Several juvenile "Psittacosaurus" have been found. Ceratopsians are separated into basal ceratopsians, including the parrot-beaked "Psittacosaurus", and neoceratopsians. Aptian age findings include a "Psittacosaurus". This is especially prominent in Ceratopsia, where only the primitive "Psittacosaurus" is bipedal. Early members of the ceratopsian group, such as "Psittacosaurus", were small bipedal animals. It was one of the first ceratopsians, or horned dinosaurs, along with "Psittacosaurus" in Mongolia. The senses of "Psittacosaurus" can be inferred from the endocast. They further suggested that some species of "Psittacosaurus" were more terrestrial than others. Another fossil from the Yixian Formation provides direct evidence of "Psittacosaurus" as a prey animal. "P. sibiricus" is the largest-known species of "Psittacosaurus". The skull of "Psittacosaurus" is highly modified compared to other ornithischian dinosaurs of its time. Two new species of "Psittacosaurus" were described by Canadian Dale Russell and Zhao in 1996. "Psittacosaurus" is one of the most completely known dinosaur genera.