A New Crocodyliform from Zos Canyon, Mongolia

2004·1.4 MB·English

by  Diego Pol

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number 3445, 36 pp., 16 figures June 2, 2004
A New Crocodyliform from
Zos Canyon, Mongolia
DIEGO POL1 AND MARK A. NORELL2
ABSTRACT
HerewereportonanewfossilcrocodyliformfromCretaceousRedbedsintheZosCanyon,
Gobi Desert, Mongolia. This new taxon, Zosuchus davidsoni, is described based on the infor-
mation provided by five specimens collected during expeditions of the Mongolian Academy
of Sciences–American Museum of Natural History. Zosuchus davidsoni is identifiable by nu-
merous characters, including a posteriorly extensive secondary palate that opens through a
secondary choana bordered by the palatines and pterygoids near the posterior edge of the
skull,andalacrimal–premaxillarycontactonthedorsalsurfaceofthesnout.Thephylogenetic
relationships of Zosuchus davidsoni are shown through a parsimony analysisinthecontextof
Crocodyliformes. This newformisfound to be a late-appearingbasalcrocodyliform,forming
a monophyletic group with two other taxa from the Early Cretaceous of China. Because of
the basal position of Zosuchus within Crocodyliformes, the marked posterior extension of the
secondary palate is most parsimoniously interpreted as a convergence with the derived con-
dition of neosuchian crocodyliforms.
INTRODUCTION lizards) are very common, crocodyliform
specimens are rare. Other expeditions have
During the last decade, expeditions from
noticed this as well, and only a few crocod-
the Mongolian Academy of Sciences andthe
yliform specimens have been collected in
American Museum of Natural History have
developed large collections of fossil verte- these beds (Osmo´lska, 1972; Storrs and Efi-
brates from Djadokhta and Djadokhta-like mov, 2000; Mook, 1924; Efimov, 1983,
rocks in the Gobi Desert (Dashzeveg et al., 1988; Osmo´lska et al., 1997). However, at
1995; Novacek, 1996, 2002). Although re- one locality, Zos Canyon, multiple corocod-
mains of reptiles (especially dinosaurs and ilian specimens have been collected. Re-
1DivisionofPaleontology,AmericanMuseumofNaturalHistory([email protected]).
2DivisionofPaleontology,AmericanMuseumofNaturalHistory([email protected]).
Copyright(cid:113)AmericanMuseumofNaturalHistory2004 ISSN0003-0082
2 AMERICAN MUSEUM NOVITATES NO. 3445
mains of three taxa have been recovered, SAM-K SouthAfricanMuseum,CapeTown,
whichareknownfromsevenspecimens.One South Africa
ofthesetaxa,Zosuchusdavidsoni,n.gen.and SMNS Staatliches Museum fu¨r Naturkunde
n.sp.,issurprisinglyprimitiveforaLateCre- Stuttgart, Stuttgart, Germany
UA University of Antananarivo, Mada-
taceous crocodyliform.
gascar
In the following report we describe the
UCMP Museum of Paleontology, Universi-
anatomy of this new form fromtheZosCan-
ty of California, Berkeley
yon locality and analyzethe phylogeneticre-
ZPAL InstytutPaleobiologiiPAN,Warsza-
lationships within the context of Crocodyli-
wa, Poland
formes. This new taxa is depicted as a basal
crocodyliform, forming a clade with two
forms from the Early Cretaceous of China. HORIZON AND LOCALITY
The following institutional abbreviations The Zos Canyon locality (figs. 1, 2) was
are used throughout the text: first visited by Mongolian Academy of Sci-
AMNH American Museum of Natural His- ences–American Museum of NaturalHistory
tory, New York field parties in 1992. It had previously been
BSP Bayerische Staatssammlung fu¨r Pa- visited by Mongolian and Russianpaleontol-
la¨ontologie und Geologie, Mu¨nich, ogists (Dashzeveg,personalcommun.);how-
Germany ever, according to Dashzeveg, little was
CNM Chongqing Natural Museum, Sich- found and nothing about this locality was
uan, People’s Republic of China
ever published. The locality has also been
DGM DepartamentodeProduc¸a˜oMineral,
referred to informally as Kholbot (Dashzev-
Rio de Janeiro, Brazil
eg, personal commun.).
GPIT Institut und Museum fu¨r Geologie
und Pala¨ontologie, Universitat Tu¨- The locality lies on the southern flank of
bingen, Tu¨bingen, Germany the Gilvent Ul, about 6 km northwest of the
GWU George Washington University, main locality at Ukhaa Tolgod (fig. 1). The
Washington, DC canyonisatthemouthofalargenorth–south
IGM Mongolian Institute of Geology, pass that transects the Gilbent Ul. Although
Ulaan Bataar, Mongolia topographically higher than the exposures at
IVPP Institute of Vertebrate Paleontology
Ukhaa Tolgod, the rocks at the Zos locality
and Paleoanthropology, Beijing,
are stratigraphically lower, as the beds dip to
People’s Republic of China
thesouth underneaththeUkhaaTolgodbeds.
