Psittacosaurids were basal to almost all known ceratopsians except `` Yinlong '' and perhaps the Chaoyangsauridae . Psittacosaurides were basal to almost all known Ceratopsians except `` Yinlong '' and perhaps the Chaoyangsauridae . Almost all Basal - Ceratopsians except `` Yinlong '' and perhaps the Chaoyangsauridae were known .