Abstract
Ceratopsia is one of the best studied herbivorous ornithischian clades, but the early evolution of Ceratopsia, including the placement of Psittacosaurus, is still controversial and unclear. Here, we report a second basal ceratopsian, Hualianceratops wucaiwanensis gen. et sp. nov., from the Upper Jurassic (Oxfordian) Shishugou Formation of the Junggar Basin, northwestern China. This new taxon is characterized by a prominent caudodorsal process on the subtemporal ramus of the jugal, a robust quadrate with an expansive quadratojugal facet, a prominent notch near the ventral region of the quadrate, a deep and short dentary, and strongly rugose texturing on the lateral surface of the dentary. Hualianceratops shares several derived characters with both Psittacosaurus and the basal ceratopsians Yinlong, Chaoyangsaurus, and Xuanhuaceratops. A new comprehensive phylogeny of ceratopsians weakly supports both Yinlong and Hualianceratops as chaoyangsaurids (along with Chaoyangsaurus and Xuanhuaceratops), as well as the monophyly of Chaoyangosauridae + Psittacosaurus. This analysis also weakly supports the novel hypothesis that Chaoyangsauridae + Psittacosaurus is the sister group to the rest of Neoceratopsia, suggesting a basal split between these clades before the Late Jurassic. This phylogeny and the earliest Late Jurassic age of Yinlong and Hualianceratops imply that at least five ceratopsian lineages (Yinlong, Hualianceratops, Chaoyangsaurus + Xuanhuaceratops, Psittacosaurus, Neoceratopsia) were present at the beginning of the Late Jurassic.
Highlights
Many Asian ceratopsians have helped clarify the early evolution of Ceratopsia in recent years, including Yinlong downsi [1], Chaoyangsaurus youngi [2], Xuanhuaceratops niei [3], Liaoceratops yanzigouensis [4], Archaeoceratops oshimai [5], Archaeoceratops yujingziensis [6], Auroraceratops rugosus [7], and Yamaceratops dorngobiensis [8]
A stem-based taxon defined as all ceratopsians more closely related to Chaoyangsaurus youngi than to Psittacosaurus mongoliensis [26] or Triceratops horridus [27]
Chaoyangsaurids may be distinguished from other ceratopsians by the following synapomorphies: semicircular ventral process near the medial face of the mandibular glenoid [3], expanded, flat dorsal surface of the squamosal with a stalked quadrate process, deep sulcus dividing the quadrate condyles, ventral margin of the angular extending laterally to form a ridge with a distinct concavity formed above the ridge, predentary reduced and much shorter than premaxillary oral margin, dorsal and ventral margin of the dentary converged rostrally more than 20% of the depth
Summary
Many Asian ceratopsians have helped clarify the early evolution of Ceratopsia in recent years, including Yinlong downsi [1], Chaoyangsaurus youngi [2], Xuanhuaceratops niei [3], Liaoceratops yanzigouensis [4], Archaeoceratops oshimai [5], Archaeoceratops yujingziensis [6], Auroraceratops rugosus [7], and Yamaceratops dorngobiensis [8]. A basal neoceratopsian, Aquilops americanus, was discovered in the Lower Cretaceous Cloverly Formation of North America, suggesting an early migration from Asia into North America [9]. Recent phylogenetic analyses still debate the origin and early evolution of Ceratopsia. New Basal Ceratopsian and Early Evolution of Ceratopsia and analysis, decision to publish, or preparation of the manuscript
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