An articulated titanosaur from Patagonia (Argentina): New evidence of neosauropod pedal evolution

Abstract

Most titanosaur dinosaurs are represented by incomplete skeletal elements lacking articulated pes. An exceptionally preserved specimen from the Late Campanian–Early Maastrichtian strata of Patagonia (Argentina) provides new data on pedal morphology and the evolutionary trends of these huge dinosaurs. This finding is one of the few articulated titanosaur pes known in the world, and shows a phalangeal formula of 2-2-2-2-0. The first three digits possess sickle-shaped claws and the articular facets of ungual phalanges, suggesting mobility in horizontal and vertical planes. A comparative analysis of available record suggests that titanosaurs had a progressive reduction of size and number of pedal phalanges in digits III and IV during the Late Cretaceous.

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Palaeoworld 17 (2008) 33–40
Research paper
An articulated titanosaur from Patagonia (Argentina):
New evidence of neosauropod pedal evolution
Bernardo J. Gonz´
alez Riga a,b,∗, Jorge Orlando Calvoc, Juan Porfiri c
aCONICET, IANIGLA-CRICYT, Centro Regional de Investigaciones Cient´ıficas y T´ecnicas, Avda. Ruiz Leal s/n,
Parque Gral, San Mart´ın, 5500 Mendoza, Argentina
bUniversidad Nacional de Cuyo, Instituto de Ciencias B´asicas, Argentina
cUniversidad Nacional del Comahue, Centro Paleontol´ogico Lago Barreales, Proyecto Dino,
Ruta Prov. 51, km 65, Neuqu´en, Argentina
Received 11 July 2006; received in revised form 6 June 2007; accepted 1 August 2007
Available online 7 September 2007
Abstract
Most titanosaur dinosaurs are represented by incomplete skeletal elements lacking articulated pes. An exceptionally preserved specimen from
the Late Campanian–Early Maastrichtian strata of Patagonia (Argentina) provides new data on pedal morphology and the evolutionary trends of
these huge dinosaurs. This finding is one of the few articulated titanosaur pes known in the world, and shows a phalangeal formula of 2-2-2-2-0.
The first three digits possess sickle-shaped claws and the articular facets of ungual phalanges, suggesting mobility in horizontal and vertical planes.
A comparative analysis of available record suggests that titanosaurs had a progressive reduction of size and number of pedal phalanges in digits
III and IV during the Late Cretaceous.
© 2007 Nanjing Institute of Geology and Palaeontology, CAS. Published by Elsevier Ltd. All rights reserved.
Keywords: Pes morphology; Sauropoda; Titanosauria; Cretaceous; Patagonia; Argentina
1. Introduction
The study of the pedal structure is essential for understand-
ing the locomotion, evolutionary trends and the behavior of
the sauropod dinosaurs. Sauropod pedes possess several unique
features: an asymmetrical pes, extreme reduction of the tarsus,
typically three large pedal claws, and expanded phalangeal artic-
ular surfaces (Bonnan, 2005). Sauropod titanoaurs have a similar
morphology, but articulated records are scarce. Up to now, more
than 40 taxa are known, but only 2 have complete and artic-
ulated pes: Opisthocoelicaudia Borsuk-Bialyncka, 1977 and
Epachthosaurus Powell, 1990 (Mart´
ınez et al., 2004). In this
context, the discovery of a new specimen herein described, with
exceptionally preserved pedal structure, is relevant from a sys-
∗Corresponding author at: CONICET, IANIGLA-CRICYT, Centro Regional
de Investigaciones Cient´
ıficas y T´
ecnicas, Avda. Ruiz Leal s/n, Parque Gral, San
Mart´
ın, 5500 Mendoza, Argentina.
E-mail addresses: bgonriga@lab.cricyt.edu.ar,bgonriga@yahoo.com.ar
(B.J. Gonz´
alez Riga), jorgecalvo@digimedia.com.ar (J.O. Calvo),
jporfiri@yahoo.com (J. Porfiri).
tematic viewpoint. This finding is, therefore, one of the few
records of an articulated titanosaur pes known in the world.
The specimen was collected by the authors during a paleon-
tological exploration at La Invernada area (Neuqu´
en Province,
Northern Patagonia) in 2005. The material came from the Allen
Formation that corresponds to Late Campanian–Early Maas-
trichtian strata of the Neuqu´
en Basin. The specimen comprises a
partial skeleton with complete manus and pes (MUCPv-1533).
In this paper, we only describe and analyze the complete and
articulated pes (Fig. 1), and other skeletal elements will be
described elsewhere.
2. Geological setting
The Neuqu´
en Basin is located in northern Patagonia,
Argentina, between 34◦40and 40◦south latitude and from 66◦
to 70◦20west longitude (Fig. 2). This basin extends between
the active margin magmatic arc along the Andes to the west,
and the Sierra Pintada System and the North Patagonian Mas-
sif to the east, respectively (Vergani et al., 1995). The Neuqu´
en
Basin covers a surface of 137 000 km2and consists in a succes-
1871-174X/$ – see front matter © 2007 Nanjing Institute of Geology and Palaeontology, CAS. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.palwor.2007.08.003

