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

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.