Ceratopsian Dinosaur Research Articles

Endocranial morphology of three early-diverging ceratopsians and implications for the behavior and the evolution of the endocast in ceratopsians

Abstract Ceratopsian dinosaurs underwent great changes, including a shift of locomotion mode, enlarged horns and frills, and increased body size. These changes occur alongside the evolution of endocranial morphology and physiology such as the size and shape of the flocculus, hearing range, olfactory ratio, and the reptile encephalization quotient (REQ). However, the evolution of endocranial structures in early ceratopsians is still unclear because of a lack of information on the earliest ceratopsians. Here, we reconstructed the endocasts of three early-diverging ceratopsians including the Late Jurassic Yinlong, and the Early Cretaceous Liaoceratops and Psittacosaurus. These ceratopsians display obvious flocculi, large and separate olfactory bulbs, long and high anterior semicircular canals, and relatively long cochlear ducts. In the evolution of the earliest ceratopsians to early neoceratopsians, changes include the increasing size of the flocculus (which is reduced or absent in late-diverging ceratopsids), the attenuation of the semicircular canals, and the heightening of the anterior semicircular canal (which is shortened in late-diverging ceratopsids). The endocranial structures suggest early-diverging ceratopsians had a higher olfactory acuity and were adapted to hearing higher frequencies than late-diverging ceratopsians. Furthermore, the REQ suggests that Yinlong and Psittacosaurus were more highly encephalized than late-diverging ceratopsians and most extant reptiles. The angle of the lateral semicircular canal suggests that heads in ceratopsians display a transition from a forward posture to a more downward posture. Our new findings are significant for understanding the physiological changes during ceratopsian evolution and also have implications for the evolution of physiology in extant tetrapods.

Osteology, phylogenetic affinities, and palaeobiogeographic significance of the bizarre ornithischian dinosaur Ajkaceratops kozmai from the Late Cretaceous European archipelago

Abstract At the climax of their evolutionary history in the latest Cretaceous, ceratopsian dinosaurs were among the most dominant components of North American and Asian land ecosystems. In other continental landmasses, however, ceratopsians were extraordinarily rare and the affinities of their proposed representatives often turned out to be inconclusive. Arguably the most significant evidence of Ceratopsia from outside North America and Asia is represented by Ajkaceratops kozmai from the Santonian (Upper Cretaceous) of Hungary. We provide a detailed osteological description of Ajkaceratops and highlight its bizarre anatomy. Ajkaceratops has been ‘traditionally’ interpreted to represent a Bagaceratops-like coronosaur, and its occurrence on the European islands was hypothesized to probably result from an early Late Cretaceous dispersal event from Asia. However, while the snout of Ajkaceratops may resemble that of some ceratopsians, closer inspection of the preserved elements indicates that these similarities are largely superficial. While it cannot be ruled out that Ajkaceratops represents a highly peculiar member of the clade, its placement is far from certain. Still, the discovery of Ajkaceratops exemplifies the importance and uniqueness of European dinosaur faunas.

Biomechanical skull performance in the meiolaniid Niolamia argentina (Testudinata: Meiolaniidae) using finite element analysis

ABSTRACT The use of horns and frills for sexual display and attack has been proposed and demonstrated in extinct taxa on several occasions, with the ceratopsian dinosaurs as the most iconic example. Niolamia argentina is a large meiolaniid turtle from Patagonia, characterized by the presence of extensive frills and massive horns in the skull. Here, Finite Element Analysis (FEA) is applied in the only known adult skull of N. argentina to assess the cranial performance simulating defensive/aggressive movements. We tested five different scenarios: (1) hitting with the snout, (2) hitting with the forehead, (3) struggling with the notch between the frill and the horn, (4) stabbing with lateral horns, and (5) caudal torsion of the frills, the last two being the most damaging and stressful scenarios. Together with the lack of skull features related to head-hitting/fighting, we find that, although the cranial structures of Niolamia argentina studied here may indicate a priori that they were for engaging in combat between males, hitting each other with the caudal frills and trying to stab each other may represent highly stressful scenarios. In this sense, we propose that the presence of frills and horns in N. argentina seems to be more suitable for sexual display than for combat behavior.

