Article: Juvenile specimens of the ornithischian dinosaur Psittacosaurus

Hitherto undescribed specimens of Psittacosaurus mongoliensis from the Oshih Fm., Mongolian Peoples' Republic, include two almost complete skulls and numerous postcranial elements. A rostral bone, present in these and other specimens of Psittacosaurus, is a cranial element otherwise known only in ceratopsians, and its presence indicates a sister group relationship for the Psittacosauridae and Ceratopsia. Each of the two new specimens of Psittacosaurus is a juvenile, and are among the smallest dinosaur specimens yet described. Parental attendance of nests was common, possibly universal among dinosaurs, but post-hatching parental care is uncertain. Juveniles of Psittacosaurus, as well as those of other dinosaurs, may have formed sibling groups.