34 B.J. Gonz´alez Riga et al. / Palaeoworld 17 (2008) 33–40
Fig. 1. Field excavation of the titanosaur skeleton at La Invernada, Patagonia (MUCPv-1533).
sion of 7000 m of sedimentary rocks from Triassic to the Early
Paleocene.
The basement rock of the Neuqu´
en Basin is commonly known
as the Choiyoi Group. It is composed by volcanic, piroclas-
Fig. 2. Map of the Neuqu´
en Basin (after Vergani et al., 1995) showing La
Invernada site (asterisk).
tic, and sedimentary complex of Permian–Triassic age (Cazau
and Uliana, 1973). From the middle-later Triassic the basin was
filled by sedimentary rocks composed mostly by continental
clastics with tuff and volcanic intercalations. During the Juras-
sic and Cretaceous, transgressive and regressive episodes were
developed, depositing continental, littoral and marine facies
(Legarreta and Gulisano, 1989).
The Malargue Group (Campanian to Danian) includes the
Allen, Jaguel and Roca formations (Fig. 3). The Allen Formation
is integrated by fluvial, lacustrine and marginal marine facies
(tidal flats, lagoon and sabkhas). These facies are related to a first
marine ingression from the Atlantic sea that covered central-
northern Patagonia toward the end of the Cretaceous Period
(Andreis et al., 1974; Barrio, 1990; Parras et al., 1998; Gonz´
alez
Riga, 1999). The fossil described here was found in the mid-
dle section of the Allen Formation, in laminated grey-greenish
pelites that are preliminarily assigned to a lacustrine environ-
ment deposited during Late Campanian–Early Maastrichtian.
The taphonomic and sedimentological aspects of this quarry,
including burial processes of the articulated specimen, will be
analyzed elsewhere.
3. Paleontology
3.1. Phylogenetic criteria and material recovered
The phylogenetic relationships of Titanosauria taxa are not
resolved yet, since most of taxa are represented by fragmentary
and disarticulated remains. Moreover, the published cladistic
analyses use different terminal taxa and are based on diverse
criteria in naming the nodes (see Curry Rogers, 2005; Wilson,
2006). For example, the name Titanosauridae, a mostly tradi-
tional titanosaurian group, is used (Salgado, 2003) or avoided
(Sereno, 1998; Wilson, 2002; Wilson and Upchurch, 2003) after