Furcatoceratops elucidans, a new centrosaurine (Ornithischia: Ceratopsidae) from the upper Campanian Judith River Formation, Montana, USA

Although ceratopsian dinosaurs have been excavated from the Judith River Formation for more than a century, their diversity within the formation remains poorly understood due to the fragmentary nature of those specimens. Here, we describe Furcatoceratops elucidans gen. et sp. nov., a new centrosaurine ceratopsid, based on a nearly complete skeleton found in the upper Judith River Formation. F. elucidans is diagnosed by several unique characters such as anteriorly oriented supraorbital horncores with slight medial curvature and anteroventral process of the nasal that laterally covers the premaxilla. Cladistic analysis recovered F. elucidans as an early-diverging centrosaurine closely related to Nasutoceratops. The holotype is inferred to be a subadult individual based on its surface textures, histological features, size and suture obliteration patterns among bones, thus providing insights into the ontogeny of ceratopsid dinosaurs. The inferred ontogenetic sequence of F. elucidans suggests that the supracranial elements co-ossified earlier than narial elements, as in the chasmosaurine Triceratops but possibly unlike in derived centrosaurines. Because most bones such as cranial and vertebral elements are disarticulated and well preserved, the holotype will serve as a useful comparative specimen for future ceratopsian research.

How Triceratops got its face: An update on the functional evolution of the ceratopsian head.

Ceratopsian dinosaurs arguably show some of the most extravagant external cranial morphology across all Dinosauria. For over a century, ceratopsian dinosaurs have inspired a multitude of cranial functional studies as more discoveries continued to depict a larger picture of the enormous diversity of these animals. The iconic horns and bony frills in many ceratopsians portray a plethora of shapes, sizes, and arrangements across taxa, and their overall feeding apparatus show the development of unique specializations previously unseen in large herbivores. Here, I give a brief updated review of the many functional studies investigating different aspects of the ceratopsian head. The functional role of the horns and bony frills is explored, with an overview of studies investigating their potential for weaponization or defense in either intraspecific or anti-predatory combat, among other things. A review of studies pertaining to the ceratopsian feeding apparatus is also presented here, with analyses of studies exploring their beak and snout morphology, dentition and tooth wear, cranial musculature with associated skull anatomy, and feeding biomechanics.

The exquisitely preserved integument of Psittacosaurus and the scaly skin of ceratopsian dinosaurs

The Frankfurt specimen of the early-branching ceratopsian dinosaur Psittacosaurus is remarkable for the exquisite preservation of squamous (scaly) skin and other soft tissues that cover almost its entire body. New observations under Laser-Stimulated Fluorescence (LSF) reveal the complexity of the squamous skin of Psittacosaurus, including several unique features and details of newly detected and previously-described integumentary structures. Variations in the scaly skin are found to be strongly regionalized in Psittacosaurus. For example, feature scales consist of truncated cone-shaped scales on the shoulder, but form a longitudinal row of quadrangular scales on the tail. Re-examined through LSF, the cloaca of Psittacosaurus has a longitudinal opening, or vent; a condition that it shares only with crocodylians. This implies that the cloaca may have had crocodylian-like internal anatomy, including a single, ventrally-positioned copulatory organ. Combined with these new integumentary data, a comprehensive review of integument in ceratopsian dinosaurs reveals that scalation was generally conservative in ceratopsians and typically consisted of large subcircular-to-polygonal feature scales surrounded by a network of smaller non-overlapping polygonal basement scales. This study highlights the importance of combining exceptional specimens with modern imaging techniques, which are helping to redefine the perceived complexity of squamation in ceratopsians and other dinosaurs.

