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The occurrence of Psittacosaurus xinjiangensis Sereno and Chow, 1988 in the Urho area, Junggar Basin, Xinjiang, People’s Republic of China

Donald B. Brinkman, David A. Eberth, M.J. Ryan, and Pei-ji Chen 0

Abstract: A partial specimen of Psittacosaurus xinjiangensis from the Urho locality in the Western Junggar basin, China, extends the known geographical range of P. xinjiangensis and documents the adult morphology in this species. Variation in the number of denticles on the dentary tooth crowns is documented, possibly representing a decrease in the number of denticles during growth, although a high denticle count remains a diagnostic feature of the species. Résumé : Un spécimen partiel de Psittacosaurus xinjiangensis provenant de la région d’Urbo dans le bassin Junggar ouest agrandit l’étendue géographique connue de P. xinjiangensis et renseigne sur la morphologie de l’adulte de cette espèce. La variation du nombre de denticules sur les couronnes des dents du dentaire est documentée, représentant possiblement une diminution du nombre de denticules durant la croissance, bien que le grand nombre de denticules demeure l’une des caractéristiques de diagnostic de l’espèce. [Traduit par la Rédaction]

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Introduction

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Psittacosaurus is a small basal member of the Ceratopsia. Seven valid species (P. mongoliensis Osborn 1924, P. sinensis, Young 1958, P. meileyingensis Sereno et al. 1988, P. xinjiangensis Sereno and Chao 1988, P. neimogoliensis Russell and Zhao 1996, P. ordosensis Russell and Zhao 1996, and P. mazongshanensis Xu 1997) are known from the Early Cretaceous of Central Asia and Japan (Manabe and Hasegawa 1991). An additional species, P. sattayaraki was described by Buffetaut and Suteethorn (1992) based on a single dentary from Thailand, but its validity in the absence of more material has been questioned (Russell and Zhao 1996). Psittacosaurus was first collected in Xinjiang during expeditions to the Western Junggar Basin by the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in 1964 (Dong 1973a, 1973b). The IVPP collected Early Cretaceous vertebrates in two areas, Urho and Delanshan (Fig. 1). In both areas, the assemblage is dominated by the pterosaur Djungaripterus, and the assemblage was referred to as the Djungaripterus fauna. The two localities differ in that dinosaurs are more abundant at the Delanshan locality, and the primitive ceratopsian Psittacosaurus, described by Sereno and Chao (1988) as Psittacosaurus xinjiangensis, was recovered at Delanshan but not at Urho. Although fragments of various specimens were collected documenting a size range, the

complete skeleton, and the type specimen of P. xinjiangensis was a juvenile. In this paper, a psittacosaur specimen collected in the Urho area during sedimentological studies of the Tugulu Group in this area is described. The specimen is of an adult individual. Although incomplete, it includes diagnostic elements that allow it to be identified as Psittacosaurus xinjiangensis. This specimen provides the first documentation of the adult morphology in this species and provides further evidence on the variation in this species. The presence of P. xinjiangensis in the Urho area reinforces the faunal similarity of the Urho and Delanshan localities. Institutional abbreviations AMNH, American Museum of Natural History, New York, N.Y.; IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Academia Sinica, Beijing, China; UGM, Urumqi Geological Museum, Urumqi, Xinjiong, China.

Systematic Paleontology Class Reptilia Order Ornithischia Suborder Certaopsia Marsh, 1890 Family Psittacosauridae Genus Psittacosaurus Osborn 1923 Psittacosaurus xinjiangensis Sereno and Chao 1988

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Received February 1, 2001. Accepted June 27, 2001. Published on the NRC Research Press Web site at http://cjes.nrc.ca on November 21, 2001.

Paper handled by Associate Editor H.-D. Sues. D.B. Brinkman,1 D.A. Eberth, and M.J. Ryan. Royal Tyrrell Museum of Palaeontology, Box 7500 Drumheller, AB T0J 0Y0, Canada. C. Pei-ji. Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 21008, China. 1

Corresponding author (e-mail: don.brinkman@gov.ab.ca).

