Udanoceratops
Udanoceratops is a genus of large leptoceratopsid dinosaur that lived during the Late Cretaceous period of Mongolia. The holotype specimen, the partial skeleton of an adult with its bones encapsulated in calcium carbonate, was discovered in the 1980s as part of the Joint Soviet-Mongolian Paleontological Expedition, and was subsequently transported to the Paleontological Institute of the Russian Academy of Sciences. In 1992, it was described by palaeontologist Sergei Kurzanov. The sole species of Udanoceratops, also the type species, is Udanoceratops tschizhovi, named after a colleague of Kurzanov, D. O. Tschizhov. Additional remains have been assigned to Udanoceratops, though these may be misattributions.
Udanoceratops is the largest known leptoceratopsid, with some estimates placing it at about in length and in weight. Its skull, though incomplete, probably was between in length, half as long again as that of the second-largest leptoceratopsid, Montanoceratops from North America. Udanoceratops
The postcranial skeleton of Udanoceratops is fragmentary, consisting primarily of a partial vertebral column and a few appendicular bones. Its tail was very tall and was somewhat compressed from side-to-side, as in many other leptoceratopsids and protoceratopsids. This and the shape of its unguals has led to suggestions that Udanoceratops was semi-aquatic, but it is more likely that the shape of its unguals was an adaptation to supporting its weight on soft substrate such as sand dunes. Similarly, while it has been suggested that Udanoceratops could have ran bipedally, this appears to be unlikely. If it did run, it likely would have done so on all fours, and would have been significantly slower than its smaller relatives. Udanoceratops is known from the Djadochta Formation of Mongolia, which is well known for the fossils of Protoceratops and Velociraptor which have been recovered there. Specifically, the Udan-Bayr locality, from which the holotype of Udanoceratops was recovered, preserves fossils of Protoceratops and the oviraptorosaur Avimimus.
Discovery
The holotype of Udanoceratops was collected in the 1980s as part of the Joint Soviet-Mongolian Paleontological Expedition. It was a mature specimen, and has been suggested to have been a male. It was recovered from the Udan-Sayr locality from a layer of dark grey sandstones designated as Bed 2. The Udan-Sayr locality is part of the Djadochta Formation of Ömnögovi Province, which has been dated to the Campanian stage of the Late Cretaceous period. PIN 3907/11 preserves a near-complete skull, several vertebrae, a scapula, a coracoid, an ilium, and at least some bones from the extremities. All of its bones were encapsulated within calcareous structures, either within concretions or as part of a single, larger mass. The bones were disarticulated and had been scattered, and the skull bones had been separated by taphonomic processes. The specimen was relocated to the Paleontological Institute of the Russian Academy of Sciences and was removed from the matrix through the use of acetic acid. In 1992, it was described by Russian palaeontologist Sergei Kurzanov, and was assigned the binomial name of Udanoceratops tschizhovi. The generic name is derived from the name of the locality in which the holotype was found and Greek ceras/κέρας meaning "horn" and -ops/ωψ meaning "face"; the species name honours D. O. Tschizhov of the Paleontological Institute, who was credited with the discovery of the specimen.In 1993 a large skull almost in length was reported from the nearby Bayan Mandahu Formation by Tomasz Jerzykiewicz and colleagues and was assigned to Udanoceratops. However, Polish palaeontologist Łukasz Czepiński in 2020 pointed out that there are no referable specimens to Udanoceratops from the Bayan Mandahu collections, and there are none in the literature which correspond to the reported remains. Therefore, he suspected that Jerzykiewicz and colleagues confused Udanoceratops with the contemporary Protoceratops hellenikorhinus.
In 2004 Viktor S. Tereschenko referred a juvenile specimen to Udanoceratops aff. tschizhovi, from the Baga Tariach locality in Dornogovi Province, which Tereschhenko attributed to the Djadochta Formation. Geological analyses carried out across fossil-bearing localities of the Gobi Desert, published by Mahito Watabe and colleagues in 2010, indicates that this locality correlates best with the Maastrichtian-stage Barun Goyot Formation. The status of PIN 4046/11 has varied since then. It has been classified as "Udanoceratops" sp. and ?Udanoceratops sp, and some studies, including one authored by Tereschenko himself, consider it an indeterminate leptoceratopsid.
