Ascent of dinosaurs more gradual than previously believed

New fossil discoveries dating from about 215 million years ago showed some of the earliest dinosaurs lived for millions of years side by side with related animals long seen as their ancestors and precursors, scientists said on Thursday.

Many scientists had thought these reptiles -- very much like dinosaurs, but more primitive -- died out around the time of the appearance of the first true dinosaurs, which were dog-sized beasts not giants, roughly 230 million years ago.

"When dinosaurs first evolved, they were not very common and they were pretty small," said Randall Irmis of the University of California-Berkeley, who worked on the study.

"So they're not the dominant predators or creatures on land at all during most of the Triassic. And it's only really until the Jurassic when they really explode in diversity and reach these huge sizes that we're so familiar with," Irmis added.

Scientists previously hypothesized that the first dinosaurs quickly out-competed their more primitive cousins, known as "basal dinosauromorphs," condemning them to extinction. But the new findings indicate that any such competition was prolonged.