Bonitasaura salgadoi gen. et sp. nov.: A beaked sauropod from the Late Cretaceous of Patagonia

Abstract

Ornithischian and theropod dinosaurs were morphologically diverse during the Cretaceous. In contrast, sauropods were relatively more conservative. The anatomy of Bonitasaura salgadoi, a new 9-m titanosaurian sauropod from Upper Cretaceous beds of Patagonia, suggests that sauropod anatomical diversity would have included unexpected items. Its unusual, rectangular lower jaw possesses narrow, anteriorly restricted teeth and shows evidence of a sharp keratinous sheath over the nondentigerous region that probably worked to guillotine plant material. This discovery definitely demonstrates that titanosaurs acquired a mandibular configuration similar to that of some basal diplodocoids, as had already been suggested by the lower jaw of the controversial genus Antarctosaurus. This oral configuration, plus the beak-like structure and the skull shape, resemble some traits more commonly seen in Laurasian ornithischians, mostly unexpressed in southern continents. A high sauropod morphological diversity seems to be in agreement with the poorly represented ornithischian clades of the southern hemisphere.

Full text

Naturwissenschaften (2004) 91:493–497
DOI 10.1007/s00114-004-0560-6
SHORT COMMUNICATION
Sebastin Apestegua
Bonitasaura salgadoi
gen. et sp. nov.: a beaked sauropod
from the Late Cretaceous of Patagonia
Received: 29 April 2004 / Accepted: 4 August 2004 / Published online: 10 September 2004
 Springer-Verlag 2004
Abstract Ornithischian and theropod dinosaurs were
morphologically diverse during the Cretaceous. In con-
trast, sauropods were relatively more conservative. The
anatomy of Bonitasaura salgadoi, a new 9-m titano-
saurian sauropod from Upper Cretaceous beds of Patag-
onia, suggests that sauropod anatomical diversity would
have included unexpected items. Its unusual, rectangular
lower jaw possesses narrow, anteriorly restricted teeth and
shows evidence of a sharp keratinous sheath over the non-
dentigerous region that probably worked to guillotine
plant material. This discovery definitely demonstrates that
titanosaurs acquired a mandibular configuration similar
to that of some basal diplodocoids, as had already been
suggested by the lower jaw of the controversial genus
Antarctosaurus. This oral configuration, plus the beak-
like structure and the skull shape, resemble some traits
more commonly seen in Laurasian ornithischians, mostly
unexpressed in southern continents. A high sauropod
morphological diversity seems to be in agreement with
the poorly represented ornithischian clades of the south-
ern hemisphere.
Introduction
After the purported global extinction of diplodocoid
sauropod dinosaurs during the Cenomanian–Turonian
(Bakker 1986), titanosaurs became the sole surviving
latest Cretaceous sauropods. They successfully evolved in
southern continents by radiating in a wide range of forms
that later recolonized some Laurasian areas.
Derived titanosaurs are said to parallel diplodocoids
(Salgado and Calvo 1997; Wilson 2002) in their ‘horse-
like’ skulls, restriction of the cylindrical, narrow-crowned
teeth to the anterior part of the snout, comb-like dentition
(Coria and Chiappe 2001), squared symphysis, and nos-
trils retracted to the top of the head. The high femur/
humerus ratio (a reversal) and an incipient whiplash tail
could also be regarded as convergences. The lower jaw
seems also to be problematic in titanosaur anatomy.
Based on it, the Patagonian titanosaur Antarctosaurus
wichmannianus (von Huene 1929), was interpreted as a
chimera which included a diplodocoid lower jaw (Jacobs
et al. 1993; Wilson and Sereno 1998; Upchurch 1999),
and was later specified to be a rebbachisaurid (Sereno et
al. 1999).
A new partially articulated sauropod titanosaur
(Fig. 1c, d) was recovered in Santonian rocks of the Bajo
de la Carpa Formation top (Hugo and Leanza 1999) at Ro
Negro Province, Argentina. Among the collected bones, a
right dentary bearing several teeth provides new insights
into the morphology of Late Cretaceous herbivorous di-
nosaurs on southern continents.
Systematic paleontology
Saurischia Seeley 1888
– Sauropoda Marsh 1878
– Titanosauria Bonaparte and Coria 1993
– Bonitasaura salgadoi gen. et. sp. nov.
Etymology
The generic name is derived from the “La Bonita” hill,
the name of the quarry, and saura, a female reptile. The
species, salgadoi, honors Leonardo Salgado, the Argen-
tinian paleontologist who gave new perspectives to sauro-
pod research.
Communicated by G. Mayr
S. Apestegua (
)
)
Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”,
Av. ngel Gallardo 470, 1405 Buenos Aires, Argentina
e-mail: paleoninja@yahoo.com.ar
Tel.: +54-11-49826595
Fax: +54-11-49826595

