Early ornithischian dinosaurs: the Triassic record

Abstract

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.

Keywords:

• Ornithischia
• Triassic
• Pisanosaurus
• Revueltosaurus
• Dinosauria
• Archosauria

Acknowledgements

We thank Andrew Heckert (NMMNH), Sankar Chatterjee (TTUP), Patricia Holroyd (UCMP), Scott Williams (PEFO), Carl Mehling (AMNH), Jaime Powell (Instituto Miguel Lillo, Tucumán), Ricardo Martinéz and Oscar Alcober (Museo de Ciencias Naturales, San Juan), José Bonaparte (Museo Argentinas Ciencias Naturales, Buenos Aires), Ana Maria Báez (Universidad de Buenos Aires), Jorge Ferigolo and Ana Maria Ribeiro (Museu de Ciências Naturais, Fundação Zoobotânica do Rio Grande do Sul, Porto Alegre), and Max Langer (Universidade São Paulo, Ribeirão Preto) for access to collections within their care. Petrified Forest National Park and the Federal Recreational Act Fee Program provided funding to WGP for fieldwork in PEFO. Funding for RBI to travel to Argentina and Brazil was provided by H.D. Montgomery and the Samuel P. and Doris Welles Research Fund. Paul Olsen graciously provided the illustration of the Wolfville “ornithischian” specimen in Figure 5. Discussions with Kevin Padian, Max Langer, Claudia Marsicano, Nathan Smith, and Alan Turner were extremely helpful. Reviews by Paul Barrett and David Weishampel greatly improved the manuscript. This is UCMP contribution no. 1907 and PEFO paleontological contribution no. 13.