Protoceratops (meaning “First Horned Face”) is a genus of sheep-sized (1.8 m long) herbivorous ceratopsian dinosaur, from the Upper Cretaceous Period (Campanian stage) of what is now Mongolia. It was a member of the Protoceratopsidae, a group of early horned dinosaurs. Unlike later ceratopsians, however, it was a much smaller creature that lacked well-developed horns and retained some primitive traits not seen in later genera.
Protoceratops had a large neck frill which was likely used as a display site to impress other members of the species. Other hypotheses about its function include protection of the neck and anchoring of jaw muscles, but the fragility of the frill and the poor leverage offered by possible attachment sites here makes these ideas implausible. Described by Walter W. Granger and W.K. Gregory in 1923, Protoceratops was initially believed to be an ancestor of the North American ceratopsians. Researchers currently distinguish two species of Protoceratops (P. andrewsi and P. hellenikorhinus), based in part by their respective sizes.
Protoceratops was a very common animal in the late Cretaceous lowland habitats of Mongolia. Fossilized remains of this dinosaur were among the most abundant fossils found on the American Museum of Natural History expeditions to Mongolia that Roy Chapman Andrews led between 1922 and 1925. The abundance of these fossils has led scientists to believe that Protoceratops was a highly social animal that lived in herds.
The American expeditions also discoveered eggs and nests belonging to Protoceratops. These were the first dinosaur nests ever found, and the discovery was widely publicized. Another famous find was that of a Protoceratops skeleton interlocked with that of a Velociraptor. Whether or not this find represents an actual act of predation, Velociraptor, along with the larger theropods such as Tarbosaurus, would almost certainly have been among the main predators on a small plant-eater such as Protoceratops.
Protoceratops was a quadrupedal dinosaur that was partially characterized by its distinctive neck frill at the back of its skull. The frill itself contained two large parietal fenestrae (holes in the frill), while its cheeks had large jugal bones. The exact size and shape of the neck frill varied by individual; some specimens had short, compact frills, while others had frills nearly half the length of the skull. The frill consists mostly of the parietal bone and partially of the squamosal. Some researchers, including Peter Dodson attribute the different sizes and shapes of these bones to sexual dimorphism, as well as the age of the specimen, at the time of death.
Protoceratops was the first named protoceratopsian and hence gives its name to the family Protoceratopsidae, a group of herbivorous dinosaurs more derived than psittacosaurids, but less derived than ceratopsids. The group is characterized by their similarities to the Ceratopsidae but with more cursorial limb proportions, generally smaller frills, and lack of large horns.
In 1998, Paul Sereno defined Protoceratopsidae as the branch-based clade including “all coronosaurs closer to Protoceratopsthan to Triceratops.” Some studies placed Bagaceratops, Breviceratops, Graciliceratops, Lamaceratops, Magnirostris, Platyceratops, and Serendipaceratops within Protoceratopsidae, but in 2006, Makovicky and Norell published a new phylogeny which removed several genera from Protoceratopsidae; several other phylogenies also exist. Bainoceratops may be synonymous with Protoceratops.