ARTICLE IN PRESS Proc. R. Soc. B 1 doi:10.1098/rspb.2006.3566 65 2 66 3 67 4 68 5 A basal ceratopsian with transitional features from 69 6 70 7 the Late Jurassic of northwestern China 71 8 1,2,* 3 4 5,6 72 9 Xing Xu , Catherine A. Forster , James M. Clark and Jinyou Mo 73 10 1Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, 74 11 142 Xiwai Street, Beijing 100044, People’s Republic of China 75 12 2American Museum of Natural History, New York, NY 10024, USA 76 13 3Department of Anatomical Sciences, State University of New York, Stony Brook, NY 11794, USA 77 14 4Department of Biological Sciences, George Washington University, Washington, DC 20052, USA 78 15 5Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, People’s Republic of China 79 16 6Natural History Museum of Guangxi, Nanning 530012, People’s Republic of China 80 17 81 18 Although the Ceratopsia and Pachycephalosauria, two major ornithischian groups, are united as the 82 19 Marginocephalia, few synapomorphies have been identified due to their highly specialized body-plans. 83 20 Several studies have linked the Heterodontosauridae with either the Ceratopsia or Marginocephalia, but 84 21 evidence for these relationships is weak, leading most recent studies to consider the Heterodontosauridae 85 22 as the basal member of another major ornithischian radiation, the Ornithopoda. Here, we report on a new 86 23 basal ceratopsian dinosaur, Yinlong downsi gen. et. sp. nov., from the Late Jurassic upper part of the 87 24 Shishugou Formation of Xinjiang, China. This new ceratopsian displays a series of features transitional 88 25 between more derived ceratopsians and other ornithischians, shares numerous derived similarities with 89 26 both the heterodontosaurids and pachycephalosaurians and provides strong evidence supporting a 90 27 monophyletic Marginocephalia and its close relationship to the Heterodontosauridae. Character 91 28 distributions along the marginocephalian lineage reveal that, compared to the bipedal Pachycephalosauria, 92 29 which retained a primitive post-cranial body-plan, the dominantly quadrupedal ceratopsians lost many 93 30 marginocephalian features and evolved their own characters early in their evolution. 94 31 Keywords: Ceratopsia; Marginocephalia; Ornithischia; Late Jurassic; Shishugou Formation 95 32 96 33 97 34 98 35 1. INTRODUCTION Ceratopsia Marsh, 1890. 99 36 The Shishugou Formation of the Junggar Basin, Yinlong downsi gen et sp nov. 100 37 Xinjiang, China was deposited during the Middle–Late 101 38 Jurassic (Chen 1996; Eberth et al. 2001), a period that (a) Etymology 102 39 is critical to the origins and early evolution of the major ‘Yin’ and ‘long’ mean ‘hiding’ and ‘dragon’ in Chinese, 103 40 dinosaurian lineages, including birds (Sereno 1999). respectively, derived from the movie ‘Crouching Tiger, 104 41 Since 2001, we have prospected in this formation and Hidden Dragon’ which was filmed in the locality where the 105 42 collected numerous vertebrate specimens (Clark et al. holotype was found; the specific name is in memory of Mr 106 43 2004), among them a nearly complete ornithischian Will Downs, who joined many palaeontological 107 44 skeleton we here identify as a basal ceratopsian expeditions in China including the one with us in 2003, 108 45 dinosaur. The two other possible Jurassic ceratopsians shortly before his death. 109 46 (Zhao et al. 1999, in press) are from the Tuchengzi and 110 47 Houcheng Formations of China. However, a recent (b) Holotype 111 48 radiometric sample from the Tuchengzi Formation IVPP V14530, a nearly complete skeleton missing only the 112 49 places its upper part in the Early Cretaceous (Swisher distal tail (figure 1 and figures 1 and 2 of electronic 113 50 et al. 2002). Consequently, the Shishugou ceratopsian supplementary material). 114 51 represents the first unquestionable Jurassic ceratopsian 115 species. 52 (c) Locality and horizon 116 53 Wucaiwan, Junggar Basin, Xinjiang, China; upper part of 117 54 Shishugou Formation, correlated with the Oxfordian stage 118 55 2. SYSTEMATIC PALAEONTOLOGY of the early Late Jurassic (Chen 1996; Eberth et al. 2001). 119 56 Ornithischia Seeley, 1887. 