of
small and well-defined polygonal figures, sepa-
rated by anastomosed sulci. In some sections the
polygones became smaller, conferring the plate
a rugose texture. Preserved costal plates are in-
completely preserved. They show ornamentation
similar to that of peripheral plates, but polygones
are more elongate.
Comments: In spite of the incomplete nature,
available specimens can be referred with confi-
dence to Chelidae, based on the combined presence
of free peripheral plates lacking firm contact with
the costal ones, and external surface decoration
consisting of dichotomizing sulci and polygones
(Broin & De la Fuente, 1993; Lapparent de Broin
& De la Fuente, 2001; Lapparent de Broin, 2003).
Plates here described are indistinguishable from
those reported by Broin (1987) under the name
of “Chelidae indet. Nº 3”, and by Gasparini & de
la Fuente (2000) as “Chelidae indet. Nº5”, from
the Maastrichtian Los Alamitos and La Colonia
formations (Río Negro and Chubut provinces, re-
spectively).
Squamata Oppel, 1811
Serpentes Linnaeus, 1758
Genus and species indeterminate
Referred material: MPM 21522, partial mid-

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Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
posterior vertebra (locality 4) (Figure 3).
Description: MPM 21522 is a small vertebra,
7mm long, consisting of centrum, left prezygopo-
physis, and ventral region of the neural canal.
As in most snakes, it shows an anteroventrally
facing articular cotyle, subcentral ridges, grooves
and foramina and synapophyses. This element
is identified as a precloacal vertebra because of
the absence of pleurapophyses, lymphapophyses,
and hemapophyses (LaDuke, 1991). It shares a
combination of characters present in middle and
posterior precloacals (wide and shallow haemal
keel, elongated vertebral centrum, and synapo-
physes with a strong lateral component ventrally
located).
In anterior view the articular cotyle is sub-
circular in contour. The neural canal was dorso-
ventrally low and with a subtriangular contour in
anterior view. There are two well-defined paraco-
tylar foramina, located close and at mid height
of the cotyle. The foramina are situated within
shallow paracotylar fossae, adjacent to the left
side of the cotyle. The prezygapophysis has the
articular facet inclined about 20° above the hori-
zontal plane, and barely surpass the lateral mar-
gin of the paradiapophysis (=synapophysis). The
ventral margin of the synapophysis is lower than
the ventral margin of the articular cotyle.
In dorsal view, the prezygapophyseal facet is
oval-shaped, with its main axis obliquely orien-
ted, about 40-50° from the sagittal plane. At the
base of the facet there is a well-defined and small
fossa. The prezygapophiseal accessory process is
absent and the interzygapophyseal rim is sha-
llow.
The posterior articular condyle is below the
level of paradiapophysis. The neural canal seems
taller than in anterior view, and is sub-triangular
in contour. A small portion of the posterodorsal
surface of the vertebral centrum is preserved,
and bears no medial keel.
The centrum has a parallelogram-shaped
profile in lateral view, with the posterior region
in a lower level than the anterior region. The pa-
radiaphophysis is massively built, with no clear
differentiation from the both dia- and parapo-
physes. The paradiapophysis is sub-rectangular
in contour and is postero-ventrolaterally faced,
with a strong lateral component. Posterior to it,
at mid-length of the vertebral length, there is a
lateral foramen. The diapophyseal region is wi-
der and more posteriorly placed than the parapo-
physeal region. Posterior to the paradiapophysis
there is a moderately developed subcentral ridge
that laterally surrounds the subcentral fossa.
The haemal keel is low and ventrally concave,
and the subcentral foramina are not visible due
the great depth of the subcentral fossa.
In ventral view, the vertebral centrum is sub-
triangular shaped, its lateral margins converging
Figure 3. Serpentes indet. (MPM 21522) partial mid-posterior vertebra in anterior (B), posterior (B), lateral (C),
ventral (D) and dorsal (E) views. Abbreviations: ct, cotyle; izr, interzygapophiseal ridge; hk, haemal keel; lf,
lateral foramen; prz, prezygapophysis; pk, pseudo keel; scf, subcentral foramen; scr, subcentral ridge; sy, syn-
apophisis; vf, ventral foramen. Scale bar: 5 mm.

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
226
posteriorly. The haemal keel is transversely wide
and is hourglass shaped, with the anterior region
transversely wider than the posterior half. The
subcentral fossae are well developed and latera-
lly delimit the haemal keel. On the ventral surfa-
ce of the haemal keel there are present two fora-
mina. Between them there is a pseudo-keel that
extends all the length of the haemal keel. This
pseudo-keel is poorly-developed, but distinctive.
Comments: Albino (Albino, 1986, 1987, 1994;
see also Martinelli & Forasiepi, 2004) described
several serpent taxa based on isolated vertebrae
from the Latest Cretaceous (Maastrichtian)
Los Alamitos Formation, Rio Negro province.
These specimens belong to basal snakes of the
“madtsoiid” grade. More recently, Gómez et al.
(2008) described from coeval beds the “anilioid”
Australophis anilioides from northern Patagonia.
In spite of this relative rich record, they restrict
to the NW corner of Patagonia. Thus, present
report extends paleogeographic distribution of
these reptiles up to the southern region of South
America.
In spite of being incomplete, the specimen
MPM 21522 shows a combination of characters
unique among snakes. Presence of a paracotylar
foramen is shared with madtsoiids, Dinilysia,
several specimens of Colombophis (Simpson,
1933; Albino, 1986, 1994, 2000; Rage & Wagner,
1999; Scanlon, 2006), boiids (Albino, 2010), and
it is variably present among anilioids (Hsiou et
al., 2019). Paracotylar foramina are absent in
Najash, Menarana, and most anilioiids (Rage,
1984, 1998; Gómez et al., 2008; Zaher et al., 2009;
La Duke et al., 2010). As occurs in the madts-
oiids Madtsoia, Adinophis, and Alamitophis,
MPM 21522 presents two paracotylar foramina
(Simpson, 1933; Albino, 1994; La Duke et al.,
2010; Pritchard et al., 2014).
Resembling Dinilysia, and the madtsoiids
Alamitophis (Albino, 1986; MACN-RN 27,38),
Nidophis, Menarana and Madtsoia pisduren-
sis (La Duke et al., 2010; Mohabey et al., 2011;
Vasile et al., 2013), the prezygapophysis of MPM
21522 extends slightly beyond the lateral mar-
gin of the synapophysis. In Colombophis and
anilioiids (Rage, 1984, 1998; Hsiou et al., 2010)
the prezygapophyses extend much further than
the synapophisys, whereas the reverse condi-
tion applies for Najash, Adinophis, Gigantophis,
Nanowana, Patagoniophis and several Madtsoia
species (Scanlon, 1997; Rage, 1998; Zaher et
al., 2009; La Duke et al., 2010; Pritchard et al.,
2014). The absence of a prezygapophiseal acces-
sory process is a plesiomorphic feature shared by
MPM 21522, madtsoiids, and Najash (Rage &
Wagner, 1999; Albino, 2000; Scanlon, 2006; La
Duke et al., 2010; Mohabey et al., 2011; Zaher et
al., 2009).
The haemal keel present in MPM 21522 is
low and wide, similar to the condition of poste-
rior precloacal vertebrae in most madtsoiids and
anilioiids. It is flanked by two small subcentral
foramina, as occurs in posterior precloacal verte-
brae of Dinilysia (MACN-RN 116). Furthermore,
MPM 21522 differs from all known madtsoiids
and other fossil snakes because of the presence
of a pseudo-keel, flanked by two foramina loca-
ted on slits, on the ventral surface of the haemal
keel. A somewhat similar condition is found in
Colombophis and in the supposed tropidophiid
Dunnophis (Rage, 1984), where the haemal keel
is wide and flat and two subcentral foramina
are present on the ventral surface of the keel;
however, there is no presence of a pseudo-keel
(Rage, 1984; Hsiou et al., 2010). Rage (2013) re-
ported for Paraungaliophis a ventral longitudi-
nal keel producing a shallow sagittal ridge, but
this keel differs from MPM 21522 in being sharp
and restricted to the posterior end of the keel.
Another distinguishable feature of MPM 21522
is that the foramina present on the ventral sur-
face are placed in a slit, similar to the anilioiid
Hoffstetterella, where the subcentral fossa of the
anterior and mid precloacal vertebra are located
within slits (Rage, 1998).
The synapophyses of MPM 21522 is well de-
veloped and the diapophyseal and parapophyseal
regions are plesiomorphically poorly differentia-
ted. This condition is also present in Alamitophis,
Colombophis, Anilius and Coniophis (Rage, 1984;
Albino, 1994; Hsiou et al., 2010). On the contrary,
the synapophyses in most of the madtsoiids and
the anilioiid Hoffstetterella are well-defined into
para- and diapophyseal regions (Simpson, 1933;
Rage, 1998; Zaher et al., 2009; La Duke et al.,
2010; Mohabey et al., 2011; Vasile et al., 2013;
Pritchard et al., 2014; Rio & Mannion, 2017), a
derived condition found in all alethinophidian
snakes (Rieppel et al., 2002; Apesteguía & Zaher,
2006).
As in Dinilysia, Najash, and the madts-
oiids Madtsoia madagascarensis, Nidophis and
Menarana (Zaher et al., 2009; La Duke et al.,
2010; Vasile, 2013), the subcentral ridges and
fossae in MPM 21522 are well developed. This
contrast with some madtsoiids (e.g., Madtsoia,
Gigantophis, Adinophis) and most anilioids
(Simpson, 1933; Rage, 1998; Gómez et al., 2008;
Mohabey et al., 2011; Pritchard et al., 2014; Rio

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Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
& Mannion, 2017; Hsiou et al., 2010) which have
poorly defined subcentral ridges and fossae.
In lateral view, a lateral foramen can be obser-
ved in most madtsoiids and anilioiids (Simpson,
1933; Rage, 1998; Albino, 2000; Zaher et al.,
2009; La Duke et al., 2010; Vasile et al., 2013;
Pritchard et al., 2014).
The interzygapophyseal constriction in MPM
21522 is shallow as in Dinilysia, Madtosoia pis-
durensis and most anilioiids (Rage, 1998; Gómez
et al., 2008; Mohabet et al., 2011), which differ
from Najash, Colombophis and most madtsoiids,
where it is notably deep (Simpson, 1933; Rage,
1998; Zaher et al., 2009; La Duke et al., 2010;
Vasile et al., 2013; Rio & Mannion, 2017; Hsiou
et al., 2010).
In spite of its distinctiveness, MPM 21522 is
here regarded as an indeterminate Serpentes be-
cause of its fragmentary condition.
cf. Rionegrophis madtsoioides Albino,
1986
Referred materials: MPM 21523, partial ver-
tebra (locality 2) (Figure 4).
Description: Description: MPM 21523 is an in-
complete vertebra, approximately 14 mm long as
preserved. As in most snakes, the cotyle faces an-
teroventrally, the subcentral ridges and grooves
are well-defined, and synapophysis are present.
The vertebra corresponds to the precloacal re-
gion because of the absence of pleurapophyses,
lymphapophyses, and hemapophyses (LaDuke,
1991). Due to the tall and narrow haemal keel,
and the absence of a hypoapophysis, this element
corresponds to the mid-precloacal region.
The cotyle is notably deep and dorsolaterally
flanked by a deep paracotylar foramen. The distal
end of the prezygapophysis is missing, thus the
presence of a prezygapophyseal process remains
unknown. In dorsal view, the prezygapophysis
shows a subtriangular contour, its major axis
forming an angle about 45° with the sagittal pla-
ne. The paradiaphophysis is widely exposed; the
diapophysis region of the synapophysis is more
developed and more posteriorly located than the
parapophyseal region of the synapophysis.
The condyle is prominent, sub-circular sha-
ped in posterior view, and slightly dorsally poin-
ted. The ventral surface of centrum exhibits on
its posteriormost region an haemal keel, flanked
by two small depressions, probably representing
the posterior portion of the subcentral grooves.
Subcentral ridges are present.
Comments. Due to the fragmentary na-
ture of MPM 21523, comparisons are limited.
However, its size and general morphology closely
resembles Rionegrophis madtsoioides (Albino,
1986). MPM 21523 shows a well-developed and
narrow haemal keel, similar to some madts-
oiids and the anilioiid Hoffstetterella brasiliensis
(Scanlon, 1997; Rage, 1998; Vasile et al. 2013),
whereas Dinilysia and Najash show a notably
wider haemal keel (Zaher et al. 2009; MACN-RN
2019). Further, the synapophysis of MPM 21523
resembles most madtsoiids synapophyses becau-
se the diapophysis is wider than the parapophy-
sis.
Presence of a well-developed keel present
on the dorsal surface of the centrum is shared
with Alamitophis and Rionegrophis madtsoioi-
des (Albino, 1986; MACN-RN 27, 32). However,
MPM 21523 differs from Alamitophis and resem-
bles Rionegrophis in the presence of a subtrian-
gular-shaped centrum when viewed dorsally or
ventrally.
In gross-morphology and main features MPM
21523 resembles Rionegrophis madtsoioides.
However, its fragmentary nature precludes a
clear determination, and thus is identified here
as cf. Rionegrophis madtsoioides.
Squamata Oppel, 1811
Mosasauridae Gervais, 1852
Genus and species indeterminate
Referred material. MPM 21524, seven isolated
teeth (locality 5) (Figure 5).
Description. Seven mosasaur teeth have been
collected from the upper levels of the Chorrillo
Formation, all of them coming from a single fos-
sil spot which also yielded ornithopod remains
(described below). We assume these teeth, found
in close association, belong to a single mosasaur
individual. Available teeth share the following
combination of characters supporting their refer-
ral as to Mosasauridae (Hornung & Reich, 2015):
1) crowns bearing distal and mesial carinae, 2)
different level of enamel striation, 3) crown po-
lygonal-shaped in cross-section, and 4) crown tip
posteriorly curved.
Available teeth can be sorted into two di-
fferent types, in accordance with size, enamel
ornamentation and morphology. One type is re-
presented by fragments of three large teeth, the
best preserved of which has an almost complete
crown (Figure 5A-C), being 44 mm in total height
and 20 mm in diameter at its base. The basal
quarter of these teeth is devoid of enamel, stria-
tions, and carinae, indicating proximity to bone