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DOI: 10.1139/cjes-38-12-1781

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Fig. 1. Locality map of the Junggar Basin of western China showing the areas of exposure of the Lower Cretaceous sediments. Based on Dong (1992, Fig. 91). Spelling of place-names follows Dong (1992).

Referred specimen UGM XG94Kh201. A partial skeleton, including skull and jaw fragments, atlas, centrum, much of the presacral vertebral column and complete sacrum, partial right forelimb, and complete right hind limb.

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Locality and stratigraphy The specimen was collected 10 km east-southeast of Urho (Fig. 1) from the middle of the Lower Cretaceous Tugulu Group (Bureau of Geology and Mineral Resources of Xinjiang Uygur Autonomous Region 1985). Whereas the Tugulu Group is recognized regionally across the Junggar, Tarim and Turpan basins, it is subdivided in different ways at different locations (e.g., Hendrix et al. 1992). The 1 : 200 000 geological survey map of the Xinjiang Autonomous Region (Division 2 1978) recognizes four subgroups in the Tugulu of northwestern Xinjiang that are identified in ascending stratigraphic order as a–d. The subgroups consist of discrete facies associations that indicate changes in depositional style during the Early Cretaceous in the internally drained, seasonally hot-and-dry Junggar Basin. Paleoenvironments inferred from the facies associations include distal braidplain (Tugulu a), braided fluvial and ephemeral floodplain (Tugulu b and d), and alluvial–lacustrine (Tugulu c). The specimen described here was collected from Tugulu c subgroup. In the Urho area, the Tugulu c subgroup consists of decimetre-to-metre-scale interbedded sheets of red, massive-tolaminated mudrocks, grey shales, and very fine-grained sandstones with intraformational mudstone pebbles and desiccation-mudchip clasts. This facies association is interpreted as the deposits of splays, sheet floods and slack water in a low-gradient, seasonally arid setting that was dominated by ephemeral ponds, extensive floodplains, and marshes (Eberth et al. 1995). The specimen was collected from a sideritized, silty-to-very-fine-grained, massive, flat-based sandstone sheet, 20–30 cm thick. This deposit can be reasonably interpreted as resulting from high rates of surface runoff during either a splay or sheet-flood event.

Description and comparison Specimen UGM XG94Kh201 is a mature individual, similar in size to the type specimen of P. mongoliensis (AMNH 6254; Osborn 1924, fig. 1). Additionally, the neural arches, where visible, appear to have fused to the centrum and have been broken off at their fused bases. The skull is represented by a few isolated fragments, including a partial jugal preserving the base of the jugal horn; articulated fragments of the occipital, quadrate, and squamosal portions of the left maxilla; and the basioccipital– basisphenoid portion of the braincase. A portion of the left jugal is preserved (Fig. 2A). As seen in all psittacosaurs, the lateral surface of the jugal is divided into two planes. The jugal is thickened through the midline ridge, and a distinct horn would have projected from the ventral margin of the bone. As in the type specimen of P. xinjiangensis, and in contrast to other species in the genus, the anterior surface of the horn is flattened. The jugal horn of specimen UGM XG94Kh201 differs from that of the type specimen in lacking the fine, radiating grooves described by Sereno and Chao (1988). We suspect that the striations in the type specimens are a feature similar to the long-grain bone texture that Sampson et al. (1997) described as feature of juvenile ceratopsids that is lost during growth. Thus this difference is a result of differences in the stage of development of the two specimens. The posterolateral portion of the left squamosal (Fig. 2B) is preserved in articulation with a fragment of the occipital portion of the parietal and the proximal surface of the left quadrate. A portion of the posterior articulating surface for the postorbital is preserved. All these fragments are of typical Psittacosaurus shape. The basioccipital–basisphenoid fragment (Figs. 2C, 2D) preserves the ventral portion of the foramen magnum and the occipital condyle. Lateral to the condyle two foramina are present. Following Xu (1997), these are identified as foramen for cranial nerves X–IX, and XI. This braincase fragment is not diagnostic for P. xinjiangensis. Portions of both maxillae are preserved (Fig. 3A). The left maxilla appears to preserve the complete tooth row of eight teeth, but all their crowns are broken off. The right maxilla preserves parts of five teeth, tentatively identified as numbers one to five, with the best preserved teeth, numbers two and three, exhibiting lateral surface features below their broken apices. Also, a single, small, isolated maxillary tooth is present. As in the type specimen P. xinjiangensis (Sereno and Chao 1988), the crown of each maxillary tooth is broad and the lateral surface is divided into three prominent lobes. The anterior lobe is flat, with the two more posterior lobes being convex anteroposteriorly. The anterior-most lobe is the broadest, with the middle and posterior lobes becoming progressively smaller. The posterior lobe has at least two small apical tips preserved along the lateral margins of the cutting surface. These tips do not wrap around onto the medial surface to the same degree as in the type of P. xinjiangensis. The type specimen of P. xinjiangensis has as many as 14 denticles on the maxillary crowns whereas other psittacosaurs bear 8–11 (Sereno and Chao 1988). This isolated tooth of UGM XG94Kh201 has nine denticles preserved, and up to two more may have been present on the broken portion of the posterior lobe. However, its small size relative © 2001 NRC Canada