Holotype
MPCA 300 (Museo Provincial “Carlos Ameghino”,
Cipolletti, Ro Negro, Argentina), consists of a partially
articulated, subadult skeleton (Fig. 1a– c). The material
includes a left frontal, left parietal, right dentary with 15
teeth, lacking at least three or four alveoli distal to the
symphysis, two cervi cal, six dorsal, and 12 caudal ver-
tebrae, two chevrons, several cervical and dorsal ribs,
humerus, radius, two metacarpals, femur, tibia, two me-
tatarsals.
Locality and geological setting
“La Bonita” hill fossil quarry, Cerro Polica, Ro Negro
Province, NW Patagonia, Argentina. The specimen was
found in a fluvial sandstone (Hugo and Leanza 1999)
Fig. 1 A The ‘La Bonita’ hill, about 100 km south of Cipolletti,
Argentina. B Stratigraphic position (arrow) of the fossil discovery.
C Skeletal reconstruction and body shape of Bonitasaura. Pre-
served bones are indicated as dashed zones. The preserved dentary
is actually the right one. D Quarry map. Scale bars are 1 m in
length. Arrow in D points north. Skull bones are labelled as fr
(frontal), lj (lower jaw) and pa (parietal)
494

which belongs to the uppermost layers of the Bajo de la
Carpa Formation (Santonian; Hugo and Leanza 1999).
Diagnosis
Bonitasaura differs from other titanosaurs in the follow-
ing combination of features: dentary alveoli reduced in
number (three in the main ramus, one in the angle, and up
to seven in the anterior region); middle and posterior re-
gion of the dentary edentulous and forming a sharp dorsal
edge, with a profusely vascularized lateral side; very ro-
bust, diagonal neural arch pillars and bulging neural spine
summits on anterior dorsal vertebrae. More diagnostic
characters might emerge once all of the postcranial ma-
terial has been prepared.
Description
The preserved skull remains of Bonitasaura include right
frontal and parietal, and an incomplete lower jaw (Fig. 2a,
b). However, only the latter will be described here.
Bonitasaura (Fig. 2) bears an anteroposteriorly straight
mandibular ramus that turns medially at almost a right
angle to meet the opposite ramus in a transverse sym-
physis. The anteroventral margin of the dentary is vertical
and lacks a chin-like process, thus differing from diplo-
docoids (Salgado and Calvo 1997; Upchurch 1999; Wil-
son 2002). The depth of the dentary is fairly homogene-
ous except for the posteriormost preserved region, but it is
not possible to see whether it gradually deepens as in
Rapetosaurus (Curry Rogers and Forster 2004). As in the
latter, the lateral plate is substantially higher than the
medial one. Although only ten complete alveoli were
preserved, each ramus would have borne around 13 al-
veoli, which decrease in size posteriorly. All but the last
two alveoli are associated with large neurovascular fo-
ramina. The anteriormost preserved alveolus bears up
to three teeth, the same number as recorded for Antarc-
tosaurus and other titanosaurs (Powell 1979; Coria and
Chiappe 2001), whereas diplodocoids reached a higher
number, as in the rebbachisaurid Nigersaurus, where up
to seven narrow, striated replacement teeth are present
(Sereno et al. 1999).
Posterior to the tooth-bearing region, the dorsal border
of the jaw bears an abrupt elevation that terminates in a
long, thin, horizontal edge, only preserved for its first
65 mm (Fig. 2c, e). The jaw widens ventrally up to three
times. The upper 15 mm of this exquisitely preserved
surface, especially on the labial side, is profusely pierced
by small neurovascular foramina and furrows, as in the
regions on bird dentaries sheathed by the rhamphotheca
(Norell et al. 2001). This unusual morphology of the mid-
to posterior region of the dentary, where bite-forces are
higher, and its presumably keratinous cover, would have
provided a strong cropping device for cutting tough plant
material.
Fig. 2 Bonitasaura, MPCA
300, holotype. A Right frontal
in ventral and dorsal views. B
Left parietal in anterior, dorsal
and posterior views. C Labial
view of the cutting mandibular
crest on the posterior dentary
and its densely pitted lateral
surface. D Reconstruction of the
head of Bonitasaura showing
the position of the guillotine-
like mandibular crest covered
by a dark rhamphoteca. E Right
dentary in lingual and labial
(reversed) views. F Dentary in
ventral and dorsal views. Scale
bars: A, B, E, F 50 mm; C
30 mm. Abbreviations: fcb
frontal central “bump”; fpc
frontoparietal concavity; gc
guillotine crest; mg meckelian
groove; nvf neurovascular fo-
ramina; sr sculptured rim; vr
ventral ridge
495

