Psittacosaurus major, Sereno & Xijin & Brown & Lin, 2007

Sereno, Paul C., Xijin, Zhao, Brown, Lorin & Lin, Tan, 2007, New psittacosaurid highlights skull enlargement in horned dinosaurs, Acta Palaeontologica Polonica 52 (2), pp. 275-284 : 275-277

Psittacosaurus major sp. nov.

Derivation of the name: From the Latin, major , meaning greater.

Holotype: LH PV1, articulated skull and nearly complete postcranial skeleton ( Figs. 1–3 ). The skull is nearly complete lacking only the right, and most of the left, palpebral, and portions of the right postorbital and squamosal. The postcranial skeleton lacks the left pubis, right radius and portions of the right manus, left tibia and fibula,and portions of the left pes.

Type locality: Near Beipiao City, Liaoning Province, China.

Type horizon: Lower portion of the Yixian Formation, Lower Cretaceous (Berriasian to Valanginian in age; Wang et al. 1998).

Diagnosis.—Psittacosaurid with a large skull relative to its postcranial skeleton (30% larger than Psittacosaurus mongoliensis and most other species); transversely narrow dorsal skull roof as a result of the narrowest proportions of the nasals and frontals among psittacosaur species; the most prominent dentary flanges of any psittacosaur species with a depth approximately one−third that of the mandibular ramus; ventrolaterally projecting jugal horn; absence (closure) of the external mandibular fenestra as in Psittacosaurus sinensis and P. neimongoliensis ; seven sacral vertebrae (one dorsosacral added with rib attachment to the distal end of the preacetabular process), one more than in other psittacosaur species.

Description.—Almost all of the diagnostic features of Psittacosaurus major sp. nov. reside in the skull ( Figs. 1 , 2 ). In lateral view, the oval profile of the skull of P. major is the result of the relatively short snout and pronounced dentary flanges ( Fig. 1A ). Preorbital length is approximately 33% of skull length, similar to that in P. sinensis and intermediate between that in P. meileyingensis (27%) and P. mongoliensis (37%; Sereno 1987). In profile P. major most closely resembles P. meileyingensis ( Sereno and Zhao 1988) as both species have pronounced, projecting mandibular flanges and an angular with very deep proportions (deeper than the surangular). In the posteroventral corner of the skull, the jugal and quadrate and the quadratojugal and squamosal approach each other more closely than in other species but do not establish sutural contact. The mandibular fenestra, which is retained as a small opening between the dentary, surangular and angular in all other psittacosaurid species, is closed in P. major as preserved on both sides of the skull.

In dorsal view, the skull is very similar to that of P. mongoliensis except for the narrow proportions of the nasals and frontals ( Fig. 1B ). Only a narrow median exposure of the nasals separates the prefrontals, and the sides of the snout are exposed to each side of the nasals in dorsal view. The frontals do not expand broadly posterior to the orbit. As a result, more of the laterotemporal region is exposed in dorsal view than in P. mongoliensis . The junction of the postorbital and frontal is swollen. Along the postorbital bar, the edge of the postorbital is everted but not swollen into a boss as in P. sienesis . The ventrolaterally projecting jugal horns are more prominently developed than in P. mongoliensis and P. meileyingensis and do not project as strongly laterally as in P. sinensis and P. xingjiangensis (Sereno et al. 1988) . In lateral view, the tip of the jugal horn is situated ventral, rather than anterior, to the laterotemporal fenestra.

The maxillary and dentary crowns, as exposed in lateral view, are very similar to that in Psittacosaurus mongoliensis ( Sereno and Zhao 1988) . The lateral aspect of each maxillary crown is divided into three parts—a central low primary ridge and two lobes to either side. Low secondary ridges are present on the lobes, the anterior narrower than the posterior lobe. the lateral aspect of the dentary crowns is smooth. A bulbous primary ridge on the medial side is exposed along the apical wear facet in cross−section.

The vertebral column is nearly identical in form and size to that in P. mongoliensis ( Table 1). The axis is not noticeably longer or more robust despite the relative increase in the size of the skull. The sacrum has incorporated an additional dorsal vertebra for a total of seven ( Fig. 3 ). There are only six sacral vertebrae in other psittacosaurid species in which the sacrum is known. In these species, the rib of the first sacral contacts the base of the preacetabular process. In P. major , in contrast, the rib of the first sacral sacral articulates with a raised articular facet close to the distal end of the preacetabular process, and the second sacral contacts the base of the process ( Fig. 3 ). An additional dorsosacral, thus, appears to have been added in P. major . The centrum of this dorsosacral vertebra is fully coossified with that of the successive sacral.

The postcranial skeleton is remarkably conservative in species within the genus Psittacosaurus , and P. major is no exception. The major limb bones (proximal two segments) are approximately 10–15% longer that respective bones in the holotype of Psittacosaurus mongoliensis ( Table 1). The additional length is matched by added girth, so the long bones have very similar internal proportions. The proportions of the major long bones within fore and hind limbs are also similar to that in P. mongoliensis . The unguals on the manus are intermediate in form between a recurved claw and flattened hoof, as is the case in other species of Psittacosaurus . Thus there is no evidence from the manual unguals to suggest that P. major emphasized quadrupedal posture any more than other psittacosaurids. The manual unguals in neoceratopsians, in contrast, are usually broader and more hoof−shaped corresponding with an habitual quadrupedal stance ( Fig. 4 ).

Figures

Fig. 1. Skull of Psittacosaurus major sp. nov., LHPV 1, Near Beipiao City, Liaoning Province, China, lower portion of the Lower Cretaceous Yixian Formation, in lateral (A) and dorsal (B) views. A1 and B1, photographs; A2 and B2, explanatory drawings of the same. In A2 and B2 the cross−hatching indicates the broken bone and the tone the matrix.

Fig. 2. Relative size of the skull of the adult holotypic specimens of Psittacosaurus mongoliensis (black) and Psittacosaurus major sp. nov. (grey) in lateral (A) and dorsal (B) views.

Fig. 3. Sacrum and articulated ilia of Psittacosaurus major sp. nov., LHPV 1, Near Beipiao City, Liaoning Province, China, lower portion of the Lower Cretaceous Yixian Formation. A. Photograph of in ventral view. B. Explanatory drawing of the same, showing broken bone (cross−hatching) and areas of matrix (tone).

Fig. 4. Skull size in ceratopsians. A. Human silhouette (1.68 m) compared to an adult skull of the frilled ceratopsian Pentaceratops sternbergi (after Lehman 1998). B. Relationships among ceratopsians showing the shift to an obligate quadrupedal posture among neoceratopsians and stepwise increase in skull size (skull size shown as a percent of trunk length).