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
228
Figure 4. cf. Rionegrophis madtsoiids (MPM 21523), one partial vertebrae in dorsal (A, C), anterior (B), lateral
(D), posterior (E) and ventral (F) views. Abbreviations: ct, cotyle; cn, condyle; dk, dorsal keel; pcf, paracoty-
lar foramina; prz, prezygapophysis; scr, subcentral ridge; sy, synapophisis; vk, ventral keel. Scale bar: 5 mm.
attachment (Figure 5). The crowns are conical-
shaped, being only slightly distally curved in side
view. Tooth crowns lack striations on their api-
cal quarter, but show a gently convex wear facet.
Crown bases are elliptical in cross-section, with
the labial side strongly convex and lingual side
slightly more flattened than the opposite side.
Mesial and distal carinae are sharp and devoid
of serrations, being the distal carina stronger
than the mesial one. Applying the terminology
for enamel ornamentation used by Hornung &
Reich (2015) these crowns have primary and se-
condary striae with no ramifications, with some
of the primary striae converging adapically with
secondary striae. Labially, the crowns exhibit
three main convex prism faces (sensu Hornung
& Reich, 2015), which are absent on the lingual
surface.
The second tooth type is represented by an
almost complete tooth crown (Figure 5D,E). It is
small sized (total height 15mm, basal diameter
10mm), subtriangular-shaped in side view, and
elliptical in cross-section. Both mesial and distal
margins are slightly convex. The labial side of the
crown is smooth and devoid of ornamentation,
whereas the lingual side bears fine basal striae.
The base of this tooth is devoid of enamel.
Comments. The Campanian-Maastrichtian fos-

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Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
sil record of mosasaurs described for southern
Patagonia and Antarctic Peninsula includes re-
mains referred as to mosasaurines, halisaurines,
tylosaurines, and plioplatecarpines (Ameghino,
1893; Gasparini & Del Valle, 1981; Martin et al.,
2002; Otero et al., 2017). Most of them are based
upon shed teeth and isolated vertebrae, and have
been considered of dubious affinities by Novas
et al. (2002). Currently, only two taxa have been
confidently identified on the basis of cranial, den-
tal and postcranial remains: Taniwhasaurus ant-
arcticus (Novas et al., 2002; Fernandez & Martin,
2007) and Kaikaifilu hervei (Otero et al., 2017),
both of them belonging to Tylosaurinae. Teeth of
Taniwhasaurus antarcticus (Novas et al., 2002)
differ from those reported here in having slen-
derer and posterolingually curved crowns, nar-
rower striae, and feebly developed unserrated
carinae. Besides, teeth here described resemble
the large Antarctic mosasaur Kaikaifilu on their
large size, overall shape and cross section of the
crown. However, teeth of Kaikaifilu bear only a
mesial carina and are more finely striated.
Several tooth remains from the Late
Cretaceous of Antarctica have been reported as
belonging to indeterminate mosasaurines (e.g.,
Martin, 2006; Martin and Crame, 2006). These
teeth differ from those here described in being
more labiolingually compressed, with enamel
surface smooth, and prism faces greater and
flatter. Besides, isolated teeth from Antarctica
referred as to tylosaurines and platecarpines
(e.g., Martin, 2006; Martin & Crame, 2006) con-
sist on poorly preserved materials, precluding
detailed comparisons with present specimens.
Teeth of Mosasaurus aff. M. hoffmani from
the Late Cretaceous of Río Negro province
(Fernández et al., 2008) differ from those re-
ported here in having lingual surface wider, and
enamel surface smooth. Teeth of “Liodon argen-
tinus” (Ameghino, 1893), from the Cenomanian
Mata Amarilla Formation, Pari Aike, Santa Cruz
province, share with those collected in Chorrillo
beds in bearing both anterior and posterior cari-
nae, but differ in being more laterally compres-
sed, with stronger carinae and serration in the
anterior carina.
In sum, MPM 21523 could not be referred
to any of the previously listed Patagonian and
Antarctic mosasaur taxa, and thus they are con-
sidered as Mosasauridae indet.
Some authors (e.g., Lindgren, 2005;
Fernández, 2008; Lindgren & Siverson, 2002,
2004) have conferred a great taxonomic value
to mosasaur shed teeth, taking them as enough
evidence for the presence of a particular mosa-
saur family, or even mosasaur genera. However,
we agree with Caldwell and Diedrich (2005) in
that the taxonomic importance of dentition has
been overestimated. For example, the presence
of strong facets has been considered diagnostic
for mosasaurines, instead the presence of deep
striations was interpreted distinctive for plio-
platecarpines (e.g., Martin & Crame, 2006).
However, as indicated by specimens from the
Chorrillo beds, these characters are vague and
are present in many non-related taxa. It has been
shown that tooth morphology varies along den-
tal series of a single individual (see for example
Konishi et al., 2012), and some taxa display up
Figure 5. Mosasauridae indet. (MPM 21524). Tooth types 1 (A-C) and 2 (D-E) in mesial (A), distal (B), basal (C,
E), and lingual views. Abbreviations: e1, primary striation; e2, secondary striation; ca, carina; wf, wear facet.
Scale bars: 10 mm.

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
230
to four distinct tooth types along the jaws (e.g.,
Schulp et al., 2004; Thevenin, 1896; Otero et al.,
2017). Further, ontogenetic changes have been
also recognized in mosasaur literature (Lindgren
& Siverson, 2004). In sum, we consider that the
taxonomic value of isolated mosasaur teeth have
to be taken with caution.
Regarding the possible paleoenvironmental
significance of mosasaur teeth, it is outstanding
their close taphonomic association with ornitho-
pod bones. The presence of mosasaur remains
is suggestive of marine influence during the en-
tombing of basal iguanodontians. This resembles
the El Puesto “member” of the upper third of the
Dorotea Formation, which yielded hadrosaurs
remains (Jujihara et al., 2014; Soto Acuña et al.,
2014), immediately below marine levels with
aquatic fossils (including mosasaurs; Soto Acuña
et al., 2016).
Dinosauria Owen, 1842
Ornithischia Seeley, 1887
Ornithopoda Marsh, 1881
Euiguanodontia Coria & Salgado, 1996
Elasmaria Calvo, Porfiri & Novas, 2007
Isasicursor santacrucensis gen. et sp. nov.
Figure 6
Holotype. MPM 21525, proximal end of left tibia
(Figure 7). The specimen comes from locality 5.
Paratypes: MPM 21526, incomplete cervical
centrum; MPM 21527, two dorsal vertebrae;
MPM 21528, sacrum lacking sacral 3rd; MPM
21529, 30 caudal vertebrae; MPM 21530, proxi-
mal end of right scapula ; MPM 21531, distal
half of left humerus; MPM 21532, iliac process
of left pubis; MPM 21533, set of juvenile speci-
mens represented by proximal end of right femur
, three proximal end of left femora, three distal
end of left femora, and two fragments of femur
mid-shaft; MPM 21534, distal end of tibia; MPM
21535, proximal and distal end of metatarsal II;
MPM 21536, distal end of left metatarsal III;
MPM 21537, proximal and distal ends of meta-
tarsal IV; MPM 21538, nearly complete meta-
tarsal IV of a juvenile individual; MPM 21539,
pedal phalanges I-1, II-1, IV-2/IV-3; MPM 21540,
six pedal ungual phalanges; MPM 21541, pedal
phalanges II-2 and III-2 of juvenile individuals.
As for the holotype, all the referred specimens
come from locality 5.
Diagnosis. Medium-sized elasmarian ornitho-
pod having the following autapomorphies: 1)
ventrally arched sacrum; 2) peg-and-socket ar-
ticulation between the first and second sacral
vertebrae in ventral view; 3) tibia with strongly
proximally projected and thickened cnemial
crest, 4) lateral condyle of tibia with an addi-
tional anterolateral process; and 5) metatarsal II
with a proximally displaced collateral ligament
pit on its lateral surface.
The preserved elements of the hindlimb,
especially metatarsals are more robust than
other elasmarians of comparable size, such as
Morrosaurus and Talenkauen.
Horizon and Locality. Collected specimens
come from locality 5.
Etymology. Isasicursor honors the skilled
technician Marcelo P. Isasi (Conicet - MACN),
who discovered the remains of this new iguano-
dontian, and cursor, meaning runner in Latin;
santacrucensis, in regards to Santa Cruz, the
Argentine province where fossils were found.
Description
Cervical vertebra (Figure 8). The only recov-
ered cervical vertebra of Isasicursor is represent-
ed by the anterior third of a centrum. In anterior
view the articular surface is concave and roughly
hexagonal in contour. The parapophyses are rep-
resented by poorly developed processes, subtri-
angular in contour when viewed from the side.
Ventral to the parapophyses the lateral surfaces
of centrum are transversely concave. In ven-
tral view, there is a sharp ventral longitudinal
keel, a condition considered synapomorphic for
Elasmaria (Rozadilla et al., 2019).
Dorsal vertebrae (Figure 8). Two incomplete
dorsal vertebrae are preserved. The centra are
laterally compressed, conferring an hourglass
contour in ventral view. In lateral view, the ven-
tral margin of the centra possesses ventral but-
tresses near the anterior and posterior articular
surfaces, with a concave margin between them.
The anterior and posterior articular surfaces are
slightly concave and oval in contour. They are
dorsoventrally taller than transversely wide, in-
dicating these centrae correspond to the mid-dor-
sal series. The ventral surface bears a stout ven-
tral keel, as occurs in most basal ornithopods in-
cluding elasmarians (i.e., Coria & Salgado, 1996;
Martínez, 1998; Coria & Calvo, 2002; Novas et
al., 2004; Calvo et al., 2007; Coria et al., 2013;
Ibiricu et al., 2014; 2019; Cruzado-Caballero et
al., 2019; Rozadilla et al., 2019). Some vascular
foramina are present on the lateral and ventral
surfaces of the centra.
Sacral vertebrae (Figure 9). The sa-
crum is represented by six disarticulated

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Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
centra,corresponding to sacrals 1,2 and 4, 5 and
6. They lack signs of fusion and pneumatization,
and the intervertebral attachment surfaces are
ornamented by radiating grooves.
The sacrum is relatively stout, with ventral
margin is strongly arched in side view, a condi-
tion absent in other known elasmarians. The first
sacral is box-shaped and is the transversely wider
element of the series. In ventral view, its anterior
margin is notably transversely wide and presents
large, fan-like transverse processes that bear ar-
ticulation facets for the sacral ribs. This strong
lateral projection is shared with Jeholosaurus
and Anabisetia (Cambiaso, 2007; Han et al.,
2012), whereas in Macrogryphosaurus these pro-
jections are feebly developed and in Sektensaurus
are totally absent (Ibiricu et al., 2019; Rozadilla
et al., 2019). The articular surface for ribs is oval
in contour and bears a rugous surface, being dor-
solaterally facing. The anterior articular surface
of the centrum is kidney-shaped, dorsoventrally
low and transversely wide. The posterior sur-
face has a dorsoventrally oriented groove that
ends in a socket-like pit on its ventral margin.
This pit articulates with a process located on
the anteroventral surface of the second sacral,
resulting in a peg-and socket articulation. In
other elasmarians, such as Gasparinisaura and
Sektensaurus the contact between both elements
is nearly flat (Ibiricu et al., 2019; Rozadilla et al.,
2019). On the ventral surface, this sacral has a
shallow ventral keel located at the bottom of a
wide and shallow longitudinal groove. Some nu-
tritious foramina occur on the ventral and lat-
eral surfaces of the centrum.
The second sacral centrum is box-shaped and
transversely compressed, which results in an
hourglass outline in ventral view. The posterior
end of the centrum bears sacral rib facets, which
expand laterally resulting in the transversely
widest portion of the bone. The sacral rib facet
is anteroposteriorly short, with a rugose attach-
ment surface. The ventral surface of centrum
has a smooth longitudinal keel, which projects
anteriorly into a peg-like process that fits into
the socket present in the posterior surface of the
first sacral. On the sides of this ventral keel there
are subparallel and shallow longitudinal grooves.
The anterior articular surface of the centrum is
suboval in contour, with the main axis trans-
versely oriented. The posterior articular surface
is suboval in contour, being slightly transversely
wider than the anterior half of centrum.
The third sacral is not preserved. The fourth
sacral is box-shaped and its lateral surface bears
laterally projected processes for the attachment
with the sacral ribs on its anterior and posterior
ends. The anterior attachment for the sacral rib
is more dorsally located and anteroposteriorly
elongated than the posterior one. These surfaces
are concave and rugose. The ventral surface of
centrum shows a shallow median longitudinal
groove. A low median ridge exists near its ante-
rior end. The anterior articular surface of cen-
trum is rugose and sub-quadrangular in contour,
instead the posterior articular surface is hexag-
onal-shaped and transversely wider than the an-
terior one.
The fifth sacral vertebra is box-shaped, with
a trapezoidal outline in ventral view, its anterior
end transversely wider than its posterior one.
The floor of the neural canal is narrow and bears
an oval pit at mid-length. Sacral rib facets are an-
teroposteriorly elongate and located at the ante-
rior half of the bone. These articulation surfaces
are suboval in contour and show strong rugosi-
Figure 6. Isasicursor santacrucensis gen. et sp. nov. Skeletal reconstruction and body shape, indicating the disco-
vered bones in white.

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
232
ties on its attachment surface. Posterior and dor-
sal to the sacral rib facets, the centrum becomes
transversely narrower. The ventral surface of
centrum bears a shallow and wide longitudinal
groove, with a low and smooth ridge restricted
to its anterior margin. The anterior articular
surface is suboval in contour, dorsoventrally low
and transversely wide. Its dorsal edge has an an-
terior process that fits into the concavity of the
posterior surface of the preceding vertebra. The
posterior articular surface is sub-quadrangular
in contour, dorsoventrally high and transversely
narrower than the anterior articular surface.
The sixth sacral vertebra is represented by a
weathered centrum. The centrum is box-shaped,
with a wide and shallow longitudinal ventral
groove. Few nutritious foramina exist on its lateral
and ventral surfaces. The anterior articular sur-
Figure 7. Isasicursor santacrucensis gen. et sp. nov. Femur (MPM 21533a) in (A) anterior, (B) posterior, (C) late-
ral, (D) medial, (E) proximal and (F) distal views. Tibia (MPM 21525a) in (G) lateral, (H) medial, (I) anterior, (J)
posterior, (K) proximal and (L) distal views. Abbreviations: cc, cnemial crest; fib, fibular facet; ff, flexor fossa;
fh, femoral head; gt, greater trochanter; itg, intertrochanteric groove; lc, lateral condyle; lp, lateral process; mc,
medial condyle. Scale bar: 10 cm.

