Dinosaurs’ gradual rise to dominance

Fossil finds in the southwestern United States suggest that dinosaurs didn’t quickly supplant the creatures they evolved from, as many paleontologists have assumed.

The first dinosaurs evolved from reptiles called dinosauromorphs about 235 million years ago. Until recently, scientists hadn’t uncovered any dinosauromorph fossils from much after that time, fueling speculation that dinosaurs had swiftly eclipsed their ancestors, says Randall B. Irmis, a paleontologist at the University of California, Berkeley.

Now, however, Irmis and his colleagues have unearthed remains of several known dinosauromorph species, and of a previously undescribed one, from the 215-million-year-old rocks of a quarry in north-central New Mexico. All these creatures lived alongside many types of dinosaurs whose remains are in the same rock strata, the researchers note in the July 20 Science.

The team’s findings suggest that dinosaurs and dinosauromorphs coexisted in some locales for between 15 million and 20 million years, says Irmis. Excavations in rocks of similar ages at fossil-rich sites in Arizona and Texas bolster the notion that the two reptile groups shared an ecosystem for significantly longer than had been recognized.