Description
Udanoceratops was a large ceratopsian, estimated by V. S. Tereschenko to have reached nearly in length, and by Gregory S. Paul to have weighed about. It is the largest named leptoceratopsid known so far, though fragmentary remains from Kazakhstan may belong to an animal of similar body size. Writing about the genus in 1992, Kurzanov suggested that it may have been anywhere between two and three times the size of Protoceratops, to which he believed it was closely related.Skull and dentition
The skull of Udanoceratops is considerably larger, both in general size and in height, than that of other leptoceratopsids and protoceratopsids. Kurzanov noted that its skull, which measured in length, was roughly half as long again as that of the North American Montanoceratops, the second-largest leptoceratopsid. Among the ceratopsians discussed by Kurzanov, Udanoceratops was found to be most similar to Leptoceratops, though significant differences between the two were noted. The skull of Udanoceratops was significantly taller, especially around the area of the rostral bone. The rostral itself was very high and narrow, with a process on its posterior portion which was very long, covering roughly two-thirds of the lower margin of the premaxilla. The premaxillary bones of Udanoceratops were completely toothless, unlike the contemporary Protoceratops. The nasal cavity was larger than in protoceratopsids, and the nasal bones themselves were considerably more robust. UdanoceratopsUdanoceratops had an extremely deep mandible, considerably more so than that of related taxa. It is very short, although the extent of this is not entirely certain, as the predentary bone is not preserved. Based on the surrounding bone texture, it likely extended almost to the beginning of the tooth row; the gap between the most anterior teeth and the posterior edge of the predentary was only around the diameter of one alveolus. The left dentary of PIN 3907/11, the holotype, bears a prominent vertical break on its anterior third, which may be the result of intraspecies conflict. Kurzanov forwarded the idea that Udanoceratops engaged in ritual confrontations wherein two individuals, presumably males, would inflict blows on one another by swinging their heads sideways and upwards, in a way similar to that suggested for other ceratopsians. Gregory S. Paul, in 2016, instead suggested that the break may have been inflicted by the beak of another individual. Due to the overall size of its skull in relation to other basal ceratopsians, it is likely that Udanoceratops had very large mandibular adductor muscles, though not as large as those of ceratopsids; the muscle attachment sites themselves were likely fairly short, judging by other leptoceratopsids.
The teeth of Udanoceratops differed between upper and lower jaws. The number of teeth on the maxilla has variously been reported as either twelve, or fifteen. These maxillary teeth had oval-shaped crowns, with thickened central crests which had shifted posteriorly in relation to the teeth of related taxa, and roots which were relatively short anteroposteriorly and had been compressed laterally across their length. The roots of the dentary teeth, of which there were either twelve, or eighteen, were identical to those on the upper jaw, though the structure of their crowns was very different, having an oblique and very strongly developed central crest. Wear patterns indicate that when the jaw was closed, shelves on the labial side of the dentary teeth would have supported the maxillary teeth. Kurzanov suggested, consequently, that Udanoceratops
Postcranial skeleton
The postcranial anatomy of Udanoceratops was not discussed at length in its initial description. Rather, Kurzanov intended to publish a follow-up paper wherein he described it in greater detail. This paper never saw publication. V. S. Tereschenko, in 2008, noted that Udanoceratops had nine cervical vertebrae; based on other leptoceratopsids and protoceratopsids, it likely had nine thoracic, three lumbar, eight sacral, and somewhere between 38–50 caudal vertebrae. The thorax specifically would have been fairly immobile, with mobility restricted to the area between the sixth and eighth thoracic vertebrae, unlike in related taxa; that said, the torso overall appears to have been more mobile vertically than in Protoceratops. The neural spines of the caudal vertebrae were proportionally longer than those of Protoceratops, and the tail overall appears to have been laterally flattened. The anterior vertebrae, those towards the front, were strongly heterocoelous in the adult specimen, though less so in PIN 4046/11.Some appendicular characteristics of Udanoceratops