Oldest preserved umbilical scar reveals dinosaurs had ‘belly buttons’

BackgroundIn egg-laying amniotes, the developing embryo is tethered to a number of the extraembryonic membranes including the yolk sac and allantois that deliver oxygen and nutrients and remove metabolic waste products throughout embryonic development. Prior to, or soon after hatching, these membranes detach from the animal leaving a temporary or permanent umbilical scar (umbilicus) equivalent to the navel or ‘belly button’ in some placental mammals, including humans. Although ubiquitous in modern mammals and reptiles (including birds), at least early in their ontogeny, the umbilicus has not been identified in any pre-Cenozoic amniote.ResultsWe report the oldest preserved umbilicus in a fossil amniote from a ~130-million-year-old early-branching ceratopsian dinosaur, Psittacosaurus. Under laser-stimulated fluorescence (LSF), the umbilicus is revealed as an elongate midline structure delimited by a row of paired scales on the abdomen. The relatively late ontogenetic stage (close to sexual maturity) estimated for the individual indicates that the umbilicus was probably retained throughout life.ConclusionsUnlike most extant reptiles and birds that lose this scar within days to weeks after hatching, the umbilicus of Psittacosaurus persisted at least until sexual maturity, similar to some lizards and crocodylians with which it shares the closest morphological resemblance. This discovery is the oldest record of an amniote umbilicus and the first in a non-avian dinosaur. However, given the variability of this structure in extant reptilian analogues, a persistent umbilical scar may not have been present in all non-avian dinosaurs.

Bone histology on Koreaceratops hwaseongensis (Dinosauria: Ceratopsia) from the Lower Cretaceous of South Korea

Koreaceratops hwaseongensis is a basal ceratopsian dinosaur found in the Lower Cretaceous Sihwa Formation (Albian), South Korea. It was found only with the lower part of the body. The right tibia and fibula exhibit poor histological preservation due to microbial degradation. They are characterized by signs of extensive tunneling by microorganisms, which rendered the bone cortex into an amalgam of microscopical globules and foci. Such signs of biodegradation may support the hypothesis that the Lower Cretaceous Korean peninsula had a semi-arid and seasonal wet-dry climate. However, a few remaining traces of histological features like the vasculature and absence/presence of growth marks indicate that the growth pattern of Koreaceratops is very similar to that of a subadult Protoceratops andrewsi. Both species exhibit zonation with some bone remodeling in their tibiae and lines of arrested growth with an extensive bone remodeling in their fibulae. The only histological similarity between Koreaceratops and the more basal ceratopsians Psittacosaurus is the longitudinal and reticular vasculature in their fibulae. These suggest that Koreaceratops is much closer to Protoceratops than Psittacosaurus in terms of phylogeny. The fibula of Koreaceratops shows a constant osseous drift that initially drifted toward the medial axis but abruptly changed to the anterior axis in the final growth interval. In addition, the right tibia's final growth exhibiting an anteromedial osseous drift suggests that Koreaceratops experienced a change in its right hindlimb's biomechanics before death. The number of growth zones in the tibia indicates that Koreaceratops was approximately eight years old when it died.

A New Late Permian Burnetiamorph From Zambia Confirms Exceptional Levels of Endemism in Burnetiamorpha (Therapsida: Biarmosuchia) and an Updated Paleoenvironmental Interpretation of the Upper Madumabisa Mudstone Formation

A new burnetiamorph therapsid, Isengops luangwensis, gen. et sp. nov., is described on the basis of a partial skull from the upper Madumabisa Mudstone Formation of the Luangwa Basin of northeastern Zambia. Isengops is diagnosed by reduced palatal dentition, a ridge-like palatine-pterygoid boss, a palatal exposure of the jugal that extends far anteriorly, a tall trigonal pyramid-shaped supraorbital boss, and a recess along the dorsal margin of the lateral temporal fenestra. The upper Madumabisa Mudstone Formation was deposited in a rift basin with lithofacies characterized by unchannelized flow, periods of subaerial desiccation and non-deposition, and pedogenesis, and can be biostratigraphically tied to the upper Cistecephalus Assemblage Zone of South Africa, suggesting a Wuchiapingian age. Isengops is the second burnetiamorph recognized from Zambia and is part of a tetrapod assemblage remarkably similar to others across southern Pangea during the Wuchiapingian. A revised cladistic analysis of Biarmosuchia yielded over 500 most parsimonious trees that generally reaffirm the results of previous analyses for burnetiamorphs: Lemurosaurus is basal, Lobalopex and Isengops are proximate burnetiid outgroups, and Bullacephalus, Burnetia, Mobaceras, Niuksenitia, and Pachydectes are burnetiines. Furthermore, Russian biarmosuchians are scattered throughout the tree and do not form sister taxon relationships with each other. Burnetiamorphs display a wide disparity of cranial adornments and are relatively speciose (13 species), especially when compared to the number of specimens discovered to date (∼16 specimens). As has been suggested in some other tetrapod clades (e.g., ceratopsian dinosaurs), the burnetiamorph fossil record supports an inferred macroevolutionary relationship between cranial adornment and increased speciation rate.