B.J. Gonz´alez Riga et al. / Palaeoworld 17 (2008) 33–40 35
Fig. 3. Geologic column of the Neuqu´
en and Malargue Groups. The arrow and
dot show the finding of specimen MUCPv-1533.
different interpretations of the Phylogenetic Code. We think that
titanosaurian clades need a detailed study including all species
recently discovered, and a carefully revision of the numer-
ous names, both in node-based and stem-based taxa. In this
study we adopt Titanosauria and Titanosauridae names after
definitions proposed by Salgado et al. (1997a) and Salgado
(2003).
In this paper we follow the anatomical nomenclature pro-
posed by Weishampel et al. (1990). These authors use cranial and
cranially instead anterior and anteriorly, craniocaudally instead
anteroposteriorly, and caudal and caudally instead posterior and
posteriorly, respectively.
The specimen described herein (MUCPv-1533) includes a
partial caudal series and the left portion of the skeleton lack-
ing the skull and cervical vertebrae (Fig. 4). The left limbs
are complete and articulated. The specimen is still in prepa-
ration; however, a preliminary observation indicates that is a
derived titanosaurid sensu phylogenetic definition of Salgado
(2003). It has strongly procoelous caudal vertebrae with promi-
nent condyles, neural arches cranially located in middle and
distal caudal centra, haemal arches articulations open proxi-
mally, metacarpals without distal articular facets, absence of
manual phalanges and claws, and femora with a lateral bulge
below the greater trochanter.
Institutional abbreviations. MUCPv-1533, Museo de
Geolog´
ıa y Paleontolog´
ıa Universidad Nacional del Comahue,
Argentina.
3.2. Description
The astragalus (Fig. 5) is the only ossified element of the
tarsus, as in Opisthocoelicaudia (Borsuk-Bialyncka, 1977),
Epachthosaurus (Mart´
ınez et al., 2004) and Neuquensaurus
(Salgado et al., 2005). This bone is transversely reduced and has
a slightly concave lateral face for the articulation of the fibula
(Fig. 5B). The cranial face has a triangular contour; it shows
a convex surface that articulates with the proximal end of the
metatarsals (Fig. 5A). In dorsal view it has a triangular contour
with a relatively small ascending process that articulates with a
depression in the distal end of the tibia, as in Epachthosaurus
(Mart´
ınez et al., 2004). The distal face is convex and strongly
rugose (Fig. 5C). The tibial face is not strongly inclined as in
Aeolosaurus sp. (Salgado et al., 1997b, p. 47).
The pes, as in all sauropods, is typically asymmetrical. It has
five metatarsals, where the metatarsals III and IV are the longest
elements. In proximal view the articular faces of the metatarsals
II–IV have a subrectangular shape; therefore, they form a very
flat arch when articulated. A similar condition is seen in Opistho-
coelicaudia (Borsuk-Bialyncka, 1977, p. 49). In the specimen
herein described, the metatarsal V is in a laterocaudal position,
showing a strongly convex contour (Fig. 6). This morphology
is different to those described for all other neosauropods, which
Fig. 4. Skeletal disposition of bone in La Invernada site (Patagonia, Argentina): (A) map of the quarry; (B) excavation of the specimen.

36 B.J. Gonz´alez Riga et al. / Palaeoworld 17 (2008) 33–40
Fig. 5. Left astragalus in anterior (A), lateral (B), and caudal (C) views. Scale bar=5cm.
have a more linear configuration (see Bonnan, 2005, p. 356).
The length of metatarsals increases from I to IV (Fig. 7;
Table 1). Both the original position of the pes in the field
and the study of the articular surfaces have allowed docu-
menting the general disposition of metatarsals and phalanges.
The unguals are inclined 40–60◦with respect to the vertical.
This special disposition is also described in the titanosauriform
Pleurocoelous (Gallup, 1989) and Opisthocoelicaudia (Borsuk-
Bialyncka, 1977).
The twisted metatarsal I is the thickest and shortest of all
metatarsals. The proximal end has a subcircular contour, and
shows a rugose and slightly convex surface. It is inclined with
respect to the axis of the shaft. The shaft has a concave surface
for the contact with the metatarsal II. The distal end has a sub-
Fig. 6. Metatarsals of the titanosaur MUCPv-1533 in proximal view (roman numbers show digit numbers). Scale bar = 5 cm.