LACM Los Angeles County Museum, Los
Angeles No geochronologic work has commenced at
MACN Museo Argentino de Ciencias Na- Zos Canyon; however, Protoceratops sp. re-
turales, Buenos Aires, Argentina mainsarecommonthroughouttheexposures.
MAL Malawi Department of Antiquities, Interestingly, the mammalian fauna seems to
Malawi be distinct as taxa unknown at other Mon-
MB Institut fu¨r Palaontologie, Museum golian Djadokhta and Djadokhta-like beds
fu¨r Naturkunde, Humbolt-Universi-
have been recovered (e.g., ‘‘zhelestids’’
tat, Berlin, Germany
[Novacek et al., 2000]).
MCZ Museum of Comparative Zoology,
At Zos Canyon three distinct levels are
Harvard University, Cambridge,
preserved (fig. 2). The uppermost of these
MA
MLP MuseodeLaPlata,LaPlata,Argen- (theRedRumlocality)iswherethezhelestid
tina material is from. Also occurring at this lo-
MOZ Museo Profesor J. Olsacher,Zapala, cality are lizards, protoceratopsians, and di-
Argentina nosaur eggs. Sediments at Red Rum are
MUC-PV MuseodeGeolog´ıayPaleontolog´ıa, bright red and very similar to classic Dja-
UniversidadNacionaldelComahue,
dokhta rocks. Below this is a series of white
Neuque´n, Argentina
beds (Zos Canyon White beds); these rocks
PVL Instituto Miguel Lillo, Tucuma´n,
are much more similar to sediments of the
Argentina
RCL Museo de Ciencias Naturales, Pon- Nemegt Formation than they are to other
tificia Universidade Catolica de Mi- Djadokhta sediments. The beds are white,
nas Gerais, Brazil poorly sorted, and represent a series of
2004 POL AND NORELL: CROCODYLIFORMFROM MONGOLIA 3
Fig. 1. Satellite photo of the Zos and Ukhaa Tolgod localities.
streamchannelspreservedasinduratedsands REFERREDSPECIMENS:IGM100/1304,100/
and gravels. Fossils in these beds are not 1306, 100/1307, and 100/1308.
common, and when found include large di- ETYMOLOGY: Zos, from the type locality;
nosaurs (ornithischans such as hadrosaurs, suchus, Greek for crocodile; anddavidsoni,
sauropods, large theropods), turtle pieces, for Amy Davidson who prepared several of
and occasional pieces of petrified wood. Be- these specimens and has been so instrumen-
low these white beds a layer of red beds sim-
tal in the exquisite preparation of Mongolian
ilarto,butmorefluvialthan,classicDjadokhta
specimens allowing the success of our work.
beds lies atop the Paleozoic basement. The
DIAGNOSIS: Small and extremely short-
rocks have produced several poorly preserved
snouted crocodyliform with an extensive an-
dinosaur skeletons (dromaeosaurids and pro-
terodorsal lacrimal process wedgingbetween
toceratopsians) and lizards. It is in these beds
the nasal and maxilla, contacting anteriorly
that all the specimens of Zosuchus davidsoni
the posterodorsal process of premaxilla. Pal-
were collected.
atines forming a posteriorly extended sec-
SYSTEMATIC PALEONTOLOGY ondary palate, almost reaching the posterior
CROCODYLOMORPHAWALKER,1970 edge of the pterygoid flanges where the sec-
CROCODYLIFORMESHAY,1930 (SENSUCLARK, ondary choana is located. Posttemporal re-
1986) gion of the parietal extremely narrow,wedg-
Zosuchus davidsoni, n.gen. and n.sp. ing between the squamosals (paralleled in
HOLOTYPE: IGM 100/1305, isloated skull Gobiosuchus). Reduced upper dentition,pre-
and lower jaws. maxillawiththreeteethandmaxillawithfive
4 AMERICAN MUSEUM NOVITATES NO. 3445
Fig. 2. Exposures of the Zos Canyon and overlying beds. Photo: G.W. Rougier.
teeth (paralleled in some mesoeucrocodyli- here as a product of ontogenetic change.
ans). Similar ontogenetic changes in supratempor-
al fenestra shape have been noted in extant
DESCRIPTION crocodyliforms (Mook, 1921; Ka¨lin, 1933).