120 57 Heterodontosauriformes new taxon. (d) Diagnosis 121 58 Marginocephalia, Sereno (1986). A small ceratopsian with the following unique features: a 122 59 distinct fossa along the midline of the frontals, trapezoidal 123 60 124 * Author for correspondence ([email protected]). quadratojugal longer than deep, sharp ridges and grooves 61 on the anterior surface of the proximal half of the 125 Q1 The electronic supplementary material is available at http://dx.doi. 62 paroccipital process, a long basipterygoid process oriented 126 Q2 org/10.1098/rspb.2006.3566 or via http://www.journals.royalsoc.ac. 63 uk. posteroventrally, slit-like carotid canal laterally bordered 127 64 128 Received 1 December 2005 1 q 2006 The Royal Society Accepted 29 March 2006 RSPB 20063566—18/4/2006—20:06—RAMAKRISHNA—210002—XML – pp. 1–7 ARTICLE IN PRESS 2 X. Xu and others A Jurassic ceratopsian from China 129 lpe lis lt lf lm lu lh rib ls 193 130 194 131 195 132 196 133 197 134 198 135 199 136 200 137 201 138 202 139 203 140 204 141 205 142 206 143 207 144 208 145 209 5 cm 146 210 147 211 148 cv rpe rt ri rfi rf lp rm dv rh rs cev sk 212 149 213 Figure 1. Yinlong downsi (IVPP V14530, holotype). Abbreviations: cev, cervical vertebrae; cv, caudal vertebrae; dv, dorsal 150 vertebrae; lf, left femur; lh, left humerus; lis, left ischium; lm, left manus; lp, left pubis; lpe, left pes; ls, left scapula; lt, left tibia; 214 151 lu, left ulna; rf, right femur; rfi, right fibula; rh, right humerus; ri, right ilium; rm, right manus; rpe, right pes; rs, right scapula; rt, 215 152 right tibia; sk, skull. Scale bar, 5 cm. 216 153 217 154 by a lamina, a prominent tubercle on the posteroventral that continues across the surface of the postorbital and 218 155 surangular and a vertical wear facet with a basal shelf on temporal bar. The large quadratojugal is trapezoidal and 219 156 the premaxillary teeth (figures 1 and 2; figures 1 and 2 of longer anteroposteriorly than dorsoventrally. As in 220 157 electronic supplementary material). Chaoyangsaurus (Zhao et al. 1999), the quadrate slopes 221 158 slightly anterodorsally. As in Liaoceratops, the quadrate has 222 159 (e) Description a longitudinal groove along its anterolateral margin and a 223 160 The holotype is probably a sub-adult as indicated by the paraquadratic foramen at mid-height on the posterior 224 161 open neurocentral sutures on the presacral vertebrae margin. A paraquadratic foramen is also seen in Hetero- 225 162 (closed on the caudal vertebrae). The holotype is dontosaurus (Norman et al. 2004). As in Chaoyangsaurus 226 163 estimated to be 120 cm in total body length. Relatively and an unnamed basal ceratopsian (Zhao et al. 1999), the 227 164 short and slender forelimbs and very robust and long distal quadrate condyles are separated by a deep groove, 228 165 hindlimbs (forelimb less than 40% the hindlimb length) which is shallow or absent in Psittacosaurus and more 229 166 suggest that Yinlong was a bipedal animal. derived ceratopsians ( You & Dodson 2003). Yinlong 230 167 The posterior part of the skull is proportionally deep in shares with Heterodontosaurus (Norman et al. 2004)a 231 168 lateral view and extremely broad in the postorbital region post-orbital with a prominent ridge running along the 232 169 in dorsal view (figures 1 and 2). The pre-orbital region is jugal process and onto the postorbital process of the jugal. 233 170 shallow and, as in some other basal ceratopsians, is The squamosal process of the postorbital is long, forming 234 171 relatively short. The orbit lies entirely above the posterior nearly the whole length of the dorsal margin of the 235 172 maxillary tooth row. The supratemporal and infratem- infratemporal fenestra. As in pachycephalosaurians 236 173 poral fenestrae are proportionally large in size. The latter (Maryan´ska et al. 2004), the temporal bar has a broader 237 174 exceeds the orbit in diameter and is nearly circular in dorsal margin relative to the lateral margin. Similar to 238 175 shape. The small antorbital fossa has distinct margins, is most ornithischians, but differing from other ceratopsians 239 176 bordered ventrally by a prominent ridge, and is positioned (Sereno 2000; Makovicky 2001; You & Dodson 2004; 240 177 under the anterior orbit. Dodson et al. 2004), the posterior margin of the parietal is 241 178 As in all ceratopsians, a small rostral bone caps the positioned distinctly anterior to that of the squamosal and 242 179 anterior premaxillae. The palpebral has a distinct anterior its posterolateral wing has a deep distal ramus facing 243 180 process and a long posterior process reaching to the caudally rather than ventrally. These features suggest a 244 181 middle of the orbit. The skull roof bears two distinct lack of parietal contribution to the frill, though the parietal 245 182 midline fossae indenting the nasals and the frontals. The near the midline slightly overhangs over the occiput. The 246 183 Q3 former is seen in Heterodontosaurus (Barrett, personal squamosal overhangs the quadrate-squamosal articulation 247 184 communication) and some basal ornithopods (He & Cai laterally and posteriorly, forming a small squamosal frill. 248 185 1984; Peng 1992; Xu et al. 2000), and to a less degree, in Similar to pachycephalosaurians (Maryan´ska et al. 2004), 249 186 some basal ceratopsians (Xu et al. 2002). As in a row of distinct tubercles runs along the posterior margin 250 187 Heterodontosaurus (Norman et al. 2004) and some basal of the squamosal and the lateral margin of the squamosal 251 188 ceratopsians, the jugal, with a blunt anterior end, process of the post-orbital.