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| Yamaceratops dorngobiensis in Son, Lee, Zorigt, Kobayashi, ... et Lee, 2022. |
Abstract
Here we report a new articulated skeleton of Yamaceratops dorngobiensis (MPC-D 100/553) from the Khugenetjavkhlant locality at the Shine Us Khudag (Javkhlant Formation, ?Santonian-Campanian) of the eastern Gobi Desert, Mongolia, which represents the first substantially complete skeleton and the first juvenile individual of this taxon. The specimen includes a nearly complete cranium and large portions of the vertebral column and appendicular skeleton. Its skull is about 2/3 the size of the holotype specimen, based on mandibular length. Its juvenile ontogenetic stage is confirmed by multiple indicators of skeletal and morphological immaturity known in ceratopsians, such as the long-grained surface texture on the long bones, the smooth external surface on the postorbital, open neurocentral sutures of all caudal vertebrae, a large orbit relative to the postorbital and jugal, the low angle of the lacrimal ventral ramus relative to the maxillary teeth row, narrow frontal, and straight ventral edge of the dentary. Osteohistological analysis of MPC-D 100/553 recovered three lines of arrested growth, implying around 3 years of age when it died, and verified this specimen’s immature ontogenetic stage. The specimen adds a new autapomorphy of Yamaceratops, the anteroventral margin of the fungiform dorsal end of the lacrimal being excluded from the antorbital fossa. Furthermore, it shows a unique combination of diagnostic features of some other basal neoceratopsians: the ventrally hooked rostral bone as in Aquilops americanus and very tall middle caudal neural spines about or more than four times as high as the centrum as in Koreaceratops hwaseongensis, Montanoceratops cerorhynchus, and Protoceratops andrewsi. The jugal with the subtemporal ramus deeper than the suborbital ramus as in the holotype specimen is also shared with A. americanus, Liaoceratops yanzigouensis, and juvenile P. andrewsi. Adding 38 new scorings into the recent comprehensive data matrix of basal Neoceratopsia and taking into account the ontogenetically variable characters recovered Y. dorngobiensis as the sister taxon to Euceratopsia (Leptoceratopsidae plus Coronosauria). A second phylogenetic analysis with another matrix for Ceratopsia also supported this position. The new phylogenetic position of Y. dorngobiensis is important in ceratopsian evolution, as this taxon represents one of the basalmost neoceratopsians with a broad, thin frill and hyper-elongated middle caudal neural spines while still being bipedal.
Systematic paleontology
Dinosauria Owen, 1842
Ornithischia Seeley, 1887
Marginocephalia Sereno, 1986
Ceratopsia Marsh, 1890
Neoceratopsia Sereno, 1986
Yamaceratops dorngobiensis Makovicky & Norell, 2006
Holotype. IGM 100/1315, an articulated skull from an adult that lacks the rostral, premaxillae, nasals, the predentary, anterior process of the lacrimal, left elements around the temporal region, left posterior mandible, and the parietal (Makovicky & Norell, 2006).
Studied specimen. MPC-D 100/553, a reasonably complete articulated skeleton with the left proximal femur and partial foot, but no cervical vertebrae, pectoral girdles, left arm, right hand, other parts of the left leg, and the right foot. Measurements are in the Supplemental Information (Tables S1–S4).
Locality, horizon, and age. Khugenetjavkhlant, Dornogovi Province, Mongolia; middle unit of the Javkhlant Formation; Upper Cretaceous (?Santonian-Campanian). The specimen was preserved on top of a trough cross-bedding sandstone that is poorly sorted, as was described for the typical coarse-grained deposits in the middle Javkhlant Formation (Eberth et al., 2009).
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| Skeleton of Yamaceratops dorngobiensis (MPC-D 100/553) in right dorsolateral view. (A) Photograph; (B) Interpretive drawing. Bones are bounded by solid lines and colored beige; the matrix is gray. Shaded areas represent the broken surface of bones. Abbreviations: cd, caudal vertebrae; dr, dorsal ribs; dv, dorsal vertebrae; f, femur; fi, fibula; h, humerus; L, bone on the left side; p, isolated parietal; pp, pedal phalanges; R, bone on the right side; ra, radius; sk, skull; sv, sacral vertebrae; ti, tibia; u, ulna; il, ilium; is, ischium. |
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| Skeletal reconstruction of juvenile Yamaceratops dorngobiensis (MPC-D 100/553) in right lateral view. (A) Reconstruction of the skeleton with preserved parts colored in white and missing bones in gray; (B) reconstruction of the complete skeleton, restored based on other basal neoceratopsian skeletons. |
Minyoung Son, Yuong-Nam Lee, Badamkhatan Zorigt, Yoshitsugu Kobayashi, Jin-Young Park, Sungjin Lee, Su-Hwan Kim and Kang Young Lee. 2022. A New Juvenile Yamaceratops (Dinosauria, Ceratopsia) from the Javkhlant Formation (Upper Cretaceous) of Mongolia. PeerJ. 10:e13176 DOI: 10.7717/peerj.13176