Classification
Udanoceratops belonged to the clade Ceratopsia, a group of herbivorous dinosaurs with parrot-like beaks which thrived in North America and Asia during the Cretaceous Period. In his 1992 paper describing Udanoceratops, Sergei Kurzanov suggested that it was the most basal member of the family Protoceratopsidae, in which he also included Leptoceratops. He did note strong similarities between the two genera, though simultaneously drew comparisons with more basal taxa, such as Psittacosaurus, and with ceratopsids. Subsequent analyses have placed Udanoceratops within the family Leptoceratopsidae, although specific placements vary between studies. Yiming He and colleagues in 2015, for instance, recovered Udanoceratops in a very derived position within leptoceratopsids, as the sister taxon of a clade otherwise consisting of Zhuchenceratops, Gryphoceratops, and Unescoceratops, in that order; In 2019, Morschhauser and colleagues recovered Udanoceratops as part of a clade with Ischioceratops, Prenoceratops, and Zhuchengceratops.The cladogram below reflects the results of the phylogenetic analysis of basal ceratopsians by Yiming He and colleagues :
The cladogram below depicts the interrelationships of Leptoceratopsidae, as recovered by Morschhauser and colleagues in 2019:
Palaeobiology
Udanoceratops, like all ceratopsians, was a herbivore. The short, deep jaws would have given the animal a powerful bite. The toothless beak would have served to grasp and crop stems or leaves, and as in other leptoceratopsids, the teeth would have met with an action that combined shearing and crushing. The feeding adaptations seen in leptoceratopsids suggest a diet of relatively tough food items, however though little is known about the plants that grew in the Gobi Desert during the Cretaceous. The type specimen does bear an injury to its face, possibly caused by the biting of another Udanoceratops. This might also explain its strong bite force as having some sort of social or defensive function. In 2008, Tereschenko suggested that Udanoceratops was somewhat aquatic in its habits, based on its high neural spines, the lateral compression of its tail, and the shape of its ungual ; he argued the same in a paper published in 2013 co-authored with T. Singer. In a subsequent paper, however, he rejected this hypothesis, suggesting that the ungual shape of the genus instead correlated more to supporting the animal's weight on loose substrate without sinking.Palaeoenvironment
Udanoceratops is known from the Djadochta Formation in Mongolia, which dates back to the Late Cretaceous about 71 million to 75 million years ago, being deposited during a rapid sequence of polarity changes in the late part of the Campanian stage. Dominant sediments at Djadochta include dominant reddish-orange and pale orange to light gray, medium to fine-grained sands and sandstones, caliche, and sparse fluvial processes. Based on these components, the paleoenvironments of the Djadochta Formation are interpreted as having a hot, semiarid climate with large dune fields/sand dunes and several short-lived water bodies, similar to the modern Gobi Desert. It is estimated that at the end of the Campanian age and into the Maastrichtian the climate would shift to the more mesic conditions seen in the Nemegt Formation.The Djadochta Formation is separated into a lower Bayn Dzak Member and upper Turgrugyin Member; in both of these members, the related Protoceratops is a dominant part of the fauna The Bayn Dzak member has yielded the dromaeosaurids Halszkaraptor and Velociraptor mongoliensis; oviraptorid Oviraptor; ankylosaurid Pinacosaurus grangeri; Protoceratops; and troodontid Saurornithoides. Ukhaa Tolgod, a highly fossiliferous locality is also included in the Bayn Dzak member. and its dinosaur paleofauna is composed of alvarezsaurids Kol and Shuvuuia; ankylosaurid Minotaurasaurus; birds Apsaravis and Gobipteryx; dromaeosaurid Tsaagan; oviraptorids Citipati and Khaan; troodontids Almas and Byronosaurus; and a new, unnamed protoceratopsid closely related to Protoceratops. In the Turgrugyin Member, the bird Elsornis; dromaeosaurids Mahakala and Velociraptor mongoliensis; and ornithomimid Aepyornithomimus are known. The Udan Sayr site, from which Udanoceratops