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Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
face is sub-rectangular in contour. There is a small
prominence at the lateral margin that contacts a
shallow concavity present at the posterior articu-
lar surface of the preceding vertebra, resulting in
a shallow peg and socket articulation. The pos-
terior articular surface is subcircular in contour.
Caudal vertebrae (Figure 8). Thirty caudal
vertebrae, corresponding to different individu-
als, have been recovered. These vertebrae belong
to the anterior, median and posterior portions
of the tail. Anterior caudal vertebrae are robust
and slightly amphicoelous. The neural arches
preserve the base of the diapophyses, which is
Figure 8. Isasicursor santacrucensis gen. et sp. nov. Selected vertebral elements. Cervical vertebra (MPM 21526)
in (A) anterior, (B) ventral and (C) lateral. Dorsal Vertebra (MPM 21527) in (D) lateral, (E) anterior, (F) posterior
and (G) ventral views. Proximal caudal vertebra (MPM 21529) in (H) lateral, (I) anterior, (J) dorsal and (K) ventral
views. Middle caudal vertebra (MPM 21529) in (L) lateral, (M) ventral and (N) anterior views. Posterior caudal
vertebra (MPM 21529) in (O) lateral, (P) ventral and (Q) anterior views. Abbreviations: di, diapophysis; hf, hae-
mal facet; lk, lateral keel; nc, neural canal; ns, neural suture; vg, ventral groove; vk, ventral keel. Scale bar: 5 cm.

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
234
suboval in cross-section. The centra are dors-
oventrally taller than anteroposteriorly long;
they are laterally concave, resulting in a spool-
shaped profile in ventral view. In lateral view, the
ventral margin of the centra shows an extensive
haemal facet, which is sub-triangular in contour
and strongly anteroventrally oriented. The ven-
tral surface of the centra bears a deep longitu-
dinal groove. Shallow longitudinal striations are
present near both anterior and posterior articu-
lar surfaces.
Mid-caudals are represented by isolated cen-
tra. These are slightly amphicoelous, and dors-
oventrally lower and anteroposteriorly longer
than in anterior caudals. The lateral surface
shows lateral longitudinal ridges, which become
sharper towards the distal end of the tail. In
ventral view, the centra are hourglass-shaped
and show a ventral longitudinal groove, which
becomes deeper towards the distal end of the
tail. The haemal facet is relatively smaller than
in anterior caudals. The anterior and posterior
articular surfaces are subcircular in contour and
sub-equal in size and shape.
Vertebrae of the distal section of the tail re-
semble those of the middle section, excepting for
being anteroposteriorly longer and with more
prominent lateral longitudinal ridges, resulting
hexagonal-shaped in cross-section. The ventral
longitudinal groove is well developed and bound-
ed by two subparallel longitudinal ridges. Distal
caudals are indistinguishable from other basal
ornithopods (e.g. Gasparinisaura, Anabisetia,
Talenkauen, Macrogryphosaurus, Diluvicursor,
Sektensaurus, Trinisaura, Jeholosaurus; Coria
& Salgado, 1996; Cambiaso, 2007; Calvo et al.,
2007; Barrett et al., 2011; Han et al., 2012;
Barrett, 2016; Herne et al., 2018; Ibiricu et al.,
2019; Rozadilla et al., 2019).
Scapula (Figure 10). The right scapula is rep-
resented by its proximal end. It is anteroposte-
riorly extensive, with thick acromial process.
It narrows anteriorly, showing a wide and flat
dorsal surface. The acromial process is laterally
expanded, forming the deltoid rim, which delim-
its the dorsal margin of a shallow deltoid fossa.
The glenoid cavity is severely damaged. It is ven-
trolaterally oriented and its anterior margin is
delimited by a shallow supraglenoid fossa. This
fossa is subcircular in contour and is ornamented
by small striations, being relatively shallow, as
occurs in Gasparinisaura and Trinisaura (Coria
& Calvo, 2002; Coria et al., 2013), whereas in
Anabisetia, Talenkauen and Mahuidacursor it
shows a deep supraglenoid fossa (Coria & Calvo,
2002; Cruzado-Caballero et al., 2019; Rozadilla et
al., 2019). Near the coracoidal suture the medial
surface of the scapula is covered by numerous
ligament scars. The coracoidal articular facet is
sub-rhomboidal in outline and strongly rugose.
Humerus (Figure 10). The left humerus is rep-
resented by isolated and incomplete shaft and
distal end. The shaft is laterally bowed, as sy-
napomorphic for Elasmaria (e.g., Anabisetia,
Notohypsilophodon, Talenkauen, Trinisaura,
Sektensaurus, Mahuidacursor; Martínez, 1998;
Coria & Calvo, 2002; Novas et al., 2004; Coria
et al., 2013; Ibiricu et al., 2014; 2019; Cruzado-
Caballero, 2019; Rozadilla et al., 2019). The del-
topectoral crest is reduced and represented by a
low ridge, its lateral surface crossed by longitudi-
nal muscle scars, as diagnostic for Elasmaria (e.g.,
Anabisetia , Notohypsilophodon, Talenkauen,
Trinisaura, Sektensaurus, Mahuidacursor;
Martínez, 1998; Coria & Calvo, 2002; Novas et
al., 2004; Coria et al., 2013; Ibiricu et al., 2014;
2019; Cruzado-Caballero, 2019; Rozadilla et al.,
2019), in contrast with the well-developed del-
topectoral crest present in most ornithopods,
including Gasparinisaura (Coria & Salgado,
1996; Rozadilla et al., 2019). The anterior sur-
face of the humeral shaft is concave, while the
posterior one is convex. The medial surface bears
an oval nutritious foramen. The shaft is reni-
form in cross-section. The distal end of the bone
is slightly transversely expanded. The anterior
surface of the bone is deeply concave proximal
to the distal condyles. In distal view, the lateral
condyle is more extensive than the medial one.
The flexor fossa is transversely wider than the
extensor one.
Pubis. The pubis is only represented by the ili-
ac process. This process is oval in cross-section.
The articular margin is almost flat and slightly
projected ventrally in a short process, being not
fused with the obturator process, indicating a
posteriorly opened obturator foramen, resem-
bling the condition of basal ornithopods (e.g.,
Hypsilophodon, Gasparinisaura, Anabisetia,
Trinisaura; Galton, 1974a; Coria & Calvo, 2002;
Cambiaso, 2007; Barrett et al., 2011; Coria et al.,
2013).
Femur (Figures 7, 11). The femora are repre-
sented by several specimens, including a proxi-
mal end belonging to a juvenile individual. The
femoral head is not preserved, but the neck is
well-defined. The greater trochanter is com-
pletely preserved only in the juvenile specimen.
In proximal view, the lateral margin of the great-
er trochanter is sigmoidal in outline. In lateral

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Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
view, this trochanter is proximally convex, and is
medially delimited by the groove that separates
it from the lesser trochanter. This indicates that
the lesser trochanter was not fused to the major
one, a condition different from Gasparinisaura
(Coria & Salgado, 1996). The lesser trochanter
is represented by its base, indicating that to-
wards its anterior end it was notably narrow, a
condition shared with Morrosaurus, Anabisetia,
Trinisaura and Gasparinisaura (Coria &
Salgado, 1996; Coria & Calvo, 2002; Coria et al.,
2013; Rozadilla et al., 2016).
The lateral surface near the proximal end
shows a bump ornamented with longitudinal
scars, which likely represents the anchoring of
the for the M. iliofemoralis externus. The an-
terior surface between the greater trochanter
and the femoral head is concave. The femo-
ral shaft is subtriangular in cross-section, be-
ing posteriorly flat and anteriorly narrow.
Figure 9. Sacrum of Isasicursor santacrucensis gen. et sp. nov. (MPM 21528) in (A) anterior, (B) dorsal, (C)
ventral, (D) right, (E), left and (F) posterior views. I-VI indicates anteroposterior position in vertebral series.
Abbreviations: f, vascular foramen; p, peg; pit, pit; p-s, peg and socket articulation; s, scar; sr, sacral rib; vg,
ventral groove; vk, ventral keel. Scale bar: 3 cm.

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
236
The distal end of the femur is transversely
expanded. In distal view, the medial condyle is
transversely wider than the lateral one. This
asymmetry is present in many elasmarians, such
as Anabisetia, Morrosaurus and Kangnasaurus,
as well as in more derived Iguanodontia (Cooper,
1985; Coria & Calvo, 2002; Norman 2004;
Norman et al., 2004; Rozadilla et al., 2016). The
medial condyle has a sub-rectangular outline,
and slightly tapers posteriorly. In medial view,
Figure 10. Isasicursor santacrucensis gen. et sp. nov. Scapula (MPM 21530) in (A) lateral (B), medial and
(C) coracoidal views. Humerus (MPM 21531) in (D) anterior, (E) posterior, (F) medial and (G) lateral views.
Abbreviations: dc, deltopectoral crest; df, deltoid fossa; dr, deltoid rim; ef, extensor fossa; ff, flexor fossa; gl, gle-
noid fossa; hf, humeral foramen; lc, lateral condyle; mc, medial condyle; sgf, supraglenoid fossa. Scale bar: 10 cm.

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Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
Figure 11. Isasicursor santacrucensis gen. et sp. nov. Selected juvenile materials. Right femur (MPM 21533b)
in (A) anterior, (B) posterior, (C) lateral (D) medial and (E) proximal views. Left tibia (MPM 21534b) in
(F) anterior, (G) posterior, (H) lateral (I) medial and (J) distal views. Metatarsal II in (K) anterior, (L) pos-
terior, (M) lateral (N) medial and (O) distal views. Metatarsal IV (MPM 21538b) in (P) anterior, (Q) poste-
rior, (R) lateral (S) medial (T) proximal and (U) distal views. Abbreviations: cp, collateral pit; fib, fi-
bular facet; gt, greater trochanter; ig, intertrochanteric groove; lc, lateral consyle; lf, lateral fossa; lr, late-
ral ridge; lt, lesser trochanter; mf, medial fossa; pc, plantar crest; sIII, surface for mtt III. Scale bar: 3 cm.

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
238
the medial condyle is posteriorly expanded and
its medial surface is smoothly concave. The
lateral condyle is gently laterally projected, re-
sulting in a well-defined longitudinal groove on
its lateral surface. This groove separates the
lateral condyle from the tibiofibular crest. The
medial condyle is not laterally expanded as in
Morrosaurus and Kangnasaurus (Cooper, 1985;
Rozadilla et al., 2016). The anterior surface of
the bone lacks a well-developed extensor fossa,
a condition shared with many basal ornitho-
pods, such as Thescelosaurus, Hypsilophodon,
Jeholosaurus, Gasparinisaura, Trinisaura,
Notohypsilophodon and “Fulgurotherium”
(Galton, 1974a,b; Molnar & Galton, 1986; Coria
& Salgado, 1996; Coria et al., 2013; Martínez,
1998; Han et al., 2012; Ibiricu et al., 2014),
while Anabisetia, Morrosaurus, Kangnasaurus,
Tenontosaurus, Rhabdodontidae, Dryosauridae
and Ankylopollexia show a well-developed exten-
sor groove (Cooper, 1985; Sereno, 1986; Coria &
Calvo, 2002; Norman, 2004; Norman et al., 2004;
Godefroit et al., 2009; Rozadilla et al., 2016). In
posterior view, the flexor fossa is deep and lim-
ited laterally and medially by stout ridges that
end in the distal condyles.
Tibia (Figure 7, 11). The proximal end of the
tibia is anteroposteriorly expanded, its proxi-
mal surface is medially inclined and exhibits a
rugose surface. The cnemial crest is anteriorly
extended and is strongly proximally projected,
resulting in a subtriangular profile in lateral
view. This trait is unique for Isasicursor, differ-
ent from the remaining ornithopods in which
the cnemial crest does not project further
proximally than the articular surface of the
tibia (e.g. Gasparinisaura, Notohypsilophodon,
Tenontosaurus, Ouranosaurus; Coria & Calvo,
1996; Tennant, 2013; Ibiricu et al., 2014;
Bertozzo et al., 2017), or has a distally deflected
anterior margin (e.g. Anabisetia, Talenkauen,
Morrosaurus, Zalmoxes; Coria & Calvo, 2002;
Godefroit et al., 2009; Rozadilla et al., 2016,
2019). The cnemial crest of Isasicursor is
transversely thickened and shows a proximally
rounded apex. In lateral view, the lateral crest
shows a straight anterior margin. In Isasicursor
it is more extensive than in Kangnasaurus,
Talenkauen and Morrosaurus (Cooper, 1985;
Novas et al., 2004; Rozadilla et al., 2016, 2019).
The lateral condyle is subtriangular in proximal
view. It shows a well-developed process at the
anterolateral corner, being absent in other basal
ornithopods such as Gasparinisaura, Anabisetia,
Talenkauen, Jeholosaurus, Kangnasaurus,
Notohypsilophodon, Hypsilophodon, Zalmoxes,
Trinisaura, and the iguanodontian Ouranosaurus
(Galton, 1974a; Cooper, 1985; Coria & Salgado,
1996; Cambiaso, 2007; Godefroit et al., 2009;
Barrett et al., 2011; Han et al., 2012; Ibiricu et
al., 2014; Bertozzo et al., 2017; Rozadilla et al.,
2019). The lateral surface of the lateral condyle
is convex and rugose. The posterior margin of
the lateral condyle is defined by a dorsoventrally
oriented groove. There is a notch between the
lateral and the posteromedial processes. The
medial surface of the cnemial crest is posteriorly
delimited by a low prominence. The tibial shaft is
oval in cross-section on its proximal third.
The distal end of tibia of Isasicursor is trans-
versely expanded and anteroposteriorly com-
pressed. The distal malleoli are weathered. Both
have a subtriangular outline in anterior view and
are distally rounded, as occurs in related taxa
with the exception of Morrosaurus, in which the
medial condyle has a sharp trapezoidal outline
(Cooper, 1985; Coria & Salgado, 2002; Cambiaso
2007; Ibiricu et al., 2014; Rozadilla et al., 2016).
The lateral malleolus is transversely wider and
more distally projected than the medial one. The
anterior surface of the lateral malleolus bears
a mound-like longitudinal crest, as occurs in
Talenkauen and Anabisetia (Cambiaso, 2007;
Rozadilla et al., 2019). Lateral to this crest there
is a smoothly concave surface for contact with
the fibula. This crest and concave surfaces are
shallower in the juvenile specimen. The posterior
surface of the lateral malleolus is concave. In the
juvenile specimen of Isasicursor, the lateral mal-
leolus is anteroposteriorly narrower. The medial
malleolus is subtriangular in distal view, concave
anteriorly and convex posteriorly, lacking the
anterior curvature present in Morrosaurus and
Trinisaura (Barrett et al., 2011; Rozadilla et al.,
2016). The distal malleoli are separated anteri-
orly by a shallow anterior intermalleolar fossa
and posteriorly by a thick intermalleolar crest.
The anterior intermalleolar fossa is shallower in
the juvenile specimen.
Metatarsals (Figures 11, 12). Metatarsal II of
Isasicursor is represented by isolated proximal
and distal ends. The proximal end is antero-
posteriorly expanded and transversely com-
pressed a condition that was previously consid-
ered synapomorphic for Elasmaria by Rozadilla
et al. (2016, 2019). However, metatarsal II of
Isasicursor is not as expanded anteroposteriorly
as in Anabisetia and Diluvicursor (Coria & Calvo,
2002; Herne et al., 2018). The lateral surface of
metatarsal II shows a flattened surface for artic-

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Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
Figure 12. Isasicursor santacrucensis gen. et sp. nov. Metatarsal I in (A) anterior, (B) posterior, (C) la-
teral, (D) medial and (E) distal views. Proximal (above) and distal end (below) of metatarsal II (MPM
21535a) in (F) anterior, (G) posterior, (H) lateral, (I) medial (J) proximal and (K) distal views. Metatarsal
III (MPM 21536) in (L) anterior, (M) posterior, (N) lateral, (O) medial and (P) distal views. Proximal (abo-
ve) and distal end (below) of metatarsal IV (MPM 21537) in (Q) anterior, (R) posterior, (S) lateral, (T) me-
dial (U) proximal and (V) distal views. Abbreviations: cp, collateral pit; ef, extensor fossa; f, foramen; ff,
flexor fossa; ics, intercondylar sulcus; sI, surface for the mtt I; sIII, surface for the mtt III. Scale bar: 3 cm.

