 Unlike almost all other dinosaur groups , skulls are the most commonly preserved elements of ceratopsian skeletons and many species are known only from skulls . There is a great deal of variation between and even within ceratopsian species . Complete growth series from embryo to adult are known for Psittacosaurus and Protoceratops , allowing the study of <unk> variation in these species . Significant sexual <unk> has been noted in Protoceratops and several ceratopsids . 