Three-dimensional geometric morphometric analysis of the skull of Protoceratops andrewsi supports a socio-sexual signalling role for the ceratopsian frill.

Socio-sexual selection is predicted to be an important driver of evolution, influencing speciation, extinction and adaptation. The fossil record provides a means of testing these predictions, but detecting its signature from morphological data alone is difficult. There are, nonetheless, some specific patterns of growth and variation which are expected of traits under socio-sexual selection. The distinctive parietal-squamosal frill of ceratopsian dinosaurs has previously been suggested as a socio-sexual display trait, but evidence for this has been limited. Here, we perform a whole-skull shape analysis of an unprecedentedly large sample of specimens of Protoceratops andrewsi using a high-density landmark-based geometric morphometric approach to test four predictions regarding a potential socio-sexual signalling role for the frill. Three predictions—low integration with the rest of the skull, significantly higher rate of change in size and shape during ontogeny, and higher morphological variance than other skull regions—are supported. One prediction, sexual dimorphism in shape, is not supported, suggesting that sexual differences in P. andrewsi are likely to be small. Together, these findings are consistent with mutual mate choice or selection for signalling quality in more general social interactions, and support the hypothesis that the frill functioned as a socio-sexual signal in ceratopsian dinosaurs.

Femoral histology and growth patterns of the ceratopsian dinosaur Psittacosaurus sibiricus from the Early Cretaceous of Western Siberia

Femoral histology and growth patterns of the ceratopsian dinosaur Psittacosaurus sibiricus from the Early Cretaceous of Western Siberia

Endocranial anatomy of the ceratopsid dinosaur Triceratops and interpretations of sensory and motor function.

Triceratops is one of the well-known Cretaceous ceratopsian dinosaurs. The ecology of Triceratops has been controversial because of its unique morphological features. However, arguments based on brain and inner ear structures have been scarce. In this study, two braincases (FPDM-V-9677 and FPDM-V-9775) were analyzed with computed tomography to generate three-dimensional virtual renderings of the endocasts of the cranial cavities and bony labyrinths. Quantitative analysis, including comparison of linear measurements of the degree of development of the olfactory bulb and inner ear, was performed on these virtual endocasts to acquire detailed neuroanatomical information. When compared with other dinosaurs, the olfactory bulb of Triceratops is relatively small, indicating that Triceratops had a reduced acuity in sense of smell. The lateral semicircular canal reveals that the basicranial axis of Triceratops is approximately 45° to the ground, which is an effective angle to display their horns as well as frill, and to graze. The semicircular canals of Triceratops are relatively smaller than those of primitive ceratopsians, such as Psittacosaurus and Protoceratops, suggesting that sensory input for the reflexive stabilization of gaze and posture of Triceratops was less developed than that of primitive ceratopsians. The cochlear length of Triceratops is relatively short when compared with other dinosaurs. Because cochlear length correlates with hearing frequency, Triceratops was likely adapted to hearing low frequencies.

Modularity and heterochrony in the evolution of the ceratopsian dinosaur frill.

