BAGACERATOPS

Although appearing quite late in the fossil record, Bagaceratops was a bit primitive, with its short and narrow neck frill, a low tooth count—only ten per jaw in adults—and a slender build typical of early ceratopsians. For years, the largest known skull was only 17cm long, and the smallest just 4.7—with the eye socket accounting for half of that—and paleontologists thought it well within the realms of possibility that its fossils were infant specimens of Protoceratops, itself a small ceratopsian. However, closer study revealed some unusual features, such as an extra opening in its skull just in front of the eye sockets that mirrors the nostril openings in both shape and size, and the faintest whiff of a low, blunt snout "horn", even in the smallest specimens. But no sooner had Bagaceratops been confirmed as a unique critter worthy of its own name than  lumpers  arrived, eager to fold other small protoceratopsids into it.

Second only to Protoceratops in sheer abundance, Bagaceratops has been discovered at Khulsan, Khermeen Tsav and Bayan Mandahu, areas which have also yielded remains of Lamaceratops, Platyceratops, Graciliceratops, Magnirostris, and Breviceratops, and some workers considered it entirely plausible that the so-called unique features which set them all apart were merely age- or gender-related quirks, or the result of compressive force during fossilisation. Of all the suggested synonyms, though, the most unlikely is Khulsan's Breviceratops,  which was long considered a juvenile Bagaceratops, despite at least one skull rumoured to belong to a critter some 2 meters long. For a juvenile of that size to "grow into" an adult Bagaceratops would require a reverse growth curve whereby it shrank by at least a quarter while also shedding it's premaxillary teeth (blunt teeth near the tip of the upper jaw) and any trace of a nose horn! Unfortunately, that skull—along with jaws of Tarbosaurus efromovi and two skulls of Protoceratops andrewsi— were nicked from the fossil repository of the Palaeontological Institute at the Russian Academy of Sciences in August of 1996 and, as far as we know, has never been recovered.

Thanks to the restudy of dozens of small ceratopsian specimens from across Mongolia's Barun Goyot Formation by Czepinski in 2019, Bagaceratops has never been as well represented as it is now, with some referred skulls fully twice as long as the previous longest, which has upped its estimated total adult length to between 1 and 1.5 metres.