B.J. Gonz´alez Riga et al. / Palaeoworld 17 (2008) 33–40 37
Fig. 7. Pes of the titanosaur MUCPv-1533 in dorsal view (roman numbers show
digit numbers). Scale bar =5 cm.
quadrangular shape and a craniomedial small concavity for the
articulation with the first phalanx.
The metatarsal II is longer than metatarsal I. It has a
craniocaudally expanded proximal end with an elongated and
rectangular rugose surface. The distal end is quadrangular,
rugose and slightly convex.
The metatarsal III is as long as metatarsal IV. It is a slender
bone with expanded ends. The rugose proximal end has a sub-
quadrangular outline. The lateral border of the proximal end is
slightly convex cranially, but it is straight caudally. This bor-
Table 2
Proximodistal lengths (cm) of phalanges of the left pes of MUCPv-1533
Digit I Digit II Digit III Digit IV
Phalanx I 21 57 51 57
Phalanx II 113 82 76 24
der is the contact area for the metatarsal IV. The distal end is
subquadrangular, wider transversely than craniocaudally.
The metatarsal IV is a slender bone, expanded distally and
proximally. Both articular surfaces are rugose. The proximal
end has a trapezoidal outline, wider cranially than caudally. The
shaft has a triangular section with a flat dorsal (palmar) face
and pointed ventral (plantar) one. The convex distal end has a
suboval outline, transversely wider than craniocaudally.
The metatarsal V has a subtriangular outline in proximal view,
with a more acute cranial border. It is the shortest metatarsal of
the pes. Metatarsal V possesses an extremely craniocaudally
broadened fan-shaped proximal end. The distal end is reduced
to almost the half of the proximal end. The medial surface is
flat on the half proximal portion, and has a straight outline in
proximal view.
The phalangeal formula is 2-2-2-2-0 (Fig. 7). The first pha-
langes increase in length from digit I to IV (Table 1); in contrast,
the second phalanges (ungueals), reduce their size from digit I
to IV (Table 2).
The phalanx I of digit I is very reduced in relation to the oth-
ers. It has a hemi-spheric shape. This structure is very different
from the titanosaurid Epachthosaurus (Mart´
ınez et al., 2004), in
which it is a more robust element.
The first phalanx of digits II–IV are wedge-shaped and have
a trapezoidal shape in dorsal view. This surface is convex lat-
eromedially and concave craniocaudally. In proximal view, an
asymmetry is developed in these phalanges and they are deeper
medially than laterally. In dorsal view, the medial border is
longer than the lateral one.
The phalanx I of digit II is similar in shape to that of digit III,
but it is thicker and shorter. By contrast, the phalanx I of digit
IV is thinner and has a flat ventral face. Their distal ends have
a convex surface, more pronounced in the borders than in the
middle for articulation with a phalanx II.
Phalanx II forms the ungual in the digits I–III (Fig. 7;Table 2).
By contrast, in Epachtosaurus the ungueal in digit III is the
phalanx III (Mart´
ınez et al., 2004). Unguals I–III are flattened
lateromedially, and has a typical “sickle” shape, as in Mendoza-
saurus neguyelap (Gonz´
alez Riga, 2003;Fig. 6).
The phalanx II of digit IV has a structure similar to pha-
lanx I of digits I–III, but it is smaller, and wider lateromedially.
Table 1
Measurements (cm) of metatarsals of MUCPv-1533
Metatarsal I Metatarsal II Metatarsal III Metatarsal IV Metatarsal V
Length 120 137 168 172 127
Proximal width 104 104 80 67 78
Distal width 72 61 60 61 40

38 B.J. Gonz´alez Riga et al. / Palaeoworld 17 (2008) 33–40
Fig. 8. Articulated titanosaur pes in dorsal view. (A) Titanosaur MUCPv-1533 from La Invernada site, Northern Patagonia; (B) Epachthosaurus from Central
Patagonia, after Mart´
ınez et al. (2004). Scale bar= 5 cm.
Fig. 9. Schematic cladogram of the neosauropods (modified from Salgado et al. (1997a),Wilson and Sereno (1998),Wilson (2006)) showing selected taxa, pedal
structures and phalangeal formulae: Diplodocus and Camarasaurus (after Farlow, 1992), Epachthosaurus (after Mart´
ınez et al., 2004), ‘La Invernada titanosaur’
(this paper, specimen MUCPv-1533 from Argentina) and Opisthocoelicaudia (based on Borsuk-Bialyncka, 1977). The phylogenetic position of ‘La Invernada
titanosaur’, based on axial characters, is hypothetic. Observations: The Macronaria clade (Wilson and Sereno, 1998) has not the same phylogenetic definition of
Camarasaromorpha (sensu Salgado et al., 1997a).

