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Bagaceratops (meaning "small-horned face") is a genus of small protoceratopsid that lived in Asia during the Late Cretaceous, around 72 to 71 million years ago. Bagaceratops remains have been reported from the Barun Goyot Formation and Bayan Mandahu Formation. One specimen may argue the possible presence of Bagaceratops in the Djadochta Formation.
Bagaceratops was among the smallest ceratopsians, growing up to in length, with a weight about . Although emerging late in the reign of the dinosaurs, Bagaceratops had a fairly primitive anatomy—when compared to the much derived ceratopsids—and kept the small body size that characterized early ceratopsians. Unlike its close relative, Protoceratops, Bagaceratops lacked premaxillary teeth (cylindrical, blunt teeth near the tip of the upper jaw).
In 2020, Czepiński described new specimens of Bagaceratops and Protoceratops from the Udyn Sayr and Zamyn Khond localities, respectively, of the Djadochta Formation, and evaluated the implications of these specimens for correlation of fossil sites of the latter formation. He considered one of these specimens in particular, MPC-D 100/551B, as a potential evidence of an anagenetic transition from Protoceratops andrewsi to Bagaceratops rozhdestvenskyi.
In 2003 Russian paleontologist Vladimir R. Alifanov named the new taxa Lamaceratops tereschenkoi and Platyceratops tatarinovi from the Barun Goyot Formation. Material assigned by Alifanov corresponds to the holotype of Lamaceratops (PIN 4487/26; a partial small-sized skull), recovered from the Khulsan locality, and the holotype of Platyceratops (PIN 3142/4; an almost complete medium-sized skull), found in the red beds of the Hermiin Tsav locality. He also coined the family Bagaceratopidae in order to contain these new taxa and Bagaceratops. Also during 2003, You Hailu and Dong Zhiming described and named the new genus and species of protoceratopsid Magnirostris dodsoni from red beds at the Bayan Mandahu locality of the Bayan Mandahu Formation, Inner Mongolia (China). The holotype of Magnirostris, IVPP V12513, represents a nearly complete skull lacking the Neck frill region of a large individual and was collected during expeditions led by the Sino-Canadian Dinosaur Project. In 2006 Mackoviky regarded all of these ceratopsians as of Bagaceratops based on the reasoning that all exhibit anatomical traits already seen on other specimens of this protoceratopsid, and some of them are likely products of preservation. In 2008 Alifanov described and named another ceratopsian taxon from the Barun Goyot Formation, Gobiceratops minitus. Its holotype (PIN 3142/299) is represented by a very small and juvenile skull that was collected from the Hermiin Tsav locality near the end of the 1970s by the Joint Soviet–Mongolian Paleontological Expedition. Though Alifanov used this skull to erect the new Gobiceratops, it had already been displayed for several years at the Moscow Paleontological Museum under the name Bagaceratops rozhdestvenskyi.
A comprehensive study on the intraspecific variation in morphology of B. rozhdestvenskyi was conducted by Polish paleontologist Łukasz Czepiński in 2019, where he concluded that the previously named Gobiceratops minutus, Lamaceratops tereschenkoi, Platyceratops tatarinovi and Magnirostris dodsoni represent additional specimens and growth stages of B. rozhdestvenskyi and therefore, junior synonyms. Czepiński reexamined many of the specimens originally described by Maryańska and Osmólska, as well as the respective holotypes of these taxa, providing evidence that all traits used to separate them are, in fact, indistinguishably present on Bagaceratops and they fall within the large intraspecific variation of this taxon. He also considered Breviceratops to be a distinct and separate genus of protoceratopsid, from both Bagaceratops and Protoceratops, as it features a combination of basal (primitive) and derived (advanced) traits.
In 2019 Czepiński analyzed a vast majority of referred specimens to the ceratopsians Bagaceratops and Breviceratops, and concluded that most were in fact specimens of the former. Although the genera Gobiceratops, Lamaceratops, Magnirostris, and Platyceratops, were long considered valid and distinct taxa, and sometimes placed within Protoceratopsidae, Czepiński found the diagnostic features used to distinguish these taxa to be largely present in Bagaceratops and thus becoming synonyms of this genus. Under this reasoning, Protoceratopsidae consists of Bagaceratops, Breviceratops, and Protoceratops. Based on cranial characters such as presence or absence of premaxillary teeth and an antorbital fenestra, P. andrewsi is the basal-most protoceratopsid and Bagaceratops the derived-most one. Below are the proposed phylogenetic relationships within Protoceratopsidae by Czepiński:
Bagaceratops is the most common taxon across the Barun Goyot Formation, which was also home to many other , including the Saichania, Tarchia and Zaraapelta; Khulsanurus and Parvicursor; Gobipipus, Gobipteryx and Hollanda; fellow protoceratopsid Breviceratops; Kuru kulla and Shri devi; halszkaraptorine Hulsanpes; pachycephalosaurid Tylocephale; and Conchoraptor, Heyuannia and Nemegtomaia. Other taxa are represented by the Megafauna titanosaur Quaesitosaurus, and a wide diversity of and .
This formation has produced numerous , such as closely related protoceratopsid Protoceratops; ankylosaurid Pinacosaurus; alvarezsaurid Linhenykus; dromaeosaurids Linheraptor and Velociraptor; oviraptorids Machairasaurus and Wulatelong; and Linhevenator, Papiliovenator, and Philovenator. Other paleofauna from this unit comprises a variety of squamates and mammals, and .
Paleoenvironment
Barun Goyot Formation
The Barun Goyot Formation, based on , is regarded as Late Cretaceous in age (Middle-Upper Campanian) and has virtually yielded the bulk of material for which Bagaceratops is known. This formation is mostly characterized by series of red beds, mostly light-coloured (yellowish, grey-brown, and rarely reddish) that are locally cemented. Sandy (often red-coloured), , conglomerates, and large-scale trough cross-stratification in sands are also common across the unit. In addition, structureless, medium-grained, fine-grained and very fine-grained predominate in sediments of the Barun Goyot Formation. The sediments of this formation were deposited in alluvial plain (flat land consisting of sediments deposited by highland ), lacustrine, and aeolian paleoenvironments, under relatively Desert climate to .
Bayan Mandahu Formation
The Bayan Mandahu Formation, which yielded Magnirostris (now synonym of Bagaceratops), is considered to be Late Cretaceous in age, roughly Campanian. The dominant lithology is reddish-brown, poorly cemented, fine grained sandstone with some conglomerate, and caliche. Other facies include alluvial (stream-deposited) and aeolian (wind-deposited) sediments. It is likely that sediments at Bayan Mandahu were deposited by short-lived rivers and on an alluvial plain with a combination of dune field paleoenvironments, under a semi-arid climate. The formation is known for its vertebrate fossils in life-like poses, most of which are preserved in unstructured sandstone, indicating a catastrophic rapid burial.
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