"Psittacosaurus" is the type genus of the family Psittacosauridae, which was also named by Osborn in 1923. Psittacosaurids were basal to almost all known ceratopsians except "Yinlong" and perhaps the Chaoyangsauridae. While Psittacosauridae was an early branch of the ceratopsian family tree, "Psittacosaurus" itself was probably not directly ancestral to any other groups of ceratopsians. All other ceratopsians retained the fifth digit of the hand, a plesiomorphy or primitive trait, whereas all species of "Psittacosaurus" had only four digits on the hand. In addition, the "antorbital fenestra", an opening in the skull between the eye socket and nostril, was lost during the evolution of Psittacosauridae, but is still found in most other ceratopsians and in fact most other archosaurs. It is considered highly unlikely that the fifth digit or antorbital fenestra would evolve a second time.