In contrast with primitive titanosaurs (Martnez 1998),
which have broad-crowned, compressed cone-chisel-like
teeth, Bonitasaura bear narrow, pencil-chisel-like teeth
(Fig. 2), as in the derived titanosaurs Rapetosaurus (Curry
Rogers and Forster 2004), Antarctosaurus (von Huene
1929), Nemegtosaurus (Nowinski 1971), and undescribed
new forms from Patagonia. As in Antarctosaurus, Rin-
consaurus (Calvo and Gonzlez Riga 2003), and the
isolated Patagonian premaxilla MPCA 79 (Coria and
Chiappe 2001), these teeth have well-marked, non-den-
ticulated carinae on their mesial and lateral edges that
divide the labial and lingual sides. Although the teeth of
MPCA 79 are slightly larger in diameter, this difference is
expected to occur between upper and lower teeth in both
diplodocoids and titanosaurs (Nowinski 1971; Holland
1906). Rinconsaurus carinae are more primitive in being
larger and asymmetrically developed. These carinae fill a
morphological gap between the broad two-winged basal
macronarian teeth, the asymmetrically winged teeth of
basal titanosauriforms, and the completely cylindrical
teeth of advanced titanosaurs.
The abundant postcranial remains of Bonitasaura
(MPCA 300) will be described elsewhere. However,
Bonitasaura does not differ significantly in the postcra-
nium from other derived titanosaurs (Apestegua and
Gallina 2002). The dorsal vertebrae bear a well-developed
prespinal lamina. Both anterior and middle caudal verte-
brae are strongly procoelous, while the distalmost caudals
are long and biconvex. Metacarpals are relatively slender,
in contrast to the short metatarsals. Bonitasaura differs
from known titanosaur species in having anterior dorsals
with robust, bulging neural spine summits, which are
supported by remarkably robust bases.
Discussion
Although a detailed character analysis is beyond the scope
of this work, available information suggests that Boni-
tasaura is closely related to the Late Cretaceous Malagasy
titanosaur Rapetosaurus and the Mongolian taxa Nemeg-
tosaurus and Quaesitosaurus. These four sauropods share
sculptured frontal borders, a dentary symphysis that is
almost perpendicular to the mandibular rami, and narrow,
pencil-chisel-like teeth that are cylindrical in cross-sec-
tion and mostly restricted to the anteriormost portion of
the lower jaw. This set of characters may suggest that
Bonitasaura is related to the Nemegtosauridae (Wilson
2002), which is defined as including all titanosaurs more
closely related to Nemegtosaurus than to Saltasaurus.
Bonitasaura differs from Antarctosaurus in having the
guillotine crest, a less straight angle of symphysis, and a
rather flat instead sinuous posterior surface of the parietal.
Rapetosaurus substantially differs in having a dentigerous
region more extended backwards and an even more gently
curved symphysis (Curry Rogers and Forster 2004). The
presence in Antarctosaurus of an extensive edentulous
region (although devoid of a tall crest) and the Rapeto-
saurus short, rugose postalveolar ridge and the bizarre
post-dentigerous corner of the maxilla, suggest that in-
cipient guillotine-like structures could have been devel-
oped in other titanosaurs.
The unresolved phylogenetic relationships of Antarc-
tosaurus, and the fact that the record of Late Cretaceous
sauropods is only composed of derived titanosaurs and
basal diplodocoids (i.e., Rebbachisauridae), led Upchurch
(1999) to propose two possibilities for the status and
evolutionary relationships of this species: a chimera of
bones from different lineages, or a diplodocoid that ac-
quired a postcranium largely convergent with that of
derived titanosaurs. The discovery of Bonitasaura has
shed light on the systematic affinities of Antarctosaurus,
showing that its bizarre lower jaw features are not unusual
in advanced titanosaurs, which can bear a squared snout
convergent to that of diplodocoids, as originally proposed
(see von Huene 1929; Calvo 1994; Salgado 2001). This is
also supported by the clear titanosaur affinities of the
remaining Antarctosaurus bones.
Wilson (2002) remarked on the morphological diver-
sity acquired by sauropod dinosaurs. However, it is clear
that their lineages were morphologically far less disparate
than other dinosaurs (e.g., theropods include Tyran-
nosaurus rex and hummingbirds; ornithischians include
large frilled ceratopsians as well as small and fast runners
like Othnielia). Sauropod morphological variation is lim-
ited to changes in relative neck and limb proportions, loss
of manual phalanges, muscular and pneumatic variation
(inferred), armor, weapons and tail mobility, wide or nar-
row hip and limb gauges (Wilson and Carrano 1999), and
some craniodental configurations. Several workers (e.g.,
Paul 1984) have already noted the possible possession of
both beaks and teeth in prosauropods, their sister-group; a
combination that is common in ornithischians, occasion-
ally present in theropods, but was never suspected for
derived sauropods.
The feeding mechanism proposed for derived ti-
tanosaurs (Calvo 1994) involves cutting and cropping of
selected soft vegetation with the anteriormost teeth. The
device exhibited by Bonitasaura, novel for sauropods,
with the non-dentigerous zone of the dentary (and perhaps
the maxilla) developed into a cutting surface, would
have permitted effective slicing of tougher vegetation and
minimized tooth wear. On the other hand, comb-like
dental replacements have already been reported for ad-
vanced diplodocoids and purported titanosaurs (Coria and
Chiappe 2001; Powell 1979; Holland 1906).
Most nemegtosaur ids had a long, square-sno uted,
shoe-shaped, hadrosaur-like head (Curry Rogers and
Forst er 2004; Nowinski 1971), which could be present
also in the Late Cretaceous reb bachisaurid diplodocoids.
The new titanosaur Bonitasaura constitutes the first
sauro pod dinosaur yet recorded that not only possesses
squared jaws, with narrow-crowned teeth arranged in
continuous series that include at least three replacement
elements per alveolus, but additionally also a keratinous
beak to aid in cutting plant material. A keratinous cutting
structure in addition to the aforeme ntioned nemeg-
tosaurid features has previously been reporte d only in
496

