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10-Dec-2009

Contact: Science Press Package
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American Association for the Advancement of Science

Triassic dinosaur illuminates early dino evolution



Based on an analysis of how Tawa relates to other early dinosaurs, researchers hypothesize that dinosaurs originated in what is now South America and soon after diverged into ornithischians (like Triceratops), sauropodomorphs (like Apatasaurus) and theropods (like Tyrannosaurus rex), before dispersing across the Triassic world more than 220 million years ago. The theropods evolved into modern-day birds, although Tawa split off from the ancestral branch early on and was not a direct bird ancestor.
[Zina Deretsky, National Science Foundation]

A newly discovered, early dinosaur from New Mexico--a two-legged carnivore that belongs to the same lineage that later produced T. rex--suggests that the first dinosaurs spread widely around the world, perhaps originating from South America.

Compared to the fossil record from the later Jurassic and Cretaceous periods, the picture of early dinosaur life in the Late Triassic, around 230 million years ago, is quite sketchy. We know that the dinosaurs had split into three main groups, Theropoda, Sauropodomorpha and Ornithischia by this time, but the fossils from this period are generally rare, fragmentary and incomplete.



The dinosaur-bearing rocks at Ghost Ranch, northern New Mexico.
[Image courtesy of Sterling Nesbitt]

Sterling Nesbitt of the American Museum of Natural History and Lamont-Doherty Earth Observatory at Columbia University and colleagues now describe several nearly complete skeletons of a new dinosaur from the Late Triassic, which they named Tawa hallae, after the Hopi word for the Puebloan sun god (Tawa) and amateur paleontologist Ruth Hall.

The dinosaur was about 2 meters long and belonged to the group of dinosaurs called theropods.

The researchers also analyzed the evolutionary relationships among Tawa hallae and other known Triassic dinosaurs. The complexity and diversity of these relationships implies that the North American theropods were probably not "endemic," meaning they probably lived on other continents too. The study suggests that the earliest dinosaurs may have originated in what is now South America and then spread around the world.

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This research appears in the 11 December issue of the journal Science.