 In clade @-@ based phylogenetic taxonomy , Ceratopsia is often defined to include all marginocephalians more closely related to Triceratops than to Pachycephalosaurus . Under this definition , the most basal known ceratopsians are Yinlong , from the Late Jurassic Period , along with Chaoyangsaurus and the family Psittacosauridae , from the Early Cretaceous Period , all of which were discovered in northern China or Mongolia . The rostral bone and flared jugals are already present in all of these forms , indicating that even earlier ceratopsians remain to be discovered . 
 In clade @-@ based phylogenetic taxonomy , Ceratopsia is often defined to include all marginocephalians more closely related to Triceratops than to Pachycephalosaurus . Under this definition , the most basal known ceratopsians are Yinlong , from the Late Jurassic Period , along with Chaoyangsaurus and the family Psittacosauridae , from the Early Cretaceous Period , all of which <a href='uninflect' title='1'><mark>were</mark></a> discovered in northern China or Mongolia . The rostral bone and flared jugals <a href='drop_aux' title='2'><mark>are</mark></a>already present in all of these forms , indicating that even earlier ceratopsians remain to be discovered . 
In clade @-@ based phylogenetic taxonomy , Ceratopsia is often defined to include all marginocephalians more closely related to Triceratops than to Pachycephalosaurus . Under this definition , da most basal known ceratopsians r Yinlong , from the Late Jurassic Period , along with Chaoyangsaurus and the fam Psittacosauridae , from tdaEarly Cretaceous Period , alll of which was discovered in northern China or Mongolia . Thdaostral bone and flared jugals already present inallll of these forms , indicating dat even earlier ceratopsians remain to be discovered .