The fossil record provides compelling examples of heterochrony at macroevolutionary scales such as the peramorphic giant antlers of the Irish elk. Heterochrony has also been invoked in the evolution of the distinctive cranial frill of ceratopsian dinosaurs such as Triceratops. Although ceratopsian frills vary in size, shape, and ornamentation, quantitative analyses that would allow for testing hypotheses of heterochrony are lacking. Here, we use geometric morphometrics to examine frill shape variation across ceratopsian diversity and within four species preserving growth series. We then test whether the frill constitutes an evolvable module both across and within species, and compare growth trajectories of taxa with ontogenetic growth series to identify heterochronic processes. Evolution of the ceratopsian frill consisted primarily of progressive expansion of its caudal and caudolateral margins, with morphospace occupation following taxonomic groups. Although taphonomic distortion represents a complicating factor, our data support modularity both across and within species. Peramorphosis played an important role in frill evolution, with acceleration operating early in neoceratopsian evolution followed by progenesis in later diverging cornosaurian ceratopsians. Peramorphic evolution of the ceratopsian frill may have been facilitated by the decoupling of this structure from the jaw musculature, an inference that predicts an expansion of morphospace occupation and higher evolutionary rates among ceratopsids as indeed borne out by our data. However, denser sampling of the meager record of early‐diverging taxa is required to test this further.

Appendicular skeleton of Protoceratops andrewsi (Dinosauria, Ornithischia): comparative morphology, ontogenetic changes, and the implications for non-ceratopsid ceratopsian locomotion.

Protoceratops andrewsi is a well-known ceratopsian dinosaur from the Djadokhta Formation (Upper Cretaceous, Mongolia). Since the 1920s, numerous skeletons of different ontogenetic stages from hatchlings to adults, including fully articulated specimens, have been discovered, but the postcranial anatomy of Protoceratops has not been studied in detail. A new, mostly articulated subadult individual provides an excellent opportunity for us to comprehensively describe the anatomy of the limb skeleton, to compare to other ceratopsian dinosaurs, and to study the ontogenetic and intraspecific variation in this species. New data provided by the specimen shed light on the lifestyle of P. andrewsi. The young subadult individuals present an array of morphological characters intermediate between the bipedal Psittacosaurus and fully quadrupedal adult P. andrewsi. We compare these observations with a broad range of non-ceratopsid Neoceratopsia (of various locomotor adaptations) and Psittacosauridae (obligate bipeds), which gives us insight into the evolution of the skeletal characters informative for the postural change in ceratopsian dinosaurs.

Ontogeny and variation of a protoceratopsid dinosaur Bagaceratops rozhdestvenskyi from the Late Cretaceous of the Gobi Desert

ABSTRACT Bagaceratops rozhdestvenskyi is a ceratopsian dinosaur from the Late Cretaceous Baruungoyot Formation of the Gobi Desert, closely related to Protoceratops spp. Several Bag. rozhdestvenskyi skulls demonstrate a wide range of variation in their morphology and size. Here I argue that the observed variability is most likely of intraspecific nature. Specimens classified in a few allegedly distinct species from the same or near-contemporary sediments, namely Gobiceratops minutus, Lamaceratops tereschenkoi and Platyceratops tatarinovi from Baruungoyot Formation, and Magnirostris dodsoni from Bayan Mandahu, are younger subjective synonyms of Bag. rozhdestvenskyi. They plausibly represent an ontogenetic series within the latter. Breviceratops kozlowskiiis a distinct taxon. The evolutionary relationships within Protoceratopsidae are complicated by the mosaic distribution of plesiomorphic and derived features in distinct species. I suggest that taxa distribution and observed changes in morphology are an evidence for the ancestral position of Protoceratops andrewsi among protoceratopsids. It implies possible temporary separation between the geological formations of the Gobi Desert yielding distinct protoceratopsid species. The novel evolutionary scenario suggests number of convergences that occurred in Protoceratopsidae and Ceratopsoidea (reduction of the premaxillary dentition, fusion of nasals, development of the accessory antorbital fenestra). Present study reveals the significance of the intraspecific and ontogenetic variation in the study of the neoceratopsian taxonomy.

