This artist's rendering, provided by the journal Science, depicts four dinosaurs and dinosaur precursors from the Hayden Quarry of northern New Mexico. ((Donna Braginetz/Associated Press))

Dinosaurs may have once ruled the Earth, but their rise was a more gradual process than previously thought, according to a new study based on evidence from a rich fossil find in the United States.

A team of paleontologists led by two graduate students from the University of California, Berkeley, has discovered that dinosaurs co-existed with species that were their ancestors for 15 to 20 million years.

Their findings, published in the Friday issue of the journal Science, run contrary to the belief that the dinosaurs rose to prominence by quickly replacing their rivals.

"Finding dinosaur precursors, or basal dinosauromorphs, together with dinosaurs tells us something about the pace of changeover. If there was any competition between the precursors and dinosaurs, then it was a very prolonged competition," said Randall Irmis, a graduate student at the University of California, Berkeley, and co-author of the report,in a statement.

Dinosaurs rose to prominence in the Late Triassic between 235 million and 200 million years ago and came to dominate the Earth in the Jurassic, between 200 million and 120 million years ago.

But pinpointing how they rose to power has been difficult because of the paucity of fossil specimens from the Late Triassic, both of dinosaurs and particularly of their ancestors. Up until 2003, when a team in Poland uncovered a creature named Silesaurus, no dinosaur predecessor had been discovered from the Late Triassic, the authors said.

But the team reported finding 1,300 fossil specimens, including several complete bones, from the Hayden Quarry at Ghost Ranch, near the northern New Mexico town of Abiquiu.

At the quarry, Irmis and fellow graduate student Sterling Nesbitt found evidence of both early dinosaurs and dinosaur precursors, as well as bones of crocodile ancestors, fish and amphibians, all dating between 220 and 210 million years ago.

The dinosaurs and their ancestors uncovered at the quarry were quite small in comparison to the dinosaurs from the later Jurassic and Cretaceous periods such as the lumbering and long-necked sauropods or the ferocious therapods like Tyrannosaurus rex.

One dinosaur ancestor found at the quarry, the Dromomeron romeri, measured only three to five feet long and was possibly bipedal. Another unnamed specimen, about three times the size of Dromomeron, walked on four legs and ate plants with a beaked snout.