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Fig. 2. Psittacosaurus xinjiangensis. Cranial elements (specimen UGM XG94Kh201). (A) lateral view of left jugal, (B) dorsal view of left squamosal, (C) occipital and (D) ventral view of basicranium. bo, basioccipital; bs, basispenoid; cn, cranial nerve fenestra ; fm, foramen magnum; jam, jugal anterior margin; mitf, margin of infratemporal fenestra; oc, occipital condyle; po, sulcus for postorbital; q, quadrate. Scale bar = 1 cm.

Fig. 3. Psittacosaurus xinjiangensis. Cranial elements (UGM XG94Kh201). (A) medial view of right maxilla and (B) medial view of left dentary. cor, broken base of coronoid process. Scale bar = 1 cm.

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to the other preserved teeth suggests that it is from the extreme end row position, so the small denticle number may reflect variation along the tooth row. Whatever the case, this

specimen suggests that the denticle count on maxillary teeth of P. xinjiangensis is variable, either between individuals or across the tooth row, and brings the range of variation exhibited in P. xinjiangensis within the range seen in other psittacosaur species. The presence of the small posterior denticles progressing onto the medial surface remains diagnostic for this species. The lower jaw is represented by a significant portion of the dentary (Fig. 3B) and postdentary bones of the left ramus. The dentary is preserved in two parts with only a small portion, including alveoli four, missing. As in the type specimen, the dentary has a sharp ventral edge and lacks any development of a ventral flange. In inner view, the dentary is slightly concave. This surface is separated from the postdentary region by a distinct ridge. The postdentary has a rounded ventral edge preserving a portion of the Meckalian fossa. Eight tooth positions are present on the dentary, with six of these containing teeth. Two of the teeth, the third and seventh were recently erupted prior to death. Tooth three shows wear only at several of its extreme apical tips, while tooth seven shows minor wear across the crown and across its lateral surface. As in the only known dentary tooth in the type material (IVPP V7704; Figs. 5C, 5D; Sereno and Chao 1988), and the crowns of adult psittacosaur species, the crown is circular in lateral view and its medial surface is subdivided into three lobes. The median lobe is larger than either the anterior or posterior lobes, being inflated at the base of the crown and tapering to a blunt apex, which in unworn teeth, projects beyond the circular crown. In the type specimen, the denticle row forms a semicircle around the margin of the crown, extending well down both sides. In tooth number seven of specimen UGM XG94Kh201, the most completely preserved tooth in the series, the denticles are more nearly transversely arranged across the dorsal edge of the tooth. Tooth three is intermediate in its condition in that on the anterior end of the tooth, the denticles follow the anterior edge of the tooth and thus form part of a semicircle, while the posterior denticles are restricted to the dorsal edge of the crown. The lateral surface of tooth seven has a well-defined medial groove opposite the apical denticle. Poorly defined secondary ridges separated by shallow depressions are present on either side of the median groove. The type specimen of Psittacosaurus xinjiangensis has ten secondary denticles symmetrically distributed on either side of the median ridge, significantly more than is present in other species in the genus, except for P. mazongshanensis (dentary crowns bear a total of 9â&#x20AC;&#x201C;15 denticles in other species, Russell and Zhao 1996). The high number of denticles was suggested to be a derived feature linking P. xinjiangensis and P. mazongshanensis. However, specimen UGM XG94Kh201 shows that the number of denticles on the edge of the dentary teeth varies within P. xinjiangensis. Tooth seven, the best preserved tooth in the series in specimen UGM XG94Kh201, has nine denticles in front of the apical denticle and eight behind, giving a total of eighteen denticles along the edge of the tooth. Tooth three has thirteen denticles along the edge of the tooth with six on either side of the apical denticle. Thus, the number of denticles on the dentary teeth in P. xinjiangensis is variable along the tooth row, ranging from that known for other psittacosaur species, ÂŠ 2001 NRC Canada