Chemostratigraphy of the Lower Cretaceous dinosaur-bearing Xiagou and Zhonggou formations, Yujingzi Basin, northwest China

The Early Cretaceous of northwest China has yielded abundant vertebrates, invertebrates, and plant fossils from numerous intermontane basins. Developing a chronostratigraphic context for these important fossil finds is important to understanding the development of modern terrestrial ecosystems, the evolution of dinosaurs, and the Cretaceous greenhouse climate. This study utilizes carbon isotope chemostratigraphy of the fossil-rich Xinminpu Group in the Yujingzi Basin in northwest Gansu Province. Lithostratigraphic descriptions defined three facies. The lowest is a tan, coarse arkosic sandstone, overlain by gray to variegated mudstones, sandstones, and thin limestones, which transition to red sandstones, conglomerates, and mudstones. Depositional environment interpretation of these lithofacies assisted in refining 450 samples of bulk sedimentary organic carbon as well as charcoal samples to develop a composite curve to correlate to existing carbon isotope curves from the region and to better constrained Cretaceous sections globally. The carbon isotope curve is correlated based on the broad positive excursion associated with carbon isotope fluctuations of the Paquier Episode that spans the late Aptian to early Albian. Based on this correlation, the middle gray mudstone facies, which contains a diverse fauna including theropods, sauropods, and ornithopods, as well as turtles and invertebrates, is placed within the upper Aptian. The upper red sandstone facies, which contains therizinosaurs and the ceratopsian dinosaur Auroraceratops, is placed in the lower Albian.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVPCitation for this article: Suarez, M. B., T. Milder, P. Nan, C. A. Suarez, H. You, D. Li, and P. Dodson 2019. Chemostratigraphy of the Lower Cretaceous dinosaur-bearing Xiagou and Zhonggou formations, Yujingzi Basin, northwest China; pp. 12–21 in Hailu You, Peter Dodson, and Eric Morschhauser (eds.), Auroraceratops rugosus (Ornithischia, Ceratopsia) from the Early Cretaceous of northwestern Gansu Province, China. Society of Vertebrate Paleontology Memoir 18. Journal of Vertebrate Paleontology 38(Supplement). DOI: 10.1080/02724634.2017.1510412.

Bite marks on the frill of a juvenile Centrosaurus from the Late Cretaceous Dinosaur Provincial Park Formation, Alberta, Canada.

Bite marks on bones can provide critical information about interactions between carnivores and animals they consumed (or attempted to) in the fossil record. Data from such interactions is somewhat sparse and is hampered by a lack of records in the scientific literature. Here, we present a rare instance of feeding traces on the frill of a juvenile ceratopsian dinosaur from the late Campanian Dinosaur Park Formation of Alberta. It is difficult to determine the likely tracemaker(s) but the strongest candidate is a small-bodied theropod such as a dromaeosaur or juvenile tyrannosaur. This marks the first documented case of carnivore consumption of a juvenile ceratopsid, but may represent scavenging as opposed to predation.

Who is the most stressed? Morphological disparity and mechanical behavior of the feeding apparatus of ceratopsian dinosaurs (Ornithischia, Marginocephalia)