Late Cretaceous orni thischians, particularly hadrosaurs
(Morris 1970), and has been proposed as an adapt ive
response to the rise and diversi fication of flowering
plants (Bakker 1986; Salgado and Calvo 1997). How-
ever, the beak of ornithischians differs from that of
Bonit asaura in that it is at the tip of the mouth, and there
are teeth in the cheeks. Functional anatomical studies are
necessary in order to evaluate how these differences re-
flect different ways of living in the two taxa. Further-
more, the dental mechanism seems to be less complex by
far. The configuration shown by Bonitas aura is thus
unique in having a poste riorly placed beak.
Interestingly, the similarities in mouth configuration
and corporal mobility (i.e., loss of the hyposphene–hy-
pantrum complex) among rebbachisaurid sauropods and
derived titanosaurs suggest probable constraints on sauro-
pod morphology that conditioned them to take advantage
of a determinate resource. These constraints could be
related to genetical or morphological limitations, envi-
ronmental stasis, or the dominance of a particular vege-
tational food source.
Late Cretaceous sauropods in the southern he misphere
apparently entered new adaptative zones that were pre-
viously believed to have been oc cupied exclusively by
ornithischians (Powell 2003). The unusual features ex-
hibited by titanosaurs and rebbachisaurids suggest that
some Lat e Cretaceous sauropods acquired a disparate
morphological diversity. Although several ornithischian
lineages were prese nt at the same time, they were re-
stricted to rare and small-sized forms when compared
with other regions. This panorama changed when North
American ornithischians entered South America by
Late Campanian t imes. The morphological diversit y o f
Late Cretaceous southern hemisphere sauropods and thei r
adaptative capabilities may help to explain their persis-
tence into the latest Cretaceous.
Acknowledgements The work was supported by the Jurassic
Foundation and Paleogenesis. I gratefully acknowledge the assis-
tance of the Pincheira and vila families and thank them for their
kind help. Thanks to Mr. Parodi for guiding us to the fossil site and
to Marcos Zffliga and the Salinas family for logistics. P.A. Gallina,
F.L. Agnolin, G. Lio, L. Gaetano, M. Cardenas, M. de la Fuente, L.
Ballarino, F. Pose, are thanked for field support. Thanks to the
geologists Hugo Corbela and Hector Leanza for the stratigraphic
position of the specimen. I am grateful to J.A. Gonzlez for illus-
trations and M. Isasi for preparation. M. Lamanna, J.F. Bonaparte
and P.J. Makovicky substantially improved this work with useful
comments and critical reviews.
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