Moreover, this variation is congruent with
The skull of IGM 100/1305 has most of
other differences in IGM 100/1304 that are
itselementspreservedexceptforthepalatine
also subject to ontogenetic change (see be-
shelves, supraoccipital, and left side of the
low). The infratemporal fenestra is not com-
rostrum (figs. 3–5). However, most of these
pletely preserved in any of the specimens
elements are nicely preserved in IGM 100/
studied here, although it probably was small
1304, 100/1306, 100/1307, and 100/1308.
due to the broadness of the dorsal process of
The snout is extremely short, high, and
moderately broad (i.e., oreinirostral, sensu thequadratojugal.Theexternalsurfaceofthe
Busbey, 1994). The external nares are con- skullisornamentatedwithaslightpittedpat-
fluent and face anteriorly. The antorbital fe- ternasinmostmembersofCrocodyliformes.
nestrae are markedly reduced and located This sculpture ismissinginthesmallerspec-
just anterior to the orbital margin. Zosuchus imen (IGM 100/1304).
davidsoni has extremely large and laterodor- The premaxilla is a short element, yet its
sallyfacingorbitsoccupying33%ofthetotal ventraledgeoccupiestheanteriorthirdofthe
skull length (IGM 100/1305). The supratem- anteroposterior extension of the snout. It
poral fenestrae are extremely reduced and forms the ventral, lateral, and dorsolateral
oriented oblique to the longitudinal axis of margin of the external nares. It possesses an
theskullwiththeiranteriorendspointingan- extremely well-developed posterodorsal pro-
terolaterally. One of the specimens (IGM cessthatwedgesbetweenthemaxillaandna-
100/1304) shows minor differences in the sal bones on the dorsal surface of the snout
shape and size of the supratemporal fenestra (fig. 3). The suture between these two bones
(fig.6).IGM100/1304(fig.7)isthesmallest does not seem to overlap, although it is not
specimen, and this variation is interpreted interdigitated. A large vertically oriented
2004 POL AND NORELL: CROCODYLIFORMFROM MONGOLIA 5
Fig. 3. Skull of the holotype of Zosuchus davidsoni IGM 100/1305 in dorsal view.
6 AMERICAN MUSEUM NOVITATES NO. 3445
Fig. 4. Skull of the holotype of Zosuchus davidsoni IGM 100/1305 in ventral view.
2004 POL AND NORELL: CROCODYLIFORMFROM MONGOLIA 7
Fig. 5. Skull of the holotype of Zosuchus davidsoni IGM 100/1305 in lateral view.
notch which fits the canineform dentary served. The suture with the palatal branchof
toothliesatthesuturebetweenthepremaxila the maxilla extends anteromedially (fig. 4)
and the maxilla (fig. 5). A small neurovas- rather than medially as in most Crocodyli-
cular foramina islocatedonthepremaxillary formes.
wall that forms the anterior edge of the pre- The premaxillary dentition is well pre-
maxillary-maxillary notch. The palatal served in the right premaxilla of IGM 100/
branches of the premaxillae extendmedially, 1305 and consists of only three slender con-
presumably contactingeachotheratthemid- ical teeth. None of these elements has ser-
line, although this contact has not been pre- rations on anterior or posterior edges. The
Fig. 6. Skull of specimen IGM 100/1304 in dorsal (A) and ventral (B) views.
8 AMERICAN MUSEUM NOVITATES NO. 3445
Fig. 7. Skull of specimen IGM 100/1304 in left (A) and right (B) lateral views.
first premaxillary tooth is small and located between the nasal and the maxilla reaching
below the lateral edge of the external nares. the posteromedial edge of the posterodorsal
The second tooth is more than twice the size process of the premaxilla (fig. 8). Thus, the
of the more anterior teeth, causing the pre- maxilla does not contact the nasals, a unique
maxilla to bulge at this point. The third pre- condition among Crocodylomorpha.
maxillary tooth is extremely reduced and is The maxilla borders the anterior edge of a
located just before the anterior edge of the reduced antorbital fenestra located just ante-
premaxillary-maxillary notch. rior to the anterior orbital edge (fig. 5). The
The maxilla has a limited exposure on the maxilla and lacrimal are not depressed
lateral surface of the snout due to the exten- around the antorbital fenestra in contrast to
sion of the premaxillary-maxillary notch. most non-neosuchian crocodyliforms that
Dorsal to this notch the maxilla borders the have a marked antorbital fossa surrounding
premaxillary posterodorsal process. At the the antorbital fenestra (e.g., Protosuchus ri-
posterior edge of the posterodorsal premax- chardsoni MCZ 6727, Gobiosuchuskielanae
illary process, the maxilla contacts the anter- ZPAL MgR-II/70, Notosuchus terrestris
odorsalprocessofthelacrimalwhichwedges MACN-RN 1037). Posteriorly, the maxilla
2004 POL AND NORELL: CROCODYLIFORMFROM MONGOLIA 9
Fig. 8. Skull of specimen IGM 100/1307 in dorsal view.
