 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 ontogenetic variation in these species . Significant sexual dimorphism has been noted in Protoceratops and several ceratopsids . 
 Unlike almost all other dinosaur groups , skulls <a href='drop_aux' title='1'><mark>are</mark></a>the most commonly preserved elements of ceratopsian skeletons and many species are known only from skulls . <a href='dey_it' title='2'><mark>There</mark></a> 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 ontogenetic variation in these species . Significant sexual dimorphism <a href='been_done' title='3'><mark>has</mark></a> been noted in Protoceratops and several ceratopsids . 
Unlike almost all other dinosaur groups , skulls the most commonly preserved elements of ceratopsian skeletons and many species are known only from skulls . It is a great deal of variation between and even within ceratopsian species . Complete growth series from embryo to adult are known fa Psittacosaurus and Protoceratops , allowing da study of ontogenetic variation in these species . Significant sexual dimorphism done been noted in Protoceratops and several ceratopsids .