Monday, December 14, 2015

[Paleontology • 2015] Hualianceratops wucaiwanensis • A New Taxon of Basal Ceratopsian from China and the Early Evolution of Ceratopsia

Hualianceratops wucaiwanensis
Han, Forster, Clark & Xu, 2015


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.

the reconstructed skull of the holotype specimen of Hualianceratops wucaiwanensis (IVPP V18641).
 Abbreviations: an, angular; d, dentary; j, jugal; ma, maxilla; pd, predentary;
po, postorbital; q, quadrate; sa, surangular; sq, squamosal 
  DOI: 10.1371/journal.pone.0143369

An artist's interpretation of Hualianceratops wucaiwanensis, a dinosaur that lived about 160 million years ago.
illustration: Portia Sloan Rollings  ||

Systematic Paleontology

Dinosauria Owen, 1842 
Ornithischia Seeley, 1887 

Ceratopsia Marsh, 1890 

Chaoyangsauridae Zhao et al., 2006
Type Genus: Chaoyangsaurus Zhao et al., 1999

Definition: A stem-based taxon defined as all ceratopsians more closely related to Chaoyangsaurus youngi than to Psittacosaurus mongoliensis or Triceratops horridus.

Revised Diagnosis: 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.

Hualianceratops gen.nov.

Type Species: Hualianceratops wucaiwanensis gen. et sp. nov.

Etymology: Hualian” means ornamental face, referring to the texture found on most part of the skull, combined with ceratops (horned face) from the Greek, a common suffix for horned dinosaurs; “wucaiwan” (Chinese: five color bay) for the area where the specimen was discovered.

Holotype: IVPP V18641, articulated right maxilla, jugal, postorbital and partial squamosal, articulated right quadrate and partial quadratojugal, articulated left partial jugal, quadratojugal and quadrate, left partial squamosal, most of the mandible, and postcranial fragments including a nearly complete left pes (Figs 1–9; also see S1 File).

Discussion and Conclusion: 
Hualianceratops (IVPP V18641) represents the second species of basal ceratopsian present in the upper part of the Shishugou Formation at the Wucaiwan locality. Though Yinlong possesses a number of autapomorphies, the incompleteness of the Hualianceratops material does not allow all of these characters to be evaluated. While two characters have been recognized that are uniquely shared by these taxa (a deep sulcus on the ventral surface of the quadratojugal for articulation with the jugal, and a squamosal with a flat dorsal surface that expands both laterally and caudally), neither unambiguously define a sister-group relationship between these taxa (see above). Hualianceratops is distinct from Yinlong in possessing the following characters: a prominent dorsal process on the infratemporal ramus of the jugal, a robust quadrate with an expanded rostral margin above the quadratojugal facet, an expansive quadratojugal facet, a deep notch on the ventral jugal wing of the quadrate, a shallow sulcus between the quadrate condyles, and strongly rugose sculpturing on the lateral surface of the dentary. None of these characters occur in individuals of Yinlong of any size, suggesting they are not ontogenetically dependent.

“New” family tree for horned dinosaurs [simplified]

Yinlong downsi shares some derived feature with both Psittacosaurus and neoceratopsians. Interestingly, Hualianceratops shares more derived characters with Psittacosaurus than with basal neoceratopsians. This includes the divergent quadratojugal process and the flattened ventral surface of the jugal, the caudodorsally curved quadrate head, the deep and short dentary. However, the large antorbital fossa, preserved squamosal and sculpture lateral surface of most bones are quite different from that of Psittacosaurus. Additionally, the wide jugal-postorbital bar is more like basal neoceratopsians.

The age of the two Shishugou species within the dating error for the beginning of the Oxfordian coupled with the most parsimonious phylogenies imply that at least five lineage of ceratopsians were present at the beginning of the Late Jurassic (Fig 11), including the two Shishugou species. The grouping of Psittacosaurus with chaoyangsaurids (Fig 11) implies long ghost lineages for Psittacosaurus and Neoceratopsia. By comparison, if there are no morphological constraints on the phylogeny then only two ceratopsian lineages are minimally necessary at the beginning of the Oxfordian, the two Shishugou species. Furthermore, all of the alternative MPTs indicate at least three lineages of chaoyangsaurids were present (assuming the autapomorphies of the two Shishugou taxa debar them from being direct ancestors to any other taxa). Three lineages are implied when the two Shishugou taxa are sister-taxa with a Chaoyangsaurus-Xuanhuaceratops clade or when the former are paraphyletic with the latter, but four lineages are implied when Chaoyangsaurus and Xuanhuaceratops are paraphyletic to a Yinlong-Hualianceratops clade. The presence of at least five lineages at the beginning of the Late Jurassic contrasts with the previous published analyses indicating only a minimum of two lineages at this time, Yinlong and all other ceratopsians, and prior to 2006 no ceratopsians were known from the beginning of the Late Jurassic. In any case, this phylogeny implies that ceratopsian phylogenetic diversification was well established by the beginning of the Late Jurassic.

Fenglu Han, Catherine A. Forster, James M. Clark and Xing Xu. 2015. A New Taxon of Basal Ceratopsian from China and the Early Evolution of Ceratopsia. PLoS ONE. DOI: 10.1371/journal.pone.0143369

New Triceratops Cousin Had a Gnarly, Bumpy Skull via @NatGeo