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Fig. 4. Psittacosaurus xinjiangensis. Ventral view of vertebral series (UGM XGP4Kh201). c, caudal vertebra; ps, presacral vertebra; r, rib; s, sacral vertebra; sr, sacral rib. Scale bar = 2 cm.

Fig. 5. Psittacosaurus xinjiangensis. Partial right forelimb (UGM XGP4Kh201). h, humerus; ps, presacral vertebra; r, rib fragment; ra, radius; ul, ulna. Scale bar = 2 cm.

Fig. 6. Psittacosaurus xinjiangensis. Left ilium (UGM XG94Kh201). (A) lateral and (B) medial views. s, articular surface for sacral vertebra. Scale bar = 2 cm.

to an increased number diagnostic for P. xinjiangensis, with the highest counts present in the posterior region of the jaw. The vertebral column is preserved in two major sections (Figs. 4, 5). The vertebral column of Psittacosaurus typically

includes 21 presacrals, 6 sacrals, and -45 caudals, although P. neimongoliensis and P. mongoliensis vary from this in the presence of an extra presacral (Sereno 1990). Using the typical condition as a basis for comparison, the vertebrae preserved in specimen UGM XG94Kh201 are the atlas, presacrals 10â&#x20AC;&#x201C;21, the six fused sacrals of the sacrum, and the first caudal vertabra. Ossified epaxial tendons preserved on UGM XG94Kh201 extend from the most cranially preserved presacral vertebra, across the sacrum, and onto the first and only preserved caudal vertebra. In the position of the epaxial tendons, P. xinjiangensis is similar to P. mongoliensis, where tendons run from the 10th presacral to the 15th caudal, and different from P. sinensis and possibly P. meileyingensis, where no tendons span the sacral region. Both ilia are present, although incomplete. Between them, only the distal tip of the posterior process is not represented. The ilium of specimen UGM XG94Kh201 is comparable to that of the type specimen of P. xinjiangensis in that the postacetabular process is long relative to its height. In UGM XG94Kh201, the base of the process is preserved on the left

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Fig. 7. Psittacosaurus xinjiangensis. Right hind limb (UGM XG94Kh201) folded in squatting position. (A) ventral and (B) dorsal views of the leg. (C) proximal view of tarsals. II, second digit; III, third digit; IV, forth digit; as, astragalus; ca, calcaneum; dist, distal tarsal; fi, fibula; fe, femur; medt, medial tarsal; mt, metatarsal; ti, tibia; ?p, ?right pubis; r, rib fragments. Scale bar = 2 cm.

ilium, and the distal end is preserved on the right. From these two portions, the length of the postacetabular process is a minimum of 6 cm. The height midway along the process is 2 cm. In this feature, P. xinjiangensis differs from other species of Psittacosaurus, which have a relatively much deeper postacetabular process. The anterior process, incompletely preserved in the type specimen, is a long narrow blade. The preserved portion of the illustrated left ilium (Fig. 6) has a smoothly curved lateral surface without a ventral flange, such as is present in P. mongoliensis. The appendicular region is represented by a portion of the forelimb (Fig. 5) and a complete hind limb (Fig. 7). The forelimb includes the distal humerus and the proximal radius and ulna. As is typical for psittacosaurs, and to a large extend for complete specimens of Protoceratops andrewsi, the knee and ankle appear to be hyperflexed so that the femur lies almost parallel to the tibia and fibula, and the foot is plantigrade.

