Reappraisal of early dinosaur radiation

Article By:

Irmis, Randall B. Utah Museum of Natural History, University of Utah, Salt Lake City, Utah.

Nesbitt, Sterling J. Jackson School of Geosciences, University of Texas, Austin, Texas.

Parker, William G. Division of Resource Management, Petrified Forest National Park, Arizona.

Last reviewed:2011

DOI:https://doi.org/10.1036/1097-8542.YB110001

The origin and early diversification of dinosaurs occurred during the Late Triassic Period (235– 201.3 million years ago). By the beginning of the Early Jurassic (201.3–176 million years ago), dinosaurs were a dominant part of terrestrial ecosystems across the globe in terms of both diversity and abundance. Vertebrate paleontologists by the mid-1990s thought that the origin and rise of dinosaurs was fairly well understood. However, the discovery of new specimens and reevaluation of previous hypotheses during the past few years have resulted in a much better understanding of the early diversification of dinosaurs and their close relatives. Dinosaurs are defined as the most recent common ancestor of Triceratops and birds (specifically, the house sparrow, Passer domesticus), and all their descendants. Thus, by definition, dinosaurs consist of two major lineages: the Ornithischia and the Saurischia (Fig. 1). Familiar ornithischian dinosaurs include a variety of forms, such as the armored thyreophorans (stegosaurs, ankylosaurs, and their relatives), the dome-headed pachycephalosaurs, horned ceratopsians (including Triceratops), and the “duck-billed” hadrosaurs. All known ornithischians were herbivorous or omnivorous. Saurischian dinosaurs consist of two major lineages: the sauropodomorphs, which include the gigantic long-necked quadrupedal sauropods and their relatives; and the theropods, a bipedal, largely carnivorous group that includes birds (Fig. 1). Studies on the origin and initial diversification of dinosaurs focus on the earliest members of the ornithischian, sauropodomorph, and theropod groups. Renewed study of the earliest members of these groups and the closest relatives of dinosaurs (here called “basal dinosauromorphs”) has revealed new insights into the patterns and processes related to the rise of dinosaurs (Figs. 1 and 2).

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