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
240
ulation with the medial surface of metatarsal III.
The anterior surface of the bone is transversely
thicker than the posterior one and is more lat-
erally expanded. The distal end of metatarsal II
has a sub-rectangular outline in distal view. The
anterior surface is smooth and bears a rounded
distal articular surface that is distally convex.
The posterior surface has a deep flexor groove.
The lateral surface is deeply concave and bears
a distinct and deep collateral ligamental pit that
is proximally located. This condition is absent in
other elasmarians and may constitute an auta-
pomorphy of Isasicursor. The lateral condylid
is transversely thicker than the medial one. In
an available juvenile metatarsal II (MPM 21535)
both extensor and flexor grooves and collateral
ligamental pits are shallower.
Metatarsal III is only represented by its dis-
tal end. The articular surface is asymmetrical in
shape, being proximally expanded at its medial
surface. The distal condylids are separated by
a well-developed intercondylar groove. In distal
view, the condyles are asymmetrical, the medial
one being larger than the lateral one, resem-
bling the condition of Anabisetia (Coria & Calvo,
2002; Cambiaso, 2007), but differing from that of
Morrosaurus, which shows a notably larger lat-
eral condylid (Rozadilla et al., 2016). The lateral
surface of the bone has a deep and sub-circular
collateral pit, while the medial one is shallower.
Metatarsal IV is represented by several ele-
ments of different specimens. The most com-
plete metatarsal IV belongs to a juvenile speci-
men, while larger specimens are represented by
isolated proximal and distal ends. The proximal
end is transversely expanded and is subtrian-
gular in proximal view, with a tapering lateral
surface, as occurs in Gasparinisaura, Anabisetia,
Thescelosaurus and Trinisaura (Galton, 1974b;
Coria & Salgado, 1996; Coria & Calvo, 2002;
Barrett et al., 2011), whereas in Talenkauen,
Morrosaurus and Styracosterna the lateral sur-
face is rounded (Norman, 2004; Rozadilla et al.,
2016; 2019). The proximal surface is gently con-
cave and its posterior margin is more proximally
extended than the anterior one. The medial
surface is deeply concave with a narrow proxi-
mal groove that receives the lateral process of
metatarsal III. Distal to this groove, the medial
surface of the bone shows a flat contact for the
lateral surface of metatarsal III. The anterior
and posterior surfaces of the proximal end of the
bone are flattened and smooth. In cross section,
the bone is subtriangular proximally and oval
distally. The lateral margin of the shaft shows a
sharp longitudinal keel. The anterior surface of
the shaft is smooth, while the posterior one pos-
sesses an oblique ridge that runs from the medial
margin to the lateral distal condyle. The distal
end is strongly anteroposteriorly expanded. In
anterior view, the articular surface lacks a well-
defined extensor groove. In lateral view, there is
a shallow collateral pit that is suboval in contour
and with a strongly anteriorly projected anterior
articular surface. In distal view, the articular sur-
face has sub-parallelogram outline, with its an-
terior margin more laterally projected than the
posterior one. The posterior margin is medially
projected, forming a lip-like process that proj-
ects laterally, a condition shared with Anabisetia
and Trinisaura (Cambiaso, 2007; Barrett et al.,
2011). This projection posteriorly bounds the
outer collateral pit. In posterior view a shallow
flexor groove separates both condylids.
MPM 21538 is proximodistally short and
transversely stout. In related taxa, such as
Gasparinisaura, Anabisetia and Morrosaurus,
the adult specimens possess a more slender
metatarsal IV than in different-sized Isasicursor
specimens (see Coria & Salgado, 1996; Cambiaso,
2007; Rozadilla et al., 2016).
Phalanges (Figure 13). Pedal toes of Isasicursor
are represented by several damaged phalanges
belonging to several individuals. Available pha-
langes are relatively stout and short, with a flat-
tened ventral surface and wide and deep collat-
eral ligamental pits.
Phalanx I-1 is represented by its distal end.
The distal trochlea is well developed, with low
condyles divided by a deep intercondylar groove.
The preserved portion of the shaft is slender
and the distal trochlea is strongly transversely
expanded. Deep and sub-circular collateral pits
are present. Phalanx II-1 shows a concave and
subtriangular-shaped proximal articular surface.
The proximal end narrows distally. The medial
surface has several muscular scars near its proxi-
mal surface, which are continuous ventrally with
a ventral ridge. In ventral view, the proximal
portion of this phalanx possesses two collateral
ridges, in which the medial one is more robust
and ventrally extended. The lateral ridge is shal-
lower and more laterally projected. These ridges
are separated by a proximal concavity. The shaft
is sub-rectangular in cross-section. Three nota-
bly anteroposteriorly short phalanges from digit
IV were recovered. We identify these elements as
IV-2/IV-3? The proximal surface is weathered, but
shows a well-developed dorsoventral ridge sepa-
rating it symmetrically, as occurs in phalanges of

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Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
the fourth digit in other elasmarians (Rozadilla
et al., 2019). The ventral margin of the proximal
surface is proximally projected, forming a well-
developed posteroventral process. The distal tro-
chlea is well-defined and is proximally delimited
by a transversely oriented groove. In the dorsal
surface of the bone there is a small extensor pit
proximal to the distal articular surface. Distal
condyles are well developed and separated by a
deep intercondylar groove. Deep collateral pits
occur at the sides of each distal condyle. These
phalanges are anteroposteriorly short, as oc-
curs in Talenkauen and derived iguanodontians
(Norman et al., 2004; Norman 2004; Rozadilla et
al., 2019), whereas more basal ornithopods and
smaller elasmarians show proportionally longer
pedal phalanges (Coria & Calvo, 2002; Cambiaso,
2007; Coria et al., 2013).
At least six pedal unguals were recovered. We
tentatively identify the most complete elements
as unguals of the third digit. The proximal surface
is suboval in proximal view, being transversely
wider than dorsoventrally deep. This surface is
asymmetrically divided by a dorsoventral ridge,
with the lateral articular surface larger than the
medial one. The dorsal margin of the articular
surface projects proximally into a proximal lip.
The blade is dorsoventrally flattened, and slight-
ly medially curved. The dorsal surface is smooth
and convex, while the ventral one is flat. The un-
gual possesses two flattened longitudinal step-
like expansions that are dorsally delimited by a
longitudinal collateral groove. The distal end of
the ungual is notably acute. The ungual is deco-
rated with striations near its proximal and distal
ends and on the ventral surface. The flexor tu-
bercle is represented by a poorly developed and
transversely expanded bump, decorated with
muscle-scar striations. The overall morphology
of these phalanges resembles that of other basal
ornithopods (e.g., Norman et al., 2004; Canudo et
al., 2013), but contrasting with the blunt hoof-
like unguals of styracosternan ornithopods (e.g.,
Norman 2004; Horner et al., 2004).
Comments. The morphology of the proximal
end of femur and humerus shows synapomor-
phic features supporting Isasicursor belongs to
the Elasmaria (e.g., cervical vertebrae laterally
Figure 13. Isasicursor santacrucensis gen. et sp. nov.Pedal phalanx IV-2/IV-3? (MPM 21539) in (A) proximal, (B)
dorsal, (C) lateral and (D) ventral views. Ungual phalanx (MPM 21540) in (E) proximal, (F) dorsal, (G) lateral
and (H) ventral views. Abbreviations: ce, collateral expansions; cp, collateral pit; mr, median ridge; st, stria-
tions; vl, ventral lip. Scale bar: 3 cm.

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
242
compressed and with a sharp ventral keel; lat-
erally bowed humerus with a rudimentary del-
topectoral crest; femur with greater trochanter
showing a sigmoidal lateral margin in proximal
view; and metatarsal II laterally compressed
in proximal view; Rozadilla et al., 2016; 2019).
Among elasmarians, Isasicursor shows unique
features in several elements, especially the proxi-
mal end of tibia and sacrum, indicating that it
is a clearly distinctive and diagnosable taxon.
Its size is similar to that of larger taxa, such as
Sektensaurus, Morrosaurus, Talenkauen and
Macrogryphosaurus. However, the incomplete
nature and dissociated preservation of available
material precludes the recognition of the rela-
tionships of Isasicursor within Elasmaria.
The Upper Cretaceous record of basal iguan-
odontians from Argentina and Antarctica in-
cludes the Cenomanian-Turonian Talenkauen
and Anabisetia, and the Campanian Trinisaura
and Morrosaurus from Antarctica (Coria et al.,
2013), whereas Sektensaurus has a poorly con-
strained age that ranges from Coniacian to
Maastrichtian (Ibiricu et al., 2018). Previous re-
ports of Campanian-Maastrichtian basal ornitho-
pods from Northern Patagonia were dismissed
(Agnolin et al., 2010). In this way, Isasicursor
constitutes the first Maastrichtian basal iguano-
dontian from the southern cone.
Ornithopods are frequently recorded as ta-
phonomic associations composed by several indi-
viduals (e.g, Horner & Makela, 1979; Salgado et
al., 1997; Andrzejewski et al., 2019). Isasicursor
is known from several specimens corresponding
to different sizes that likely represent different
ontogenetic stages, found together in a reduced
fossiliferous spot (i.e., a bed roughly 5 m long
and 0.50 m thick). This leads to the interpreta-
tion that this taxon had a gregarious behavior,
at least at the time of their death. In South
America the finding of different individuals in
close association is common, as demonstrated by
Talenkauen santacrucensis, in which the holo-
type specimen was found associated with a neo-
natal tooth (Egerton et al., 2013), and Anabisetia
and Gasparinisaura which are known from sev-
eral individuals (Salgado et al., 1997; Coria &
Calvo, 2002). This may indicate that Isasicursor
and other elasmarians were gregarious, a condi-
tion well-known among their Laurasian counter-
parts (Andrzejewski et al., 2019).
Hadrosaurs appear in the South American
fossil record at the Campanian-Maastrichtian
time span, being represented by abundant re-
mains corresponding to different species (Brett-
Surman, 1979; Bonaparte et al., 1984; Juarez-
Valieri et al., 2010; Coria et al., 2012; Cruzado-
Caballero & Powell, 2017). These dinosaurs were
outstanding components of the South American
“Allenian vertebrate assemblage” (see Leanza
et al., 2004), which includes other herbivorous
dinosaurs such as titanosaurs and ankylosaurs
(Bonaparte, 1986). In a global context, the re-
cord of Campanian-Maastrichtian basal ornitho-
pods suggests a decline in diversity with respect
to previous times, whereas hadrosaurs exhibit
a notorious “evolutionary explosion” during
the end of the Late Cretaceous (Horner et al.,
2004). In that time span, basal ornithopods are
nearly exclusively represented by the European
Rhabdodontidae and the North American
Thescelosauridae (Weishampel et al., 2003; Boyd
et al., 2009; Ősi et al., 2012; Boyd, 2015). The
same seems to be true for South America, where
ornithopods are only represented by small to me-
dium sized elasmarians.
The discovery of diverse elasmarian in
Campanian-Maastrichtian beds of Patagonia
and Antarctica (e.g. Trinisaura, Morrosaurus,
Sektensaurus, Isasicursor; Ibiricu et al., 2010;
2019; Coria et al., 2013; Rozadilla et al., 2016)
suggest that some of these basal ornithopods co-
existed with hadrosaurs. This, together with the
much smaller size of elasmarians, suggests that
some kind of niche partitioning occurred among
Gondwanan ornithopods (see Brett-Surman,
1979; Case et al., 2000).
Sauropoda Marsh, 1878
Titanosauriformes Salgado, Calvo & Coria, 1993
Titanosauria Bonaparte & Coria, 1993
Genus and species indeterminate
Referred material: MPM 21542, three isolated
teeth (locality 4).
Description. Available teeth lack the tip of
the crown and most of the enamel. They are
pencil-like as typical for titanosaurs (e.g. García
& Cerda, 2010). The enamel is smooth and the
teeth are subcircular in cross-section. There is
no clear difference in thickness between crown
and root.
Comments. The general crown outline is similar
in the three collected teeth, including a pencil-
like general aspect with poor labiolingual com-
pression and lacking mesial and distal carinae.
These are features typical of titanosaur dentition
(García & Cerda, 2010). Further, the absence of
needle-like teeth and lack of strong enamel or-
namentation, argue against rebbachisaurid af-

