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A New Crocodyliform from Zos Canyon, Mongolia

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A New Crocodyliform from Zos Canyon, Mongolia PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024 Number 3445, 36 pp., 16 ®gures June 2, 2004 A New Crocodyliform from Zos Canyon, Mongolia DIEGO POL1 AND MARK A. NORELL2 ABSTRACT Here we report on a new fossil crocodyliform from Cretaceous Redbeds in the Zos Canyon, Gobi Desert, Mongolia. This new taxon, Zosuchus davidsoni, is described based on the infor- mation provided by ®ve specimens collected during expeditions of the Mongolian Academy of Sciences±American Museum of Natural History. Zosuchus davidsoni is identi®able 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, and a lacrimal±premaxillary contact on the dorsal surface of the snout. The phylogenetic relationships of Zosuchus davidsoni are shown through a parsimony analysis in the context of Crocodyliformes. This new form is found to be a late-appearing basal crocodyliform, 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 During the last decade, expeditions from specimens are rare. Other expeditions have the Mongolian Academy of Sciences and the noticed this as well, and only a few crocod- American Museum of Natural History have yliform specimens have been collected in developed large collections of fossil verte- these beds (OsmoÂlska, 1972; Storrs and E®- brates from Djadokhta and Djadokhta-like mov, 2000; Mook, 1924; E®mov, 1983, rocks in the Gobi Desert (Dashzeveg et al., 1988; OsmoÂlska 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- 1 Division of Paleontology, American Museum of Natural History ([email protected]). 2 Division of Paleontology, American Museum of Natural History ([email protected]). Copyright q American Museum of Natural History 2004 ISSN 0003-0082 2 AMERICAN MUSEUM NOVITATES NO. 3445 mains of three taxa have been recovered, SAM-K South African Museum, Cape Town, which are known from seven specimens. One South Africa of these taxa, Zosuchus davidsoni, n.gen. and SMNS Staatliches Museum fuÈr Naturkunde n.sp., is surprisingly primitive for a Late Cre- Stuttgart, Stuttgart, Germany taceous crocodyliform. UA University of Antananarivo, Mada- In the following report we describe the gascar UCMP Museum of Paleontology, Universi- anatomy of this new form from the Zos Can- ty of California, Berkeley yon locality and analyze the phylogenetic re- ZPAL Instytut Paleobiologii PAN, 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 (®gs. 1, 2) was are used throughout the text: ®rst visited by Mongolian Academy of Sci- AMNH American Museum of Natural His- ences±American Museum of Natural History tory, New York ®eld parties in 1992. It had previously been BSP Bayerische Staatssammlung fuÈr Pa- visited by Mongolian and Russian paleontol- laÈontologie und Geologie, MuÈnich, ogists (Dashzeveg, personal commun.); 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 Departamento de ProducËaÄo Mineral, Rio de Janeiro, Brazil referred to informally as Kholbot (Dashzev- GPIT Institut und Museum fuÈr Geologie eg, personal commun.). und PalaÈontologie, Universitat TuÈ- The locality lies on the southern ¯ank of bingen, TuÈbingen, Germany the Gilvent Ul, about 6 km northwest of the GWU George Washington University, main locality at Ukhaa Tolgod (®g. 1). The Washington, DC canyon is at the mouth of a large north±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, People's Republic of China are stratigraphically lower, as the beds dip to LACM Los Angeles County Museum, Los the south underneath the Ukhaa Tolgod beds. Angeles No geochronologic work has commenced at MACN Museo Argentino de Ciencias Na- Zos Canyon; however, Protoceratops sp. re- turales, Buenos Aires, Argentina mains are common throughout the exposures. MAL Malawi Department of Antiquities, Interestingly, the mammalian fauna seems to Malawi be distinct as taxa unknown at other Mon- MB Institut fuÈr Palaontologie, Museum golian Djadokhta and Djadokhta-like beds fuÈr Naturkunde, Humbolt-Universi- have been recovered (e.g., ``zhelestids'' tat, Berlin, Germany MCZ Museum of Comparative Zoology, [Novacek et al., 2000]). Harvard University, Cambridge, At Zos Canyon three distinct levels are MA preserved (®g. 2). The uppermost of these MLP Museo de La Plata, La Plata, Argen- (the Red Rum locality) is where the zhelestid 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 Museo de GeologõÂa y PaleontologõÂa, bright red and very similar to classic Dja- Universidad Nacional del Comahue, dokhta rocks. Below this is a series of white NeuqueÂn, Argentina PVL Instituto Miguel Lillo, TucumaÂn, beds (Zos Canyon White beds); these rocks Argentina are much more similar to sediments of the RCL Museo de Ciencias Naturales, Pon- Nemegt Formation than they are to other ti®cia Universidade Catolica de Mi- Djadokhta sediments. The beds are white, nas Gerais, Brazil poorly sorted, and represent a series of 2004 POL AND NORELL: CROCODYLIFORM FROM MONGOLIA 3 Fig. 1. Satellite photo of the Zos and Ukhaa Tolgod localities. stream channels preserved as indurated sands REFERRED SPECIMENS: IGM 100/1304, 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 petri®ed 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 ilar to, but more ¯uvial than, classic Djadokhta 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- toceratopsians) and lizards. It is in these beds terodorsal lacrimal process wedging between that all the specimens of Zosuchus davidsoni the nasal and maxilla, contacting anteriorly were collected. the posterodorsal process of premaxilla. Pal- atines forming a posteriorly extended sec- SYSTEMATIC PALEONTOLOGY ondary palate, almost reaching the posterior CROCODYLOMORPHA WALKER, 1970 edge of the pterygoid ¯anges where the sec- CROCODYLIFORMES HAY, 1930 (SENSU CLARK, 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. maxilla with three teeth and maxilla with ®ve 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; KaÈlin, 1933). The skull of IGM 100/1305 has most of Moreover, this variation is congruent with its elements preserved except for the palatine other differences in IGM 100/1304 that are shelves, supraoccipital, and left side of the also subject to ontogenetic change (see be- rostrum (®gs. 3±5). However, most of these low). The infratemporal fenestra is not com- elements are nicely preserved in IGM 100/ pletely preserved in any of the specimens 1304, 100/1306, 100/1307, and 100/1308. studied here, although it probably was small The snout is extremely short, high, and due to the broadness of the dorsal process of moderately broad (i.e., oreinirostral, sensu the quadratojugal. The external surface of the Busbey, 1994). The external nares are con- skull is ornamentated with a slight pitted pat- ¯uent and face anteriorly. The antorbital fe- tern as in most members of Crocodyliformes. nestrae are markedly reduced and located This sculpture is missing in the smaller spec- 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 sally facing orbits occupying 33% of the total ventral edge occupies the anterior third of the skull length (IGM 100/1305). The supratem- anteroposterior
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