Udanoceratops

Udanoceratops (meaning "Udan-Sayr's horned face") 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 attributed to Udanoceratops, though both of them may be misattributions.

Udanoceratops is the largest known leptoceratopsid, with some estimates placing it at about 4 m (13 ft) in length and 700 kg (1,500 lb) in weight. Its skull, though incomplete, probably was between 60–70 cm (24–28 in) in length, half as long again as that of the second-largest leptoceratopsid, Montanoceratops from North America. Udanoceratops' skull was far deeper and more robust than that of other leptoceratopsids, with larger nasal cavities and presumably a heavier beak. The projections on Udanoceratops' cheek bones would have supported a large patch of keratin. The bone of its snout above the beak was completely smooth, bearing neither the horns nor the slight tubercles of related taxa. Udanoceratops had a very deep and robust lower jaw (mandible), capable of exerting a powerful bite, possibly strong enough to cleave through bone; indeed, the holotype bears damage to its mandible which may have been caused by such a bite. The lower teeth bore shelves into which the upper set would have slotted when the jaw was closed. This would have resulted in a slicing action, similar to the system present in Archaeoceratops and Leptoceratops.

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 (the bones which would have supported its claws) 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.

The holotype (PIN 3907/11) 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 (also spelled Udyn Sayr or Üüden Sair) locality from a layer of dark grey sandstones designated as Bed 2. The Udan-Sayr locality is part of the Djadokhta 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 (Udan Sayr) 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 1 m (3.3 ft) 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, and it is most likely that they were confused with the contemporary (and relatively large) Protoceratops hellenikorhinus.

In 2004 Viktor S. Tereschenko referred a juvenile specimen (PIN 4046/11) to Udanoceratops aff. tschizhovi, from the Baga Tariach locality in Dornogovi Province, which Tereschhenko attributed to the Djadokhta Formation. Geological analyses carried out across fossiliferous localities of the Gobi Desert published by Mahito Watabe and team in 2010, indicates that this locality instead correlates best with the Maastrichtian-stage Barun Goyot Formation. The assignment of this specimen, however, has varied since then from "Udanoceratops" sp., to ?Udanoceratops sp, and some studies, including one authored by Tereschenko himself, consider it an indeterminate leptoceratopsid.

Udanoceratops was a large ceratopsian, estimated by V. S. Tereschenko to have reached nearly 4 m (13 ft) in length, and by Gregory S. Paul to have weighed about 700 kg (1,500 lb). 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.

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 60–70 cm (24–28 in) 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, Leptoceratops was found to be the most similar, though there remain significant differences between the two. 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 (rear) portion which was very long, and covered 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. Udanoceratops' jugal bones were long and low, and were oriented almost horizontally. While many ceratopsians have an additional bone on each jugal, called the epijugal, this structure was apparently absent in Udanoceratops; the same is seen in some juvenile specimens of Protoceratops. Rather, there was a small, roughly textured projection where it would have otherwise sat. A cornified area would have extended from the jugals to the quadratojugals, supported in part by this projection, which would have been covered in a sheath of keratin. Unlike many other ceratopsians, where either a full horn or a slight tubercule was present on the nasal bones, at least the dorsal (top) portions of Udanoceratops' nasals appear to have sported nothing at all, as the bone texture is completely smooth there. A rough cornified area appears to have run along the anterior portion of the nasal bones, connecting to a similar patch on an ascending branch at the anterior (front) part of the premaxillae. While much of the posterior portion of the skull has not been preserved, the shape of the lower temporal fossa suggests that Udanoceratops may have had a slightly longer frill than that observed in Leptoceratops, though it would have still been small.