Earth and Environmental Science Transactions of the Royal Society of Edinburgh

Research Article

Evaluating hypotheses for the early diversification of dinosaurs

Randall B. Irmisa1

a1 Utah Museum of Natural History and Department of Geology & Geophysics, University of Utah, 1390 E. Presidents Circle, Salt Lake City, UT 84112–0050, USA Email:


Many hypotheses have been proposed for the rise of dinosaurs, but their early diversification remains poorly understood. This paper examines the occurrences, species diversity and abundance of early dinosaurs at both regional and global scales to determine patterns of their early evolutionary history. Four main patterns are clear: (1) sauropodomorph dinosaurs became abundant during the late Norian–Rhaetian of Gondwana and Europe; (2) Triassic dinosaurs of North America have low species diversity and abundance until the beginning of the Jurassic; (3) sauropodomorphs and ornithischians are absent in the Triassic of North America; and (4) ornithischian dinosaurs maintain low species diversity, relative abundance and small body size until the Early Jurassic. No one hypothesis fully explains these data. There is no evidence for a Carnian–Norian extinction event, but sauropodomorphs did become abundant during the Norian in some assemblages. No clear connection exists between palaeoenvironment and early dinosaur diversity, but environmental stress at the Triassic–Jurassic boundary is consistent with changes in North American dinosaur assemblages. Elevated growth rates in dinosaurs are consistent with the gradual phyletic increase in body size. This study demonstrates that early dinosaur diversification was a complex process that was geographically diachronous and probably had several causes.

Key Words:

  • Dinosauromorpha;
  • Early Jurassic;
  • Late Triassic;
  • Ornithischia;
  • Sauropodomorpha;
  • Theropoda