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Comparative Study
, 273 (1598), 2135-40

A Basal Ceratopsian With Transitional Features From the Late Jurassic of Northwestern China

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Comparative Study

A Basal Ceratopsian With Transitional Features From the Late Jurassic of Northwestern China

Xing Xu et al. Proc Biol Sci.

Abstract

Although the Ceratopsia and Pachycephalosauria, two major ornithischian groups, are united as the Marginocephalia, few synapomorphies have been identified due to their highly specialized body-plans. Several studies have linked the Heterodontosauridae with either the Ceratopsia or Marginocephalia, but evidence for these relationships is weak, leading most recent studies to consider the Heterodontosauridae as the basal member of another major ornithischian radiation, the Ornithopoda. Here, we report on a new basal ceratopsian dinosaur, Yinlong downsi gen. et. sp. nov. from the Late Jurassic upper part of the Shishugou Formation of Xinjiang, China. This new ceratopsian displays a series of features transitional between more derived ceratopsians and other ornithischians, shares numerous derived similarities with both the heterodontosaurids and pachycephalosaurians and provides strong evidence supporting a monophyletic Marginocephalia and its close relationship to the Heterodontosauridae. Character distributions along the marginocephalian lineage reveal that, compared to the bipedal Pachycephalosauria, which retained a primitive post-cranial body-plan, the dominantly quadrupedal ceratopsians lost many marginocephalian features and evolved their own characters early in their evolution.

Figures

Figure 1
Figure 1
Yinlong downsi (IVPP V14530, holotype). Abbreviations: cev, cervical vertebrae; cv, caudal vertebrae; dv, dorsal 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; 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, right tibia; sk, skull. Scale bar, 5 cm.
Figure 2
Figure 2
IVPP V14530. (a) Skull and mandible in lateral view. (b) Skull in dorsal view. Abbreviations: an, angular; d, dentary; emf, external mandibular fenestra; en, external naris; f, frontal; ff, fossa on frontals; fn, fossa on nasals; itf, infratemporal fenestra; j, jugal; m, maxilla; p, parietal; pa, palpebral; pd, predentary; pm, premaxilla; po, postorbital; q, quadrate; qj, quadratojugal; r, rostral bone; sa, surangular; sq, squamosal; stf, supratemporal fenestra; tr, tubercle row; ts, tubercle on surangular. Scale bar, 2 cm.
Figure 3
Figure 3
A single most parsimonious tree recovered showing the phylogenetic position of Yinlong downsi and the interrelationships of the Cerapoda. The proposed tree plotted against geological time indicates the timing of the major cranial changes among the Ceratopsia. The Marginocephalia node is supported by 22 synapomorphies and the Heterodontosauriformes by nine synapomorphies (see electronic supplementary material).

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