borders the anteroventral margin of the en- maxillary teeth are preserved on the leftside
larged orbit and contacts the jugal before the of IGM 100/1304, although these elementes
midpoint of the orbit. are poorly preseved (fig. 6). The four poste-
The ventral edge of the maxilla is straight riormost maxillary teetharenicelypreserved
and increases its ventral extension posterior in IGM 100/1305 and have no serrations
to the premaxillary-maxillary notch. Five along their anterior or posterior edges.Allof
10 AMERICAN MUSEUM NOVITATES NO. 3445
themareconicalandsubequalinsize,except slightly along their contact with the lacrimal
for the third tooth, which is approximately and prefrontal (fig. 8). The posterior edge of
twice the size of the other maxillary teeth the nasals contacts with the frontal in an in-
(fig. 5). In contrast to neosuchian crocodyli- terdigitated transverse suture.
forms, the tooth size variation of Zosuchus The lacrimal is poorly preserved in most
davidsoni is not matched by a sinusoidal specimens. This element forms the posterior
ventral edge of the maxilla. and dorsal margins of the antorbital fenestra
The palatal branch of the left maxilla of and the dorsal half of the anterior orbital
IGM 100/1305 extends posteromedially to margin. Posterior to the antorbital fenestra,
the premaxilla-maxilla palatal contact (fig. the lacrimal is a thin columnar bar, slightly
4). Specimen IGM 100/1306 shows the pos- exposed on the lateral surface of the snout
terior region of both palatal shelves exposed (fig. 5). Therefore, the posterior edge of the
in dorsal view. These contact each other, antorbital fenestra is very close to the ante-
forming a brief and anteriorly located sec- rior orbital margin. Dorsal to the antorbital
ondary palate as well as the anterior and lat- fenestra, the lacrimal extends anteromedially
eral edges of the anterior (primary) choanal on the dorsal surface of the snout. As de-
opening (fig. 11). scribed above, this well-developed process
This condition clearly resembles the mor- wedges between the nasal and maxilla,
phology present in basal crocodyliforms reaching the posterodorsal end of the pre-
(e.g., Gobiosuchus ZPAL MgR-II/67, Fruita maxilla anteriorly. A similar, anteriorly ex-
form LACM 120455a) that lack the posteri- tended process is also present in other short-
orly extended maxillary secondary palate snouted crocodyliforms such as Simosuchus
present in mesoeucrocodylians, in which the clarki (UA 8679) or Navajosuchus mooki ((cid:53)
maxillarypalatalbranchescontacteachother ’’Allognathosuchus’’ mooki (Brochu, 1999);
medially along their entire length. Lateral to AMNH 6780), although in these forms the
this opening, the palatal branch of the max- anterior process of the lacrimal does not
illa forms the anterior and medial edges of a reach the posterodorsal process of the pre-
large suborbital fenestra. maxilla.Furthermore,thederivedsimilarities
The nasals form most of the the dorsal between these taxa and Zosuchus are limited
margin of the confluent external nares. Their to this character and the extremely short-
dorsal surfaces have different degrees of or- snouted condition.
namentation in the three specimens in which The prefrontals are elongate, longitudinal-
this region was preserved (IGM 100/1304, ly oriented bones that form the anteromedial
100/1305,and100/1307).ThenasalsofIGM edge of the orbits. The medial edge of the
100/1304 are smooth, whereas those of the prefrontal contacts the frontal and nasals.
IGM 100/1307 are the most heavily orna- Anteriorly, the prefrontal is sutured to the
mented(showingapatternofsmallandwell- lacrimal on the dorsal surface of the snout.
spaced pits). This difference in ornamenta- They are overlapped by a large anterior pal-
tion is interpreted as ontogeneticdifferences, pebral preserved in IGM 100/1304 (fig. 6).
which is also expressed in differences in the The descending process of the prefrontal is
ornamentation of other bones, the degree of not preserved in any of the specimens, and
interdigitation of sutures, and skull size thereforeitcannotbedeterminediftheycon-
among the specimens studied here. The an- tact the palate as in mesoeucrocodylians
teriorregionofthelateraledgesofthenasals (Clark, 1994).
is straight (IGM 100/2 and 100/1307) or Two large palpebrals are preserved above
slightly concave (IGM 100/1304). In this re- the orbit of specimen IGM 100/1304. The
gion the lateral edges of the nasals diverge anterior palpebral is triangular and extends
slightly posteriorly along their contact with posterolaterally from the anteromedial mar-
the posterodorsal process of the premaxilla. gin of the orbit. Its anterior end overlaps the
Posterior to this area, the nasal contacts the prefrontal and probably the lacrimal at its
anterodorsal process of the lacrimal that contact with the skull. In contrast with most
wedges between the maxilla and the nasals. crocodyliforms, the posterior palpebralisex-
The lateral edges of the nasals converge tremely large, being subequal to the anterior