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Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
finities for the collected specimens (Salgado et
al., 2004). In sum, MPM 21542 is regarded as
Titanosauria indet.
?Colossosauria González Riga, Lamanna, Otero,
Ortíz, Kellner and Ibiricu, 2019
Nullotitan glaciaris gen. et sp. nov.
Figure 14
Holotype. MACN-PV 18644 and MPM 21542,
the specimen consists of isolated cervical cen-
trum (presumably Cv3; MACN-PV 18644), frag-
mentary cervical rib shaft, dorsal rib shaft frag-
ments, several caudal vertebrae, fragmentary left
scapula, proximal and distal ends of right femur,
almost complete right tibia, fibula, and astrag-
alus. The holotype specimen comes from local-
ity 1. It also includes an isolated cervical cata-
logued with collection number MACN-PV 18644
by J. F. Bonaparte in 1981, and then catalogued
as cf. Antarctosaurus. Later, Bonaparte et al.
(2002) interpreted this titanosaur as related with
Aeolosaurus; Powell (2003), instead, referred the
material as to Titanosauridae indet.
Referred specimens. The following specimens
were collected from other nearby localities, from
levels above and below the holotype: 1) MPM
21545, complete humerus, lacking cortex of mid-
shaft, partial rib, and vertebra, which were found
preserved ex situ, about 100 meters far from the
place of the holotype specimen. These elements
were found relatively high on the slope, indi-
cating they correspond to a different individual
than the holotype. Further, the humerus belongs
to an individual smaller than the holotype. 2)
MPM 21546 (locality 1), isolated and partially
preserved distal caudal centra. 3) MPM 21547
(locality 5), sequence of five mid-caudal verte-
brae with their respective haemal arches, found
articulated in situ. This specimen still remains
in the field. 4) MPM 21548 (locality 2), isolated
complete left tibia. 5) MPM 21549 (locality 2),
proximal to mid-caudal centrum, found ex situ
some meters far from the isolated tibia.
Stratigraphic provenance. All aforemen-
tioned specimens come from the lower and middle
sections of the Chorrillo Formation, being absent
from the upper third of this unit. The upper-most
record is from locality 4, which was found from
beds lying above a thick conglomerate bank. This
later specimen is not yet numbered, still remaing
in the field. This later specimen still remains un-
numbered, waiting to be excavated from the field.
Diagnosis. Large titanosaurian sauropod diag-
nosable on the basis of the following combina-
tion of characters (autapomorphies marked with
an asterisk*): 1) anterior caudal centra notably
anteroposteriorly short, its transverse diameter
duplicating its anteroposterior length; 2) proxi-
mal and mid-caudal centra with lateral and ven-
tral surfaces profusely excavated by large blind
depressions*; 3) mid-caudal vertebrae with lat-
eral surface with a tabicate large fossa below the
transverse process; 4) centra of all available cau-
dal vertebrae lacking signs of pneumatization; 5)
mid-caudals with a deep ventral longitudinal fur-
row surrounded by two longitudinal thick ridges;
6) fibula with a pronounced sigmoid curvature
when viewed anteriorly or posteriorly*; and 7)
distal end of tibia strongly anteroposteriorly com-
pressed and more transversely expanded than in
other titanosaurs.
Etymology. Nullotitan, in honor of geologist
Francisco E. Nullo, discoverer of the holotype
specimen, and titan, powerful giant; the spe-
cific name glaciaris refers to the majestic Perito
Moreno Glacier, observable from the excavation
site.
Remarks. Fossil remains of sauropods from
Chorrillo beds, albeit fragmentary, consist of
bones, teeth and egg-shell fragments. Bones that
are here referred as to Nullotitan glaciaris were
found broken and forming different bone accu-
mulations, spread over 100 square meters on a
slope surface. All collected elements belong to
large titanosaur sauropods, and no overlapping
bones exist among these five discrete bone accu-
mulations. The available bones that were found
in situ, come from a reduced spot of hard green
sandstone exposed at the top of the slope; in
contrast, the remaining sets of bones were col-
lected down on the slope, and were exposed ex
situ, but some of them still preserving bits of the
green sandstone exposed on the top of the slope.
No dermal ossifications have been discovered in
Chorrillo beds, either isolated or in association
with bone remains.
Description. Almost all the collected titano-
saur bones belong to large sized individuals.
Estimated length of the holotype specimen al-
most probably surpassed 20 meters long, based
on extrapolations of available elements with the
fairly complete titanosaurs Dreadnoughtus and
Patagotitan (Lacovara et al., 2014; Carballido et
al., 2017).
Cervical vertebra (Figure 15). Neck verte-
brae are represented just by a single, incomplete
centrum, collected by J. F. Bonaparte in 1981

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
244
(MACN-PV 18644). It is elongate (45cm long) and
dorsoventrally low (22cm). The centrum is opist-
hocoelous. The parapophyses are tabular shaped,
dorsoventrally depressed, and anteroposteriorly
extended. The sides of the centrum are deeply
excavated between parapophysis and diapophy-
sis, bearing a large and elliptical pleurocoel at the
bottom of the excavation. In anterior view the
vertebra is cross-shaped, due to the development
of para- and diapophyses bounding the centrum.
There are subhorizontally oriented longitudinal
ridges that run from diapophyses to parapo-
physes. The ventral surface of centrum is flat-
tened and smooth, as is usual in basal sauropods
(Tazoudasaurus, Shunosaurus, Patagosaurus),
basal macronarians (Camarasaurus), and titano-
saurs (Malawisaurus, Rapetosaurus, Isisaurus,
Trigonosaurus, Neuquensaurus and Saltasaurus),
and unlike some basal Titanosauriformes
(Giraffatitan, Paluxysaurus and Tendaguria)
and diplodocoids (Apatosaurus, Diplodocus and
Limaysaurus) where it is transversely concave.
This combination of features suggests it may cor-
respond to Cv3. The internal structure is camel-
late, as occurs in some basal Titanosauriformes
(Giraffatitan), basal somphospondylians (Erketu,
Ligabuesaurus, Phuwiangosaurus), titanosaurs
(Mendozasaurus, Malawisaurus, Rapetosaurus,
Isisaurus, Trigonosaurus, Alamosaurus,
Neuquensaurus, Saltasaurus) and some diplodo-
cids (Apatosaurus and Diplodocus), and unlike
other sauropods where inner pneumaticity of the
cervical centrum is absent (e.g., Patagosaurus)
or present but with several small and complex
internal cavities (Camarasaurus, Limaysaurus).
Caudal vertebrae (Figures 16-18). The holo-
type of Nullotitan glaciaris includes different
caudal elements, corresponding to proximal and
middle section of the tails. Most of them are only
represented by incomplete centra, with limited
information regarding transverse processes and
neural arches. Location of each of these isolat-
ed elements along the caudal series is tentative
and based on the ratio between anteroposterior
length vs. transverse width of centra, contour of
centra (i.e., proportional development of primary
and secondary lateral surfaces), presence and
relative development of articular surfaces for
haemal arches, degree of convexity of the poste-
rior articulation of centrum, and profusion and
size of blind perforations on lateral and ventral
surfaces. In contrast with cervical elements, all
available caudals are apneumatic, and the inter-
nal tissue is compact.
Proximal caudals characterize for the following
combination of features: 1) anteroposteriorly
short centra; 2) centrum transversely wide and
elliptical-shaped in anterior view; 3) lateral and
ventral surfaces profusely excavated by large,
blind perforations; 4) centrum lateral surface
(below transverse process) steeply inclined ven-
trally and medially; 5) transverse processes
dorsoventrally deep and anteroposteriorly com-
pressed. Mid-caudals exhibit: 1) squared-shaped
centra in side view; 2) ball-shaped distal articu-
lar surface; 3) centra with a deep ventral longi-
tudinal furrow; 4) blind excavations reduced in
size, but still abundant in number; 5) transverse
processes reduced in size, cone-shaped, and lo-
cated at centrum mid-height. Finally, distal cau-
dals exhibit: 1) relatively elongate centra; and
2) posterior articular surface cone-shaped and
transversely wide in posterior view.
Regarding the distribution among
Titanosauria of the blind, apneumatic excava-
tions which do not enter inside the centrum,
there are some cases in which similar depres-
sions exist (see Martinelli et al., 2011). For ex-
Figure 14. Nullotitan glaciaris gen. et sp. nov. Silhouette showing the recovered elements.

245
Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
ample, in Drusilasaura the ventral surface
of Cd4? exhibits a pair of large elliptical open-
ings which were considered autapomorphic for
this species (Navarrete et al., 2011). Similarly,
Salgado (1996) reported on caudals 2 through 4
of Pellegrinisaurus the presence of small holes on
each side of the centrum. However; in Nullotitan
the excavations are larger than in other titano-
saurs, and much more abundant.
The following description of caudal elements
positions the ventral surface of centrum on the
horizontal plane. The dorsoventral axis of cen-
trum (especially in proximal caudals) is vertically
positioned, and forming a right angle with the
ventral surface of centrum. Thus, the floor of
the neural canal results inclined ventrodistally
(in side view) with respect to ventral surface
of centrum. This means that the caudal series,
when reconstructed articulated in line with the
sacrum, show the neural canal inclined, accom-
panying the dorsoventral lowering of vertebrae.
Caudal 1st? (Figure 16). It is represented by
most of its centrum and the base of the neural
arch. The later one occupies a central to cranial
position in lateral view, similar to other titano-
saurs (e.g., Patagotitan; Carballido et al., 2017).
The centrum is 40 cm in transverse width, 34 cm
in dorsoventral depth, and 22 cm in anteroposte-
rior length (but probably 25 cm when complete).
Transverse processes are broken at their bases
but revealing a prominent vertical structure.
They are dorsoventrally deep, extending over
the dorsal half of the lateral centrum surface. In
side view it is seen that the transverse process
occupies a central position on the lateral surface
of the vertebra. The anterior articular surface is
surrounded by a well-developed rim. The dorsal
part of transverse processes extends dorsally, well
above level of neural arch. Transverse processes
are cranially concave and caudally convex. On its
cranial surface, the transverse process bears a
pair of large, eye-shaped, blind excavations. On
the surface behind the transverse process, there
is only one of these excavations. Below the trans-
verse processes, the centrum becomes strongly
constricted due to the ventromedial slope of
the lateral surface. Notably, this lateroventral
surface of the centrum exhibits large and blind,
randomly distributed perforations, consisting
in fossae separated by ridges. The lateroventral
surface of centrum forms an inflexion with the
flattened ventral surface. The ventral surface of
the centrum is perforated by isolated longitudi-
nally oriented, blind depressions, which develop
on the posterior half of the bone.
The cranial articular surface is deeply con-
cave. The distal articular cone is eroded, so its
caudal projection is difficult to discern; howev-
er, it seems to have been hemispheric, as usual
among titanosaurs.
The neural canal is oval-shaped, with its floor
concave that widens posteriorly (in dorsal view).
The later region is highly vascularized, as sug-
gested by abundant foramina. On the posterior
surface of the base of neural arch, there is a me-
dian fossa immediately above the neural canal.
No facets for haemal arches are present.
Caudal 2nd? (Figure 17). It is represented by the
dorsal half of centrum and the badly preserved
bases of the neural arch, including the base of
the right transverse process. As in Cd1st, the
centrum is anteroposteriorly very short, but
Figure 15. Nullotitan glaciaris gen. et sp. nov. Cervical vertebra (MACN Pv 18644) in left lateral (A), dorsal (B),
posterior (C), right lateral (D), and longitudinal section (E) views. Abbreviations: di, diapophysis; fo, fossa; na,
neural arch; nc, neural canal; pa, parapophysis; pl, pleurocoele. Scale bar: 10 cm.

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
246
transversely wide. The transverse processes are
eroded, but the preserved portions indicate they
were dorsoventrally deep. However, they are
shallower than in Cd1st, due to the lower margin
in a higher position on the lateral surface. The
cranial and caudal surfaces of the transverse pro-
cess are concave-convex, as in Cd1st. Regarding
the blind depressions, they are similar to those of
Cd1st. The cranial articular surface seems more
concave than in Cd1st. The floor of the neural
canal is fan-shaped in dorsal view, being trans-
versely expanded towards the rear.
Caudal 4th? or 5th? (Figure 18). It is represented
by most of the centrum and base of neural arch
and base of left prezygapophysis. The centrum is
dorsoventrally and transversely smaller than the
caudals previously described, but it is anteropos-
teriorly as long as these centra. The lateral sur-
face of centrum orientates lateroventrally. The
base of right transverse process is sub-conical,
and it locates immediately below level of neural
canal. Due to the higher position of transverse
process, the lateral surface of centrum is more de-
veloped than in more proximal caudals. In sharp
difference from more proximal caudals, the base
of the transverse process reaches the anterior
margin of centrum, thus the later results devoid
of a lateral surface in front the transverse pro-
cess. Regarding the blind depressions, they are
similar to those of more proximal caudals, being
more developed towards the posterior margin of
centrum. Blind depressions on the ventral sur-
face are more elongate and deeper than those of
the lateral surface. The ventral surface of cen-
trum is almost flat.
The base of prezygapophysis indicates that
it was anterodorsally projected. The base of the
spinoprezygapophyseal lamina, the only portion
preserved, is nearly horizontal and sharp. On the
internal surface of neural arch, behind the base
of prezygapophysis, and above the intraprezyga-
pophyseal lamina (TPRL), there is an elongate
fossa. The floor of neural canal is flat; it widens
posteriorly as in the remaining proximal caudals.
Due to poor preservation, there is no evidence of
facets for articulation with the haemal arches.
Mid-caudals (Figure 18). Possible caudals 11
and 12 are preserved. Because it is not easy to
elucidate the respective position of these verte-
brae, we describe them together. They are ap-
proximately half the size of the available proxi-
mal caudals. The base of the neural arch occu-
pies most of the anteroposterior length of cen-
trum (discounting the hemispherical posterior
articular surface). The floor of the neural canal
exhibits foramina and longitudinal grooves on
its posterior half. The centrum is damaged along
the anterior articular rim. The caudal surface
is hemispherical, but slightly eccentric in side
view, with the maximum convexity on the dor-
sal half. In distal view, the contour of the articu-
lar surface is sub-quadrangular, with the major
axis vertically oriented. The lateral surface has a
large fossa below the transverse process, as well
Figure 16. Nullotitan glaciaris gen. et sp. nov. Caudal vertebrae?1st (A-D) and ?2nd (E-G) (MPM 21542) in ante-
rior (A, E), posterior (B, F), right lateral (C, G), and ventral (D) views. Abbreviations: lg, longitudinal groove;
tp, transverse process. Scale bar: 10 cm.

247
Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
as deep blind depressions, which are located on
the central region of the centra.
The ventral surface exhibits a deeply exca-
vated central depression, flanked by strong lon-
gitudinal ridges. The inner side of these ridges
exhibits randomly distributed, and longitudi-
nally elongate foramina, which are much smaller
that the blind fossae described for the proximal
caudals. Facets for articulation with the haemal
arches are present. Transverse processes are lo-
cated at level of neural canal, and the preserved
bases suggest they were robust.
Caudals 15th and 16th (Figure 18). They are
quadrangular in caudal view, being wider than
tall, different from the mid-caudals described
above. The posterior surface is less spherical
than in previous caudals, and the spherical ar-
ticulation is placed on the center of the surface
and with a flat surface surrounding it.
In caudal 15, the articulation for the haemal
arches, placed on the posteroventral border of
centrum, are closer to each other (5 cm) than in
caudal 16th (6-7 cm); the primary lateral surfac-
es (Salgado & García, 2002) are dorsoventrally
oriented and dorsoventrally deeper (7cm) than in
caudal 16th (4cm), where they are ventrally ori-
ented. According to the interpretation of Salgado
and García (2002), this would indicate that the
M. caudofemoralis extended distally as much as
caudal 16th.
Distal caudal centra (Figure 18). No complete
distal centra are available. The centra are pro-
coelous, and notably dorsoventrally compressed.
The anterior articular surface is subcircular in
contour. The posterior articular surface is kid-
ney-shaped and shows an eccentric distal cone,
Figure 17. Nullotitan glaciaris gen. et sp. nov. Caudal vertebrae ?11th (A-D), ?15th (E-H), ?16th (I-M), ?20th
(N-Q), and ?21th (R, S) (MPM 21542). The elements are figured in anterior (A, E, I, Q, R), lateral (B, H, M, P,
S), dorsal (C, L), ventral (D, G, K, O), posterior (F, J, N) views. Abbreviations: cn, neural canal; tp, transverse
process; fo, fossa; h, haemal facet; gr, groove; na, neural arch. Scale bar: 10 cm.

