Appendicular skeleton of Protoceratops andrewsi Comparative morphology, ontogenetic changes, and the implications for non- ceratopsid ceratopsian locomotion PROTOCERATOPS ANDREWSI Protoceratops andrewsi is a member of Ornithischia (‘bird-hipped’ dinosaurs), which inhabited what we call today the Gobi Desert in Central Asia, during the Late Cretaceous time (i.e., between 83 and 72 million years ago). It was discovered in 1922. Protoceratops belongs to a group of horned dinosaurs (Ceratopsia), but contrary to the large Triceratops, it was a relatively small (up to 2 m long) dinosaur and had no horns, just a medium-sized frill at the back of its head (called the parietal frill). Protoceratops fed on plants and supposedly lived in herds. Adults were rather stocky and quadrupedal animals, but when young, Protoceratops was much more agile. SUBADULT PROTOCERATOPS Our analysis of the subadult skeleton of Protoceratops andrewsi indicates that it could occasionally stand on two legs either feeding this way (to reach higher growing plants) or for behavioral reasons (checking its surroundings for potential enemies). ADULT SUBADULT DIFFERENCES Narrow Wide scapular scapular blade blade Narrow Wide glenoid glenoid Arched Straight femur femur MAJOR DIFFERENCES BETWEEN BIPEDAL AND QUADRUPEDAL NON-CERATOPSID CERATOPSIANS Tibia longer than the femur Tibia shorter than the femur Slender and pointy hand unguals Broad (hoof-like) hand unguals Narrow ilium Wider ilium Long and slender foot Broader, more stocky foot This is an open access graphic distributed under the terms of the Appendicular skeleton of Protoceratops andrewsi Creative Commons Attribution License. (Dinosauria, Ornithischia): comparative morphology, ontogenetic changes, and the implications for non- ceratopsid ceratopsian locomotion PeerJ 4:e7324 DOI: 10.7717/peerj.7324 http://peerj.com/articles/7324/.