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American Association for the Advancement of Science

Slow rise for the dinosaurs

This scene depicts four dinosaurs and dinosaur precursors from the Hayden Quarry of northern New Mexico. The coexistence during the Late Triassic of the dinosaur precursors Dromomeron romeri (lower left) and a Silesaurus-like animal (bottom center), and the dinosaurs Chindesaurus bryansmalli (top center, with crocodylomorph in its mouth) and a coelophysoid theropod (upper right), indicates that the rise of dinosaurs was prolonged rather than sudden. Artwork by Donna Braginetz.

The Jurassic period, about 200 to 150 million years ago, was the heyday for the dinosaurs, which were the most common land animals during this time. (Does the movie title Jurassic Park ring a bell?)

Scientists have wondered for a long time how the dinosaurs became so successful and what happened to their ancestors, a group of reptiles called the "dinosauromorphs."

Skeletal reconstruction of dinosaur precursors and dinosaurs from the Hayden Quarry. The small animal in the lower left foreground is Dromomeron romeri, and the four-legged creature behind it is a Silesaurus-like animal. Dinosaurs include Chindesaurus bryansmalli (to the right, second from the back) and a Coelophysis-like theropod (animal in the far back).

Did something catastrophic happen to the dinosauromorphs that gave the true dinosaurs a "jump start" and allowed them to take over quickly?

A research team has now discovered a set of fossils showing that the dinosaurs' rise to dominance was a gradual climb rather than a sudden takeover. This finding suggests that dinosaurs and their older reptile relatives lived together for a long time before the dinosauromorphs disappeared and the true dinosaurs ruled the land.

Dinosauromorphs probably looked a lot like dinosaurs, but paleontologists (scientists who study fossils), have found important differences between the bones of dinosauromorphs and true dinosaurs. For example, the hip sockets of dinosauromorphs and true dinosaurs are quite different.

This tells the paleontologists that dinosauromorphs and true dinosaurs belonged to two different evolutionary groups.

In the 20 July 2007 issue of the journal Science, Randall Irmis of the University of California, Berkeley, and his colleagues describe a collection of fossils from New Mexico that contains variety of dinosaurs, including carnivorous theropods (that's the group to which T. rex belongs) and early dinosauromorphs. They also found fossils of fishes, amphibians and crocodile-relatives.

These fossils come from the end of the Triassic period, which preceded the Jurassic. The researchers think all these animals were living together for about 15 to 20 million years before the dinosaur heyday occurred in the Jurassic.