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
248
dorsally displaced. At least in caudal 19th, the
primary lateral surfaces are constrained to the
ventral face of centrum.
Haemal arches. Haemal arches are represent-
ed by two single elements. They are rod-shaped
and proximally opened. In lateral view they show
a gentle sigmoid curvature.
Scapula (Figure 19). The central portion of a
left scapula is preserved (the following descrip-
tion assumes the long axis of scapula as vertically
oriented). This portion of the scapula lacks both
glenoidal and acromial regions. It is a plate-like
bone, medially concave and laterally convex. The
most notable feature preserved on this fragmen-
tary scapula is a medial tubercle with a muscle
scar located dorsal to the level of the acromial
process and close to the anterior margin of the
bone. The tubercle is triangular-shaped, with
the apex oriented ventrally. Such prominence
has been also reported in other titanosauriforms
(e.g., Wintonotitan, Alamosaurus; D´Emic, 2012;
Poropat et al., 2015). A ventromedial process is
observed on the ventral margin of the scapula,
as reported in Ligabuesaurus (Bonaparte et al.,
2006).
Humerus (Figure 20). A fairly complete right hu-
merus (MPM 21545) is available. It measures 114
cm in total length, 44 cm in maximum proximal
width, and 40 cm of maximum distal width (mid-
shaft is damaged, and its reconstructed maximum
width is estimated in 25 cm). The robust index (RI;
maximum distal width/total length) of Nullotitan
is estimated in 0.28, being comparable to that
of Epachthosaurus (RI=0.27), Mendozasaurus,
(R=0.24; Martínez et al., 2004; Mannion &
Otero, 2012), Patagotitan (RI estimated=0.28;
Carballido et al., 2017), and Notocolossus (RI
=0.28; González Riga et al., 2016). These hu-
meral proportions differ from the robust hu-
meri (RI more than 0.28) of Dreadnoughtus
(RI=0.33), Neuquensaurus (RI=0.35, Salgado et
al., 2005; Otero, 2010), Saltasaurus (RI=0.36;
Powell, 2003), and Opisthocoelicaudia (RI=0.41;
Borsuk-Bialynicka, 1977). Bonaparte et al. (2002)
considered the humerus of Nullotitan similar to
Aeolosaurus rionegrinus (Powell, 2003) in their
slender proportions, but the RI of the latter one
can not calculated because the humerus is in-
complete.
The humeral deltopectoral crest of Nullotitan is
markedly expanded distally as it occurs in practi-
cally all sauropods. In Nullotitan, the greater an-
terior expansion of the lower portion of the delto-
pectoral crest is placed at nearly 25% of the total
length of the bone beginning from the top. This
means a shorter deltopectoral crest, with values
more similar to those of basal titanosaurs such as
Paralititan and Epachthosaurus the ratio is near-
ly 31% (Smith et al., 2001; Martínez et al., 2004).
In other forms, in turn, the deltopectoral crest
is more distally extended, such as in Elaltitan
(36%; Mannion & Otero, 2012), Dreadnoughtus
(37%), Patagotitan (33.6%; Carballido et al.,
2017), Narambuenatitan and Mendozasaurus
Figure 18. Nullotitan glaciaris gen. et sp. nov. Distal caudal vertebrae (MPM 21542) in dorsal (A), lateral (B),
ventral (C), posterior (D), anterior (E) views. Abbreviations: cn, neural canal; co, articular cone; ri, ridge.
Scale bar: 10 cm.

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Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
(35%; Filippi et al., 2011; González Riga et al.,
2018).
Femur (Figure 21). The femoral head and neck
are poorly preserved. They seem to be proximo-
medially elongate, describing an obtuse angle
with the longitudinal axis of femur, a condition
typical for titanosauriforms (González Riga et
al., 2019).
The femur shaft shows a minimum width
of about 44 cm, and the distal end of the right
femur is 53 cm in maximum transverse diam-
eter, with globe-shaped, sub-equal articular con-
dyles, 28 cm in anteroposterior diameter. They
are separated through a marked anteroposterior
constriction, visible in distal aspect. This separa-
tion is also evident on the cranial surface, which
exhibits a shallow distal concavity. In correla-
tion, the cranial surface of distal femur forms a
wide, shallow but deeply grooved, extensor sul-
cus. Articular condyles are not extended on the
cranial surface of the distal end. Subequal distal
condyles may indicate that Nullotitan is close to
the lithostrotian clade (Upchurch et al., 2004),
but the lack of dorsomedial beveling of the dis-
tal condyles onto the cranial side of the shaft
excludes it from Saltasaurinae (Wilson, 2002).
The medial surface of distal femur is flat but
strongly decorated by proximodistally oriented
ridges almost probably corresponding with the
insertion of adductor muscles. The tibial condyle
is more conical-shaped (in cranial view) than the
more rounded lateral condyle. Also, the later
one is carved by deep sulci, giving a brain-like
aspect, different from the tibial condyle with a
less marked decoration. Both the tibiofibular
crest and the posterior projection of the medial
condyle are broken at their bases. The distal
end of femur described above resembles that of
Mendozasaurus. Notably, the femur shaft (imme-
diately proximal to the distal condyles) is highly
compressed anteroposteriorly (10cm).
Tibia (Figure 22). The tibia of the holotype
(MPM 21542) consists of a right element, broken
and slightly distorted. It measures 105 cm in to-
tal length, and 44 cm in maximum distal width.
It exhibits remarkable features interpreted as
diagnostic for the species (see below). A second
tibia (MPM 21548), discovered in isolation, is
tentatively referred as to the same genus and
species, although its features do not completely
agree with those of the holotype.
The tibia of Nullotitan is robust. As in other ti-
tanosaurs, the distal end of tibia of Nullotitan is
transversely expanded to twice midshaft breadth
(Ullman & Lacovara, 2016) and the proximal
end is anteroposteriorly expanded. However,
the distal end of tibia of Nullotitan is strongly
anteroposterior compressed and more trans-
Figure 19. Nullotitan glaciaris gen. et sp. nov. Left scapula (MPM 21542) in lateral (A), and medial (B) views.
Abbreviations: bl, blade; tu, tubercle; vp, ventral process. Scale bar: 10 cm.

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
250
versely expanded than in other titanosaurs (e.g.,
Mendozasaurus; González Riga et al., 2019), sug-
gesting that may constitute an autapomorphic
trait of Nullotitan.
Fibula (Figure 23). A nearly complete right fibu-
la is available. The fibula of Nullotitan measures
109 cm long, being a bit longer than the fibula of
Dreadnoughtus (103 cm; Lacovara et al., 2014).
The fibula of Nullotitan is rubust (RI=0.4), more
than Neuquensaurus (RI=0.172-0.232; Otero,
2010), but less than in Uberabatitan (0.6).
The anterior margin of the bone is straight,
whereas the posterior one is strongly sigmoid,
much more than in other titanosaurs (for in-
stance, Uberabatitan, Mendozasaurus, Elaltitan;
Salgado & Carvalho, 2008; Silva Junior et al.,
2019; González Riga et al., 2018; Mannion &
Otero, 2012). The fibular shaft is also strongly
sigmoidal in posterior view, with the lateral sur-
face markedly convex at level of the biceps tu-
bercle (Figure 23C). In general terms, the fibula
is similar to that of Dreadnoughtus (Ullman &
Lacovara, 2016). The sigmoid curve present in
both Nullotitan and Dreadnoughtus is exaggerat-
ed in lateral view, for the notable anteroposterior
expansion of both proximal and distal ends of the
bone. In lateral view, the proximal margin of the
fibula is nearly horizontally oriented, as occurs
in Dreadnoughtus (Ullman & Lacovara, 2016),
Mendozasaurus (González Riga et al., 2018),
Saltasaurus (Powell, 2003), and Neuquensaurus
(Otero, 2010), and unlike other titanosaurs such
as Uberabatitan (Salgado & Carvalho, 2008;
Silva Junior et al., 2019) in which it is posteri-
orly inclined. On the lateral surface, there is the
tubercle for the M. iliofibularis.
Astragalus (Figure 24). As usual among sauro-
pods, the astragalus is conical-shaped, with an
excavated lateral surface for articulation with
the calcaneum and the fibula, and a pointed end
oriented medially, which is partially broken and
coincident with the medial expansion of the dis-
tal end of tibia. Apparently, the medial expansion
of astragalus was more developed in Nullotitan
Figure 20. Nullotitan glaciaris gen. et sp. nov. Right humerus (MPM 21546) in anterior (A), lateral (B), posterior
(C), medial (D), distal (E), and proximal (F) views. Abbreviations: dp, deltopectoral crest; hh, humeral head;
lc, lateral condyle; mc, medial condyle. Scale bar: 10 cm.

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Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
than in Uberabatitan (Salgado & Carvalho, 2008;
Silva Junior et al., 2019). The ascending process
of the astragalus is low.
Comparisons. Because of the paucity of the
available materials, the phylogenetic position
of Nullotitan among titanosaurs is difficult to
discern. This taxon exhibits features diagnosing
Titanosauria, as well as characteristics of less in-
clusive clades such as Lithostrotia, Colossosauria,
and Lognkosauria (González Riga et al., 2019).
In addition, Nullotitan lacks features diagnosing
the derived titanosaur clades Saltasauridae and
Aeolosaurini.
Nullotitan exhibits the following titanosau-
rian features: caudal vertebrae procoelous, with
neural arches located on the anterior half of
centrum; femur medially bowed on its proximal
end; presence of a prominent lateral bulge distal
to the greater trochanter; and femoral shaft an-
teroposteriorly compressed (Salgado et al., 1997;
Mannion et al., 2013; González Riga et al., 2019).
Furthermore, presence of subequal-sized con-
dyles on distal femur may suggest lithostrotian
affinities for Nullotitan (Upchurch et al., 2004).
Characters of Colossosauria present in Nullotitan
are: deltopectoral mediolateral thickness of ante-
rior attachment surface with distal half medio-
laterally expanded relative to proximal half and a
value near 0.15 of minimum mediolateral width
vs. proximodistal length (González Riga et al.,
2019).
Proximal caudals of Nullotitan are notably
anteroposteriorly short, showing awl-like trans-
verse processes and a slightly convex posterior
articular surface. This combination of traits is
also shared with most lognkosaurians (González
Figure 21. Nullotitan glaciaris gen. et sp. nov. Proximal (A-B) and distal (C-D) ends of right femur (MPM 21542)
in anterior (A, C), and posterior (B, D) views. Abbreviations: eg, extensor groove; lc, lateral condyle; lbu, lat-
eral bump; mc, medial condyle; ne, neck. Scale bar: 10 cm.

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
252
Riga et al., 2019).
As follows, we compare Nullotitan with other
gigantic members of the colossosaurian clade
Lognkosauria (sensu Carballido et al., 2017;
González Riga et al., 2019). This clade is actu-
ally composed by the genera Argentinosaurus,
Futalognkosaurus, Mendozasaurus,
Notocolossus, Patagotitan, Puertasaurus,
Dreadnoughtus and Drusilasaura (Carballido
et al., 2017; González Riga et al., 2018, 2019).
Patagotitan, Futalognkosaurus, and Notocolossus
show the first caudal transverse processes with
a notably dorsoventrally narrow base, a condi-
tion not observed in Nullotitan (Carballido et al.,
2017). In addition, Patagotitan shows a ventral
pocket at the base of centrum, which is absent in
n Nullotitan. Notocolossus, Futalognkosaurus,
Dreadnoughtus and Puertasaurus show verte-
brae that are proportionally anteroposteriorly
longer than Nullotitan, with a distal articular
surface more spherical. Also, in Notocolossus
and Futalognkosaurus the transverse pro-
cesses are more dorsally located on the sides
of centra (Lacovara et al., 2014). As occurs in
Patagotitan, the mid-caudals show a large fos-
sa below transverse process (Carballido et al.,
2017). However, in Nullotitan this fossa is tabi-
cated by an obliquely oriented ridge of bone.
Figure 22. Nullotitan glaciaris gen. et sp. nov. Tibia (A-F) right tibia of MPM 21542 specimen and (G-J) left tibia
of MPM 21548 specimen. The elements are figured in anterior (A, G), medial (B, H), posterior (C, I), lateral (D,
J), proximal (E) and distal (F) views. Abbreviations: aspa, ascending process for astragalus; cc, cnemial crest;
pc, proximal condyle; pvp, posteroventral process; ri, ridge. Scale bar: 10 cm.

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Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
As already said, proximal caudal centra dis-
tinguish for the presence of profuse blind exca-
vations. These are considered autapomorphic for
the new species. Small vascular foramina were
reported in caudal vertebrae of some titanosaurs
(Mannion & Calvo, 2011; Mannion et al., 2013),
but are much smaller than in Nullotitan. The
excavations in caudals of Nullotitan are elon-
gate and subparallel to the main axis of the tail.
On proximal caudals, the excavations are rela-
tively large (i.e., 5 cm long in a centrum 22 cm
long), but are reduced or absent in more distal
caudals. Such excavations are randomly distrib-
uted over centrum surface, being different from
the symmetrical ventral perforations present in
some titanosaurs, such as Pellegrinisaurus and
Drusilasaura (Salgado, 1996; Navarrete et al.,
2011). Clearly, the perforations do not invade
the vertebral centrum. Such pattern of blind
randomly distributed excavations, resemble the
excavations observed on the nuchal crest of felids
(Duckler, 1997), which are produced by muscu-
lar activity, particularly overstretched muscles
that create depressions (due to bone necrosis)
at attachment sites. In humans, this pattern is
evidenced in athletic individuals who place high
strain on selected active muscles and have skele-
tal responses at attachment sites in consequence
(Duckler, 1997). Our best guess is that such large
and elliptical-shaped blind excavations on the
proximal caudals of Nullotitan (as well as other
titanosaurs) may have anchored thick tendons
of hypaxial and epaxial muscles. If this interpre-
tation proves to be correct, then it represents a
different anatomical adaptation for tail support
and movement control than that known in other
dinosaurs.
Humeral proportions of Nullotitan (RI=0.28)
are more gracile in comparison with the ro-
bust humerus exhibited by Dreadnoughtus
(RI=0.33), Elaltitan and Argyrosaurus (Huene,
1929; Powell, 2003; Mannion & Otero, 2012;
Lacovara et al., 2014; González Riga et al., 2019).
Although Nullotitan exhibits a RI similar to that
of Notocolossus, this genus exhibits a humerus
with markedly asymmetrical proximal margin
in anterior view (nearly straight laterally but
strongly expanded and rounded proximomedi-
ally) (González Riga et al., 2019), features absent
in Nullotitan.
The proximal end of the femur is strongly me-
dially bent in Nullotitan, forming a large convex
surface that is not present in Dreadnoughtus,
Mendozasaurus, and Patagotitan. In respect
to robustness of femur, Nullotitan resembles
more Dreadnoughtus than Patagotitan and
Mendozasaurus, which show a more elongate
and narrow femur. The fibula of Nullotitan is
Figure 23. Nullotitan glaciaris gen. et sp. nov. Right fibula (MPM 21542) in anterior (A), lateral (B), posterior (C), me-
dial (D), proximal (E), and distal (F) views. Abbreviations: it, iliofibularis tubercle; mf, medial fossa. Scale bar 10 cm.

