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221 15 Comments on the Basicranium and Palate of Basal Ceratopsians PETER DODSON, HAI-LU YOU, AND KYO TANOUE the anatomy of basal ceratopsians has been studied for more than eight decades, but certain anatomical regions of the skull have been neglected due to problems of preservation and especially of preparation. In the past decade many new specimens of basal ceratopsians, including both psittacosaurids and basal neoceratopsians, have been discovered in China, and a number of new taxa have been described. Although the Jehol fauna is justifiably famous for its feathered theropods and birds, it is also the source of important psittacosaurid material, especially Psittacosaurus lujiatunensis and Hongshanosaurus houi, and basal neoceratopsians such as Liaoceratops yanzigouensis. The excellent preservation and preparation of this fossil material now allows for new detailed studies. We present the first detailed descriptions of the palate and basicranium of selected basal ceratopsians. A striking contrast is found between psittacosaurids and basal neoceratopsians in these regions. In caudal view the basioccipital of psittacosaurids is bilobate , while that of basal neoceratopsians is quadrilateral. The basioccipital-basisphenoid synchondrosis is exposed ventrally in psittacosaurids and is covered by the pterygoids in basal neoceratopsians. In psittacosaurids the basipterygoid processes of the sphenoid are long and directed rostrally, while those of basal neoceratopsians are shorter and directed ventrally or ventrolaterally. The central plate of the pterygoid of psittacosaurids is rostral in position at the midlength of the skull and the median joint is short, while the central plate of this element in basal neoceratopsians is caudally positioned with an elongate median joint. In psittacosaurids the vomer is nearly horizontal. The conspicuous choanae are very large, and open ventrally into the roof of the oral cavity. Air flow through the dorsally positioned naris into the nasal cavity is predominantly vertical. In basal neoceratopsians the vomer is inclined caudodorsally. The choanae are reduced to slits at the rostral end of the oral cavity, and are difficult to view due to their steep inclination . The air flow through the nasal cavity is much more nearly horizontal. Psittacosaurids exhibit a large number of autapomorphies in the nasal cavity. These and other related features add characters that provide us with new data that help us to understand the evolution of the Ceratopsia. Introduction Ceratopsian dinosaurs are distinctive herbivorous dinosaurs of the Cretaceous of Asia and North America. The roots of their radiation may be sought in the Jurassic. Recently a putative basal ceratopsian, Yinlong, was reported from the Late Jurassic of northwestern China (Xu et al. 2006). Yinlong joins Chaoyangsaurus and Xuanhuaceratops (Zhao et al. 1999, 2006) as basalmost Ceratopsia. For many years, only three taxa gen- 222 dodson, you, & tanoue FIGURE 15.1. Phylogenetic tree of the Ceratopsia compiled from You et al. (2003, 2005); Chinnery (2004); Makovicky and Norell (2006); and Xu et al. (2006). erally characterized basal ceratopsians: Leptoceratops from Alberta (Brown 1914), and Psittacosaurus (Osborn 1923) and Protoceratops (Granger and Gregory 1923), both from Mongolia. Apart from the fragmentary Microceratops described by Bohlin (1953) from Gansu Province, no new basal ceratopsian taxon was described until Bagaceratops was described by Maryańska and Osmólska in 1975. The past decade has revealed a plethora of new basal ceratopsians including Archaeoceratops (Dong and Azuma 1997; You and Dodson 2003); Chaoyangsaurus (Zhao et al. 1999); Liaoceratops (Xu et al. 2002); Hongshanosaurus (You and Xu 2005; You et al. 2003); Auroraceratops (You et al. 2005); Xuanhuaceratops (Zhao et al. 2006); and Yinlong (Xu et al. 2006), all from China. In recent years, there has also been renewed interest in Psittacosaurus, in including the description of new species: P. xinjiangensis (Sereno and Chao 1988); P. meileyingensis (Sereno et al. 1988); P. neimongoliensis (Russell and Zhao 1996) and P. ordosensis (Russell and Zhao 1996), both from Inner Mongolia; P. mazongshanensis (Xu 1997) from Gansu; P. sibiricus from Siberia (Averianov et al. 2006); P. lujiatunensis (Zhou et al. 2006) from Liaoning; and P. major (Sereno et al. 2007), also from Liaoning. A general understanding of the phylogeny of basal ceratopsians is shown in Fig. 15.1. Ceratopsians have distinctive cranial morphology, and readily yield characters that permit phylogenetic analysis (Fig 15.1.; Sereno 2000; Makovicky 2001; You and Dodson 2003, 2004; Xu et al. 2006). Not only have new materials been described , but in many cases, new preparation practices have provided access to structures such as the palate and the basicranium that previously could not be viewed. The new material invites reexamination of some areas of anatomy that...

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