Early ornithischian dinosaurs: the Triassic record

Ornithischian dinosaurs are one of the most taxonomically diverse dinosaur clades during the Mesozoic, yet their origin and early diversification remain virtually unknown. In recent years, several new Triassic ornithischian taxa have been proposed, mostly based upon isolated teeth. New discoveries of skeletal material of some of these tooth taxa indicate that these teeth can no longer be assigned to the Ornithischia using unambiguous synapomorphies. The Triassic record of ornithischian dinosaurs now comprises only three probable occurrences: Pisanosaurus and an unnamed heterodontosaurid from Argentina, and an unnamed specimen from the uppermost Triassic of South Africa. This revised Triassic record suggests that ornithischians were not very diverse or abundant through the Triassic, and there are large gaps in the Triassic ornithischian fossil record. Moreover, traditional living analogues for interpreting the feeding ecology of early ornithischians from their tooth morphology are generally inappropriate, and “herbivorous” archosaur teeth such as those found in early ornithischians are not necessarily diagnostic of herbivorous feeding.