Ceratopsians were herbivorous dinosaurs that dominated many of the terrestrial ecosystems in Asia and North America during the Cretaceous. The bizarre variety of skulls and lower jaw morphologies as well as the inferred ecological abundance of many species in this clade indicate that Ceratopsia was a successful group. Here we analyzed 126 lower jaws from 50 ceratopsian species, using two-dimensional geometric morphometrics and finite element analysis to investigate differences in shape and structural performance of this part of the feeding apparatus across Ceratopsia. Morphological differences in lower jaws across ceratopsian clades are said to originate from feeding adaptation.Our results show that the stress (physical loadings modeled in response to biting) in lower jaws was quite similar between “basal” and “derived” taxa, whereas major differences among clades occur for stress values associated with the coronoid process. The basal ceratopsians Hualianceratops and Yinlong had a highly stressed and primitive lower jaw, indicating that those animals may have fed on relatively soft foliage and fruits. A similar condition was found for basal neoceratopsians and protoceratopsids. Psittacosaurids possessed a well-integrated and compact lower jaw able to withstand high stress, at the cost of having a highly stressed coronoid process. Leptoceratopsids were characterized by the opposite condition. Taxa such as Leptoceratops, Prenoceratops, Zhuchengceratops and Cerasinops appear to have had a comparatively efficient feeding apparatus. Ceratopsidae represents the clade with the most efficient masticatory apparatus within Ceratopsia, even if the horizontal ramus of the lower jaw appears less able to withstand high levels of stress as compared with other ceratopsians. Additionally, we found the dentary and surangular–angular complex co-evolved to generate a masticatory apparatus able to withstand high stress, particularly in Protoceratopsidae and Triceratopsini. The major phenotypic evolutionary rate and morphological changes occurred during the mid- to Late Cretaceous, when intense climate change and angiosperm diversification could have affected the evolution of ecological diversity and feeding biomechanics in Ceratopsia.

Dental microwear reveals mammal-like chewing in the neoceratopsian dinosaur Leptoceratops gracilis.

Extensive oral processing of food through dental occlusion and orbital mandibular movement is often cited as a uniquely mammalian trait that contributed to their evolutionary success. Save for mandibular translation, these adaptations are not seen in extant archosaurs or lepidosaurs. In contrast, some ornithischian dinosaurs show evidence of precise dental occlusion, habitual intraoral trituration and complex jaw motion. To date, however, a robust understanding of the diversity of jaw mechanics within non-avian dinosaurs, and its comparison with other vertebrates, remains unrealized. Large dental batteries, well-developed dental wear facets, and robust jaws suggests that neoceratopsian (horned) dinosaurs were capable chewers. But, biomechanical analyses have assumed a relatively simple, scissor-like (orthal) jaw mechanism for these animals. New analyses of dental microwear, presented here, show curvilinear striations on the teeth of Leptoceratops. These features indicate a rostral to caudal orbital motion of the mandible during chewing. A rostrocaudal mandibular orbit is seen in multituberculates, haramiyid allotherians, and some rodents, and its identification in Leptoceratops gracilis is the first evidence of complex, mammal-like chewing in a ceratopsian dinosaur. The term circumpalinal is here proposed to distinguish this new style of chewing from other models of ceratopsian mastication that also involve a palinal component. This previously unrecognized complexity in dinosaurian jaw mechanics indicates that some neoceratopsian dinosaurs achieved a mammalian level of masticatory efficiency through novel adaptive solutions.

Cranial anatomy of Yinlong downsi (Ornithischia: Ceratopsia) from the Upper Jurassic Shishugou Formation of Xinjiang, China

ABSTRACTYinlong downsi, from the Upper Jurassic Shishugou Formation, Xinjiang, northwestern China, is the oldest known ceratopsian dinosaur. Here we provide a detailed description of the skull and mandible based on the holotype, three partial skulls, and disarticulated materials from several other specimens. Yinlong can be diagnosed by six autapomorphies: a distinct fossa along the midline of the frontals; a slit-like carotid canal bordered by laminae; premaxillary teeth with a vertical wear facet and a basal shelf; a deep sulcus on the ventral surface of the quadratojugal; large oval nodules concentrated on the lateral surface of the jugal; and a squamosal with an expanded dorsal surface and a long, constricted quadrate process. A detailed comparison with other basal ceratopsians suggests that Yinlong downsi may belong within Chaoyangsauridae. Yinlong also shares many derived characters both with Psittacosaurus and neoceratopsians, suggesting that character evolution in early ceratopsians is complex. Additionally, a caniniform premaxillary tooth is present in one small specimen (IVPP V18636), which may suggest sexual dimorphism or individual variation.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVPCitation for this article: Han, F.-L., C. A. Forster, J. M. Clark, and X. Xu. 2015. Cranial anatomy of Yinlong downsi (Ornithischia: Ceratopsia) from the Upper Jurassic Shishugou Formation of Xinjiang, China. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2015.1029579.