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
254
different from that of other large titanosaurs,
including Dreadnoughtus and Argentinosaurus
(Bonaparte & Coria, 1993), in being strongly sig-
moidal in both anterior and lateral views.
In the case of Drusilasaura, the few overlap-
ping elements preclude proper comparisons with
Nullotitan. Both taxa share proximal caudals
strongly anteroposteriorly compressed with awl-
like transverse processes that extend towards
the vertebral centrum, a longitudinal ventral
groove delimited by thick ridges, and distal cau-
dals dorsoventrally compressed (Navarrete et al.,
2011). However, Drusilasaura apomorphically
shows two large ventral foramina that are ab-
sent in Nullotitan. Further, Drusilasaura lacks
the blind fossa present along the lateral surface
of caudal vertebrae of Nullotitan.
Nullotitan differs from saltasaurids (sensu
Gonzáles-Riga et al., 2019) in having proximal
caudals anteroposteriorly short, with posterior
articular surfaces slightly convex, and apneumat-
ic centra. These are plesiomorphic traits indicat-
ing that Nullotitan is not related with saltasau-
rids, which usually have caudal vertebrae with
dorsoventrally depressed and strongly procoelous
centra, and highly pneumatic centra and neural
arches (e.g., Powell, 1993; Salgado et al., 1997;
Bonaparte et al., 2000; Salgado & Azpelicueta,
2000; Rose, 2007; Taylor, 2009; González Riga
et al., 2019). In this regard, the ventral aspect
of the caudal vertebrae of Nullotitan is unique
in that the proximal and distal caudals show
a relatively flat ventral surface of centrum,
whereas mid-caudals exhibit a deep longitudi-
nal furrow surrounded by two thickened ridges.
Presence of deep ventral furrow in Nullotitan
should not be confused with the deep pneumatic
excavations present in derived titanosaurs such
as Saltasaurus and Rocasaurus (Powell, 2003;
Salgado & Azpelicueta, 2000).
Besides, caudal vertebrae of Nullotitan dif-
fers from the caudal type reported for aeolosau-
rines (i.e., Aeolosaurus patagonicus and A. col-
huehuapensis; Powell, 2003; Casal et al., 2007;
Martinelli et al., 2011) because in the latter ones
the neural arches are strongly inclined anteriorly
and have a more developed procoelous condition.
Thus, the sum of features of the caudal vertebrae
of Nullotitan clearly distinguishes it from other
Late Cretaceous derived titanosaurs from South
America.
In sum, although the scarcity of currently
available materials of Nullotitan precludes a
clear taxonomic referral, its anatomical details
(mainly from the caudal vertebrae), support
it as a valid gen. et sp.. Also, the large size of
Nullotitan, in joint with the lack of saltasaurid
and aeolosaurine features, plus all the character-
Figure 24. Nullotitan glaciaris gen. et sp. nov. Right astragalus (MPM 21542) in dorsal (A), ventral (B), anterior
(C), lateral (D), posterior (E), and medial (F) views. Abbreviations: ap, ascending process; df, dorsal fossa; fc,
calcaneum facet; tf, tibial facet. Scale bar: 10 cm.

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Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
istics listed above suggest its placement among
colossosaurian titanosaurs.
Theropod dinosaurs
Remains of theropod dinosaurs (including
birds) collected in the Chorrillo beds include sev-
eral isolated bones and a single tooth identified
as Megaraptoridae indet., as well as some pedal
phalanges referred with doubts to Noasauridae
and Unenlagiidae. Besides, eggshell fragments
are described as belonging to Theropoda indet.
Available evidence, albeit fragmentary, clearly
shows that the dinosaur fauna from the Chorrillo
Formation was taxonomically diverse, formed
by clades also documented in other regions of
Patagonia and Gondwana, but with an apparent
dominance of megaraptorids over other theropod
groups. From the equivalent Dorotea Formation
in Chile, Manriquez et al. (2019) mention the dis-
covery of theropod teeth, but no details on their
phylogenetic affinities was given.
Coelurosauria Huene, 1914
Megaraptora Benson, Carrano and Brusatte,
2010
Megaraptoridae Novas et al., 2013
Megaraptoridae gen. et sp. indet. 1
Referred material. MPM 21545, fragmentary
specimen composed of a posterior dorsal cen-
trum, transverse process of a dorsal vertebra,
partial neural arch of a caudal vertebra, frag-
ments of two indeterminate vertebrae, rib frag-
ments and proximal end of pubis (locality 3)
(Figure 25). The specimen was found in a 5x3 m
surface area.
Horizon. Base of the upper third of the Chorrillo
Formation, lying above a thick bank of conglom-
erates.
Description. Available elements belong to a
megaraptorid similar in size to Aerosteon (Sereno
et al., 2008), approximately 8-9 meters in whole
length.
Dorsal vertebrae. The isolated centrum is in-
complete, lacking most of its dorsal portion. It
probably corresponds to a posterior dorsal verte-
bra, with transverse width vs maximum length
ratio of 1.16, almost the same as dorsal 10th of
Aerosteon. The anterior articular surface of cen-
trum is concave, but the posterior one is flat.
The lateral surfaces are remarkably concave, as
occurs with other big-sized theropods (Brochu,
2003; Sereno & Brusatte, 2008). Ventrally, the
centrum is smooth and transversely convex. This
condition is observed in other big-sized megarap-
torids as Aerosteon and Murusraptor, in which
these surfaces become progressively deeper pos-
teriorly. Large paired pleurocoels are present on
both sides of the centrum; they are separated
by a wide and oblique septum, a diagnostic fea-
ture of Megaraptoridae (Novas et al., 2013). The
pleurocoels penetrate the centrum medially and
slightly ventrally. The internal structure of the
centrum is camellate, as occurs in other meg-
araptorids (e.g, Orkoraptor, Megaraptor; Calvo et
al., 2004; Novas et al., 2008; Benson et al., 2010,
2012; Porfiri et al., 2014).
An isolated transverse process is interpreted as
belonging to a dorsal vertebra. It is oval in cross
section, and with camellate internal structure.
Distally the process bears a concave and oval
surface for articulation with the rib tuberculum.
The articular surface of the process is proportion-
ally much bigger than in the cervical vertebrae of
other theropods.
Caudal vertebra. This element is represented
by the right side of a neural arch, including the
base of the transverse process and the base of
the postzygapophysis. The presence of a tall neu-
ral arch suggests it corresponds to the proximal
third of the tail. As in other vertebral elements,
this neural arch possesses a camellate internal
structure. The preserved part of the neural arch
appears to be dorsoventrally more depressed
than in Aoniraptor and Murusraptor (Coria &
Currie, 2016; Motta et al., 2016), but similar to
Megaraptor and Orkoraptor (Novas, 1998; Novas
et al., 2008). The transverse process is robust,
dorsoventrally tall and posterolaterally directed.
Below it there is the anterior centrodiapophyseal
lamina, which is robust and anteroventrally di-
rected, delimiting anteriorly the prezygadiapo-
physeal-centrodiapophyseal fossa, and posterior-
ly the centrodiapophyseal fossa. The presence of
two laminae delimiting three fossae is observed
in other megaraptorids such as Aoniraptor,
Orkoraptor, Murusraptor, and Megaraptor
(Calvo et al., 2004; Novas et al., 2008; Coria &
Currie, 2016; Motta et al., 2016). In contrast, in
other theropod groups (e.g., abelisaurids, car-
charodontosaurids, tyrannosaurids) the neural
arch lacks these laminae below the transverse
process. Interestingly, the anteroventral limit
of the prezygadiapophyseal-centrodiapophyseal
fossa is made up by an anteroposteriorly wide
and anteriorly inclined lamina. This robust lam-
ina is observed in Aoniraptor, Murusraptor and
Megaraptor, but not in Aerosteon and Orkoraptor.
The postzygapophysis is small and slightly lon-
ger than wide. Above it, the postspinal laminae

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256
Figure 25. Megaraptoridae indet. (MPM 21545). (A-D) Partial dorsal centrum in dorsal (A), ventral (B), posterior
(C) and right lateral (D) views. (E-G) Isolated dorsal transverse process in anterior (E), posterior (F) and distal (G)
views. (H-K) Partial caudal neural arch in left lateral (H), dorsal (I), right lateral (J) and anterior (K) views. (L-O)
Proximal end of right pubis in lateral (L), medial (M), anterior (N) and proximal (O) views. Abbreviations: acdl,
anterior centrodiapophyseal lamina; pcdl, posterior centrodiapophyseal lamina; pl, pleurocoel. Scale bars 3 cm

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Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
ascends vertically and delimits the postspinal
fossa. This fossa is narrow and does not show in-
terspinous ligament tuberosity.
Pubis. The pubis is represented by the proxi-
mal end of the right bone. Its lateral surface is
convex, while the medial one is both anteropos-
teriorly and dorsoventrally concave. The medial
surface shows vertical striations probably for
ligament attachment. The anterior margin is
transversely thick and convex in cross-section,
whereas the posterior margin is thin and sharp.
The pubis is oval in proximal view, with a rugose
articular surface for the iliac pedicle, and seems
morphologically less complex than in other meg-
araptorans, such as Aerosteon or Murusraptor
(Sereno et al., 2008; Coria & Currie, 2016). The
proximal margin of pubis seems simple, lacking
differentiation into acetabular, ischiadic and iliac
articular surfaces present in another megarap-
torids.
Discussion. MPM 21545 exhibits the following
megaraptorid synapomorphies: 1) dorsal cen-
tra with two pleurocoels separated by a septum
(Novas et al., 2013); and 2) two centrodiapophy-
seal laminae separating three fossae below the
transverse process of the anterior-middle caudal
vertebrae (Calvo et al., 2004). This morphology
is absent in other theropods such as ceratosaurs,
spinosaurids, allosauroids, tyrannosauroids, and
ornithomimosaurs.
Megaraptoridae gen. et sp. indet. 2
Referred material. MPM 21546, isolated dor-
sal centrum (locality 4) (Figure 26).
Description. This small vertebra measures 2.72
cm long, 2.43 cm tall, and 2.79 cm wide. It is in-
ternally pneumatized through a complex pattern
of lateral pleurocoels, similar to those of meg-
araptoran theropods (e.g., Sereno et al., 2008).
Based on extrapolations with complete theropods
(e.g., Allosaurus; Madsen, 1976), we estimate
this specimen being approximately three meters
long. The maturity of the specimen is uncertain,
but the small size of the element together with
the lack of fusion with the neural arch suggests
it corresponds to a juvenile. This vertebra is even
smaller than those of the juvenile specimen of
Megaraptor namunhuaquii described by Porfiri
et al. (2014).
The centrum has a slightly concave anterior
articular surface and a flat posterior one, as oc-
curs in posterior dorsal vertebrae of megarapto-
rids (e.g., Murusraptor, Aerosteon; Sereno et al.,
2008; Coria & Currie, 2016). The lateral and ven-
tral surfaces are concave. Laterally, the centrum
shows a rounded fossa with an anterior pleuro-
coel and a posterior pleurofossa separated by an
oblique septum, as it occurs in other megarap-
torans (e.g., Fukuiraptor, Aerosteon, Megaraptor,
Murusraptor, Tratayenia). The centrum is quad-
rangular in side view, devoid of articular surfaces
for both ribs and haemal arches, thus indicating
it may correspond to the posterior dorsals. Also,
the ventral surface is smooth and transversely
convex, contrasting with the caudal vertebrae
of megaraptorans, which possess strong ventral
keels (Méndez et al., 2012; Motta et al., 2016).
Furthermore, in this element the posterior ar-
ticular surface is flat but the anterior one shows
a moderate concavity.In dorsal view, the floor of
the neural canal is shallow and has a constant
transverse width. This condition is observed
in isolated dorsal centra of the megaraptorids
Aerosteon and Murusraptor (Sereno et al., 2008;
Coria & Currie, 2016), while the caudal centra of
these theropods have a neural canal constricted
around mid-length. In lateral view, the centrum
is squared-shaped in outline, contrasting with
other megaraptorids, in which the centrum is
shorter and taller (e.g., Megaraptor Aerosteon,
Murusraptor, Tratayenia; Sereno et al., 2008;
Coria & Currie, 2016; Porfiri et al., 2014, 2018).
However, Wilson et al. (2016) noted that this fea-
ture seems size-dependant, and may be related
with the fact that all of the above mentioned
megaraptorids are 8-10 meters long.
Comments. Up to now, the Patagonian record
of Megaraptoridae corresponds to large-sized
animals, approximately 8-9 m long. For their
size and relative abundance, these theropods
may have constituted the main predators during
the Early Maastrichtian in southern Patagonia.
Even juveniles may have played an important
ecological role in these southern dinosaur fau-
nas, considering that the specimen here reported
attained a whole length of, at least, 2 m. This
means that megaraptorids probably preyed upon
different sized herbivore dinosaurs (small to me-
dium sized elasmarians, and bigger titanosau-
rids) in the course of their ontogeny.
Megaraptoridae gen. et sp. indet. 3
Referred material. MACN-Pv 19066, iso-
lated shed tooth crown, presumably com-
ing from the lower levels of the Chorrillo
Formation (Figure 27). Collected by J. F.
Bonaparte in 1981 on the same large area were
the holotype of Nullotitan glaciaris was found.