B.J. Gonz´alez Riga et al. / Palaeoworld 17 (2008) 33–40 39
This phalanx has a convex proximal end that articulates with
the concave face of the distal end of the phalanx I of digit
IV.
4. Discussion
The fossil preservation of titanosaurian pedes is very rare
due to taphonomic processes of disarticulation. For this rea-
son, most pes reconstructions are made with certain degree of
uncertainty. For instance: Janenshia robusta Fraas, 1908 has a
complete right pes (Fraas, 1908, plate XII) with a phalangeal for-
mula 2-3-3-2-1; however, Bonaparte et al. (2000, p. 38) stated
that “digit IV and V either lack ungual phalanges, or they could
have been lost due to taphonomic factors” but they draw the
complete pes (Bonaparte et al., 2000;Fig. 8A). Another exam-
ple is the pes of a titanosauriform from Russia (Averianov et al.,
2002) reconstructed with few bones.
A complete titanosauriform pes belongs to Gobititan (You
et al., 2003) from the Albian of China. The phalangeal for-
mula is 2-2-2-2-2, and it has a phalanx in the digit V, absent
in all titanosaurs recorded. Although it was assigned to a
Titanosauria, it does not have synapomorphies of this group of
sauropods. Therefore, and based on the diagnosis of Gobititan,
we can only consider it as a titanosauriform, not as a titano-
saur.
The calcaneum is present in titanosauriforms such as Bra-
chiosaurus and Gobititan; however it is absent in the specimen
herein described and in other known titanosaurids.
Up to now, just two taxa have provided information about
the pes structure in titanosaurids: Opisthocoelicaudia skarzyn-
skii from Mongolia and Epachtosaurus sciutoi (Mart´
ınez et al.,
2004) from Chubut Province.
The specimen described (MUCPv-1533) includes a partial
caudal series and the left portion of the skeleton but lacks
the skull and cervical vertebrae. The pes has five metatarsal
and the phalangeal formula is 2-2-2-2-0. The first three dig-
its possess sickle-shaped claws and the articular faces of
ungual phalanges suggesting mobility in horizontal and ver-
tical planes. In particular the first phalanx of the digit I
is a much reduced structure (Fig. 7). In contrast, with this
pedal structure, Epachthosaurus has a phalangeal formula 2-
2-3-2-0 (Fig. 8) and Opisthocoelicaudia has 2-2-2-1-0. It is
showing a wide diversity of pedal morphology in titanosaurid
sauropods.
In general, the evolution of the sauropod pes shows a pha-
langeal reduction trend (Carrano, 2005), but this aspect has not
been well documented in titanosaurs. In this context, phyloge-
netic analyses of titanosaurs indicate that Epachthosaurus is a
basal taxon, and Opisthoceolicaudia is a derived genus closely
related to Saltasaurus (Salgado et al., 1997a; Wilson, 2006).
This record suggests that titanosaurs had a progressive reduc-
tion in the size and number of pedal phalanges in digits III and IV
toward the end of the Cretaceous (Fig. 9). The new material from
Patagonia herein described could be considered as a titanosaurid
more derived than Epachthosaurus, based on its pedal structure.
This preliminary hypothesis may be confirmed with a detailed
study of the rest of skeleton.
5. Conclusions
An exceptionally preserved titanosaurid specimen from the
late Campanian–Early Maastrichtian strata of Argentina pro-
vides new evidence of pedal morphology in these dinosaurs. As
far as we know, this is the third record of a titanosaur articulated
pes, and shows a phalangeal formula of 2-2-2-2-0. Morpholog-
ical analysis of this structure suggests that titanosaurs had a
progressive reduction in the size and number of pedal phalanges
in digits III and IV, from the Late Cenomanian–Early Turonian
to the Late Campanian–Early Maastrichthian in age.
Acknowledgements
We thank the organizers of the 2nd International Paleontolog-
ical Congress (China) for allowing us publishing this paper. We
very much appreciate the review of this manuscript by M. Bon-
nan and I. Canudo. The excavation was carried out by a team of
Laboratorio de Paleovertebrados del IANIGLA-CRICYT (Men-
doza) and the Centro Paleontol´
ogico Lago Barreales (Dino
Project, Universidad Nacional del Comahue, Neuqu´
en) and sup-
ported by the Wintershall Company. We are also grateful for
the supports through the projects of Unversidad Nacional de
Cuyo (2007), Agencia Nacional de Promoci´
on Cient´
ıfica y Tec-
nol´
ogica de Argentina (PICT 2005-33984), Consejo Nacional
de Investigaciones Cient´
ıficas y T´
ecnicas de Argentina (CON-
ICET PIP-5222) and Universidad Nacional del Comahue I-122,
2005-2007). Finally, we are grateful to the Chevron S.A., who
supported the second author’s (JOC) travel from Argentina to
China to present this paper.
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