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Psittacosaur species are distinguished primarily on the shape of the jugal horn, ornamentation of the dentition, and shape of the ilium. Specimen UGM XG94Kh201 is referred to P. xinjiangensis by the following features; the anteriorly flattened jugal horn, increased number of dentary denticles over other species, and the possession of an elongate iliac postacetabular process. It differs from the type specimen in that the dentary teeth have fewer denticles (maximum 18 rather than 21) and that the number of denticles on the teeth appears to be variable along the tooth row. This difference is

assumed to represent variation within the species. Since the smaller individual has a higher denticle count, it may be that this represents a decrease in the number of denticles during ontogeny. However, in P. mongoliensis, the denticle count increases from juveniles to adults (Sereno 1990). Additional specimens are needed to resolve whether the lower number of denticles in UGM XG94Kh201 is truly a result of ontogenetic change or simply reflects variation within adults of the species. The two specimens also differ in that striations on the jugal horn are absent. This may be a reflection of the relative stage of development of individual specimens. As described by Sampson et al. (1997), in ceratopsids, the presence of striations on the cranial bones is a juvenile feature that is lost during growth. The presence of striations in the type specimen, a juvenile individual, and their absence in UGM XG94Kh201, an adult, suggests that psittacosaurs may show a similar pattern of ontogenetic development. Specimen UGM XG94Kh201 provides the first documentation of Psittacosaurus xinjiangensis at the Urho locality. Previously the only ornithischian identified from the Urho locality was Wuerhosaurus. The occurrence of Psittacosaurus in this area is not surprising given the general similarity in the assemblages of the Urho and Delanshan localities (Dong 1973a, 1973b). The greater abundance of dinosaur remains in the Delanshan locality has been interpreted as a result of ecological factors related to the environment of deposition of these areas. Dong (1992) interpreted the sediments in the Urho area as lacustrine, and suggested that the fossil assemblages of the two areas differed because the Delanshan area preserved a shoreline assemblage. However, examination of the sediments at the Urho area has revealed that they were ÂŠ 2001 NRC Canada

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deposited in a semi-arid, fluvial–lacustrine setting characterized by at least, seasonal subaerial exposure, similar to the modern environments in this area and other basins across northern China (cf. Eberth 1993). Thus, the difference in faunal composition between Delanshan and Urho is more likely a reflection of differences in other aspects of the terrestrial paleoenvironments, climate, and (or) preservational processes. One striking feature of UGM XG94Kh201, as preserved, is the posture of the hind limb. This posture is frequently encountered in both Psittacosaurus and basal ceratopsids such as Protoceratops. Given the apparent hyperextension of the knee and ankle joints that this entails, it has been assumed that this is an artificial position. However, the frequency with which this position is encountered raises the possibility that this is a normal resting posture, possibly exaggerated by the compression of the elements resulting from the loss of the soft tissue. Jerzykiewicz et al. (l993) concluded that many of the articulated Protoceratops specimens that are oriented subhorizontally and belly down are the remains of animals that died in situ and were probably buried by sandstorm events. Fastovsky et al. (1997) determined that the articulated Protoceratops specimens that they encountered in the south-central Gobi Desert of Mongolia, in which the limbs were drawn up to their bodies, were the result of animals dying on the lee side of prograding eolian dunes, presumably overwhelmed by sandstorms. The crouched position of UGM XG94Kh201, common amongst other psittacosaurs and articulated Asian neoceratopsians, can be interpreted as a natural position that would have reduced their body profiles during inclement weather and might have been a natural position for other activities such as feeding or nest incubation.

Acknowledgments The specimen described here was discovered during a joint Royal Tyrrell Museum of Paleontology – Paleontological Institute of Nanjing expedition to the Junggar Basin, aimed at studying the stratigraphy and environments of the basin, and was supported by a grant from the National Geographical Society. Irene Hughes drew Figs. 2 through 7. Chad Kerychuk of Digital Dream Machine (Edmonton, Alberta) assisted with figure formating. We would also like to thank Peter Makovicky and Dale Russell for reviewing the manuscript.

References

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