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
258
Description. MACN-PV 19066 is represented
by a shed tooth crown that lacks its apex, a por-
tion of the mesial margin, and some denticles in
the distal margin. The crown is slightly curved
to the right in distal view and the mesial carina
is displaced to the right of the distal one, thus in-
dicating that the tooth represents a left anterior
maxillary or dentary dental piece. The crown has
an apicobasal height of 19.0 mm, a basal labio-
lingual width of 6.8 mm, and a basal mesiodistal
length of 10.7 mm.
The crown is distinctly recurved in labial
and lingual views, with the apex being located
distally to the base, as occurs in other meg-
araptorans (e.g. Fukuiraptor: Azuma & Currie,
2000; Australovenator, Hocknull et al., 2009;
Megaraptor, Porfiri et al., 2014; Orkoraptor,
Novas et al., 2008; Murusraptor, Coria & Currie,
2016). The entire labial surface of the crown is
apicobasally and mesiodistally convex, whereas
the distal one-third of the lingual surface is api-
cobasally concave. This concavity produces the
slight curvature of the crown in distal view. The
rest of the lingual surface is homogeneously con-
vex. The distal margin of the crown possesses
a sharp carina that begins 1 mm from the base
and reaches the apicalmost preserved portion
of the crown. This carina is serrated along its
entire length, with a rather constant density of
five denticles per mm. Thus, the density of den-
ticles of MACN-PV 19066 is higher than that in
Fukuiraptor kitadaniensis (3−4 denticles per
mm; Azuma & Currie, 2000), Orkoraptor burkei
(3−4 denticles per mm; Novas et al., 2008) and
Megaraptor namunhuaiquii (3−4 denticles
per mm; MUCPv 595). The denticles are mesi-
odistally deeper and apicobasally tall and their
main axis is orthogonal to the distal edge of the
crown, resembling the condition in the above
mentioned megaraptorans. The distal denticles
of MACN-Pv 19066 have a rounded edge and are
separated from each other by short interdenticu-
lar sulci that do not extend as blood grooves, as
occurs in megaraptorids (e.g., Australovenator,
Megaraptor, Orkoraptor, Murusraptor; Hocknull
et al., 2009; Porfiri et al., 2014; Novas et al., 2008;
Coria & Currie, 2016). In contrast, at least some
check tooth crowns of the non-megaraptorid
Fukuiraptor have well-defined, oblique -basally
oriented- blood grooves (Azuma & Currie, 2000).
The mesial surface is considerably labiolingual-
ly broader than the distal one and has a carina
which extends along the total length of the crown.
The carina reaches the apicalmost preserved por-
tion of the crown and lacks denticles, as occurs
in Megaraptor (Porfiri et al., 2014), Orkoraptor
(Novas et al., 2008) and Murusraptor (with ex-
ception of the premaxillary teeth of this species;
Coria & Currie, 2016). In contrast, Fukuiraptor
(Azuma & Currie, 2000) and Australovenator
Figure 26. Megaraptoridae indet. (MPM 21546). (A-F) Posterior dorsal centrum in left lateral (A), ventral (B),
posterior (C), right lateral (D), dorsal (E) and anterior (F) views. Abbreviations: pl, pleurocoel. Scale bar: 3 cm.

259
Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
(Hocknull et al., 2009) possess denticles on both
mesial and distal carina. As far as it is preserved,
the enamel of MACN-Pv 19066 is smooth, with-
out wrinkles or wear facets. In contrast, exten-
sive wear facets, but not wrinkles, are usually
present in other megaraptorids (e.g. Novas et
al., 2008; Hocknull et al., 2009). The base of the
crown is sub-oval in cross-section, with a labial
margin distinctly more convex than the lingual
one. There is no labiolingual constriction of the
base of the crown, contrasting with the eight-
shaped cross-section of the base of the crowns
of the megaraptorids Orkoraptor (Novas et al.,
2008), Murusraptor (Coria & Currie, 2016) and
Megaraptor (Porfiri et al., 2014).
Comments. MACN-PV 19066 can be referred to
Megaraptoridae because of the presence of the
following combination of characters: strongly
distally recurved crown, with apex located distal
to the base, and absence of mesial serrations. The
Chorrillo Formation megaraptorid tooth differs
from those of other megaraptorids in its higher
density of distal denticles and the absence of an
8-shaped cross-section at the base of the crown
(Novas et al., 2013). Although MACN-PV 19066
suggests the presence of a previously unknown
megaraptorid taxon we refrain of erecting a new
species because of the fragmentary condition of
the megaraptorid specimens currently known
from the Chorrillo Formation.
Abelisauroidea Bonaparte and Novas, 1985
Noasauridae Bonaparte and Powell, 1980
Gen. et sp. indet.
Referred material. MPM 21547, isolated right
Figure 27. Megaraptoridae indet. (MACN-Pv 19066), isolated shed tooth crown in (A), labial; (B), lingual; (C),
mesial; (D), distal; (E), apical; and (H), basal views. F, G, selected anatomical details. Abbreviations: ds, distal
serrations; mc, mesial carina. Scale bar: 1 cm.

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
260
pedal phalanx IV-2? (locality 4) (Figure 28).
Description. The non-ungual pedal phalanx is
almost complete, lacking the ventral portion of
the distal condyles. The bone is 15 mm long and 8
mm high proximally. The element is proximodis-
tally short, dorsoventrally deep, and transversely
narrow. The proximal articular surface of the
phalanx has a dorsoventral keel separating two
concavities, the medial one being slightly wider
than the lateral one. Proximally, the phalanx has
a well-developed ventral projection, sub-triangu-
lar in contour as viewed from above. The phalanx
is ventrally flat and smooth. Dorsally is trans-
versely narrower with respect to the ventral sur-
face, a condition resembling other abelisauroids
(e.g., Novas & Bandyopadhyay, 2001; Novas et al.
2004; Brissón Egli et al., 2016). Distally, the me-
dial condyle is more developed than the lateral
one. The lateral collateral ligament pit is wider
and deeper than the medial one.
Comments. The pedal phalanx MPM 21547 re-
sembles Noasauridae in being transversely nar-
row and dorsoventrally deep, a condition shared
with Velocisaurus and Vespersaurus (Brissón
Egli et al., 2016; Langer et al., 2019). Further,
a well-developed, transversely thick, and pos-
teriorly extended posterodorsal process is also
shared with noasaurids, including Noasaurus,
Ligabueino, Velocisaurus and Vespersaurus
(Langer et al., 2019), constituting a synapo-
morphic feature of Noasauridae (Agnolin &
Chiarelli, 2010). However, in the case for the
phalanx here described the ventral projection
proximally surpasses the level of the dorsal lip,
contrasting with known noasaurids in which
the projections are sub-equal in proximal ex-
tension (e.g., Laevisuchus, Noasaurus; Novas
& Bandyopadhyay, 2001, Sampson et al., 2001,
Novas et al., 2004; Agnolin & Chiarelli, 2010;
Brissón Egli et al., 2016).
Noasaurids have been recorded in differ-
ent Late Cretaceous localities of Gondwana, in-
cluding India (Novas & Bandyopadhyay, 2001;
Novas et al., 2004), Africa (Sampson et al., 2001)
and South America (Bonaparte & Powell, 1980;
Bonaparte, 1991; Brissón Egli et al., 2016; Langer
et al., 2019; Martinelli et al., 2019). Present speci-
men constitutes the southernmost record for this
theropod family, and forms part of a small-sized
theropod fauna also integrated by megaraptorids
and unenlagiids (see below).
Paraves Sereno, 1997
Unenlagiidae Bonaparte, 1999
Gen. et sp. indet.
Referred material. MPM 21548, pedal ungual
of digit II; MPM 21549, phalanx III-2. (locality 4)
(Figure 28).
Description. MPM 21548 consist on the proxi-
mal portion of the ungual phalanx of right pedal
digit II, lacking its proximodorsal lip. The ungual
is laterally compressed, being 9.9 mm in trans-
verse width, comparable in size with the corre-
spondent ungual of Neuquenraptor argentinus
(MCF PVPH 77; Novas & Pol, 2005; Brisson et
al., 2017). It is elliptical-shaped in cross-section,
with collateral grooves asymmetrically placed, as
typical of paravians (Rauhut & Werner, 1995).
The lateral surface is flat, while the medial one
is dorsoventraly convex. The proximoventral
flexor tubercle is well-developed and shows ru-
gosities on its ventral surface, as typically oc-
curs among paravian theropods (e.g., Ostrom,
1969). The proximal articular surface presents
a well-defined dorsoventral keel separating two
sub-equals concavities. The ventral margin of
ungual forms a cutting edge that is continuous
with the lateral surface of the ungual blade. The
cutting edge is medially displaced, as it also oc-
curs in other paravians (e.g., Neuquenraptor,
Deinonychus, Velociraptor, Buitreraptor; Ostrom,
1969; Norell & Makovicky, 1999; Makovicky et
al., 2005; Brisson Egli et al., 2017). The longi-
tudinal collateral grooves are proximally forked,
depicting the characteristic horizontal oriented
“Y”-shaped groove.
Besides, specimen MPM 21549 is a complete
pedal non-ungual phalanx, probably correspond-
ing to pedal phalanx III-2. It was found nearby
MPM 21548 and their sizes are congruent to
refer them to a single individual. Phalanx MPM
21549 measures 45 mm long and 19 mm high, be-
ing larger than that of Neuquenraptor (Brisson
Egli et al., 2017).
The proximal articular surface lacks the me-
dial keel and shows a sub-triangular contour,
with a straight ventral margin. The proximodor-
sal lip has a sub-triangular profile in dorsal view,
proximally surpassing the level of the articular
surface. The proximal articular surface is gen-
tly concave. The proximal portion of the ventral
surface has a rugose and flat shelf. Distally, the
extensor fossa is wide. The distal condyles are
circular-shaped in lateral view and have wide
collateral ligament pits. In lateral view, the dis-
tal condyles slightly surpass the dorsal margin of
the body of the phalanx.
Comments. Unenlagiids were recorded in NW
Patagonia and NW Argentina (Agnolin and
Novas, 2013). MPM 21548 and MPM 21549 con-

261
Novas et al.: Paleontology of the Chorrillo Formation at Santa Cruz province
stitute the southernmost unenlagiid record for
South America. This unenlagiid, similar in size
and shape to Neuquenraptor, expands the mea-
gre record of Maastrichtian unenlagiids from
Argentina, up to the date represented by the
large bodied Austroraptor (Novas et al., 2009).
Aves Linnaeus, 1758
Ornithurae Haeckel, 1866
Kookne yeutensis nov. gen. et sp.
Holotype. MPM 21550, incomplete right cora-
coid lacking sternal end and proximal end dam-
aged (locality 2) (Figure 29).
Diagnosis. Medium-sized derived ornithurine
diagnosable on the basis of the following unique
combination of characters (autapomorphies
marked by an asterisk*): 1) robust coracoidal
shaft with well-defined and proximodistally ex-
tended procoracoid process; 2) ligamentum cora-
coscapulare ventralis forming a well-defined scar,
resulting in a notch that separates the scapular
cotyla from the facies articularis humeralis; 3)
Figure 28. Theropod pedal elements. (A-C), isolated noasaurid right pedal phalanx IV-2? (MPM 21547) in me-
dial (A-B) and dorsal (C) views; (D-H), Unenlagiid pedal elements. Right pedal phalanx III-2 (MPM 21549, D)
and right pedal ungual of digit II (MPM 21548, E-G) in lateral (D), ventral (E, F), and medial (G-H) views.
Abbreviations: cp, colateral pit; ft, flexor tubercle; mvc, medioventral crest; pdp, posterodorsal process; pvp,
posteroventral process. Scale bar: 10 cm

Revista del Museo Argentino de Ciencias Naturales, n. s. 21(2), 2019
262
facies articularis humeralis ovoidal in shape,
with its distal half more transversely expanded
than the proximal half; 4) cup-shaped impression
for the acrocoracohumeral ligament, with thick-
ened and well-defined margins*.
Etymology. “Kookne”, mythological swan com-
panion of the Aonikenk hero Elal; and yeutensis,
from “yeut”, “mountain” in Aonikenk language.
Description. Kookne yeutensis is represented by
an isolated coracoid lacking the sternal end and
showing badly damaged acrocoracoidal process.
The coracoid is robust, and belongs to a medium-
sized bird, the size of a tern (maximum preserved
proximodistal length of coracoid is 23.4 mm, mi-
nor coracoidal transverse width is 4.8 mm, and
the preserved transverse width at level of scapu-
lar cotyla is 6.3 mm). The distal end of the bone
exhibits a transverse linear ridge for the impres-
sion of the M. sternocoracoidei. This ridge is
obliquely oriented and runs from the laterodistal
to the medioproximal edges of the bone.
The procoracoid process is present, but lacks
its distal tip. It is represented by an acute lamina.
This process is relatively dorsoventrally short,
distally extending as a narrow but well-defined
ridge, constituting the medial edge of the distal
half of the coracoid. Distal to the procoracoid pro-
cess there is a strap-like impression for the M.
subscapularis, as is observed in modern anseri-
forms (e.g., Anas) and Maaqwi (McLachlan et al.,
2017). At the base of the procoracoidal process
there is a small, ellipsoidal-shaped foramen for
the N. supracoracoidei. This foramen penetrates
the bone dorsally and leaves the bone medially,
within the supracoracoidei groove. This foramen
is distal to the medial margin of the scapular
cotyla, and is slightly recessed.
The scapular cotyla is roughly subtriangular
in contour. The margins of the cotyle are nota-
bly thick, especially the lateral and distal ones.
Laterally, the distal margin of the cotyle is de-
limited by a notch formed by the scar of the
ligamentum coracoscapulare ventralis. This scar
separates the scapular cotyle form the articular
surface for the humerus. The humeral articular
surface is relatively large, roughly ovoidal in con-
tour and its surface is slightly concave. Its distal
half is transversely wider than its proximal half.
The lateral margin is slightly thickened. Its dis-
tal end is more proximally located than the distal
level of the scapular cotyla and is delimited by
the scar of the ligamentum coracoscapulare ven-
tralis. The humeral articular surface is laterodor-
sally oriented and slightly proximally tilted. The
proximal edge of the humeral articular surface
is narrow and bifurcates in two main ridges that
delimit a well-defined and cup-shaped impressio
for the acrocoracohumeral ligament. The dorsal
crest becomes proximally prominent and medi-
ally wraps the triosseous canal.
The lateral surface of the coracoid shows a sub-
triangular flattened surface with its tip pointing
distally. This surface is ventrally delimited by a
very narrow but conspicuous ridge corresponding
with the M. supracoracoidei. Distal to this ridge,
there is preserved the proximalmost portion of
the intermuscular ridge between the M. coraco-
brachialis caudalis and M.supracoracoideus (see
Jasinoski et al., 2006).
In medial view the coracoid shows a very deep
and wide triosseal cana