WASHINGTON (Reuters) - The ascent of the dinosaurs to the throne of the animal kingdom may have been more gradual than previously believed, scientists said on Thursday.

This illustration shows four extinct animals -- two dinosaurs and two more primitive dinosaur relatives -- found at a site in northern New Mexico. Shown are the dinosaur precursors Dromomeron romeri (lower left) and a Silesaurus-like animal (bottom center), and the dinosaurs Chindesaurus bryansmalli (top center, with prey in its mouth) and a coelophysoid theropod (upper right). REUTERS/Science/Artwork by Donna Braginetz/Handout

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.

The newly found fossils from New Mexico dating from the Triassic period showed that the first dinosaurs co-existed with these animals -- “dinosaur wannabes,” as one scientist called them -- for perhaps 15 to 20 million years.

“For the first time, we’re finding the earliest dinosaurs and their closest relatives together,” paleontologist Kevin Padian of the University of California-Berkeley, one of the researchers, said in a telephone interview.

“That tells us that the transition to the beginning of the age of dinosaurs was not a very-rapid affair and that, therefore, it wasn’t instant competitive superiority.”

Irmis said these dinosaur precursors are not thought to have been direct evolutionary ancestors of the dinosaurs but rather having shared a close common ancestor.

NEWLY DISCOVERED BEASTS

The scientists discovered new dinosaur precursors including one 3 to 5 feet long called Dromomeron and another unnamed one about three times larger that walked on four legs and ate plants with a beaked snout.

Relatively small bipedal carnivorous dinosaurs also were found, including Chindesaurus, which measured about 6 feet (2 meters) long, as well as remains of an apparent close relative of the well-known Triassic dinosaur carnivore Coelophysis.

The fossils were found at the Hayden Quarry at Ghost Ranch, a site that over the decades has yielded many exquisite fossils. For example, hundreds of Coelophysis fossils were found in the 1940s at Ghost Ranch, making it among the best documented of all dinosaurs.

These early Triassic dinosaurs were not the bullies and behemoths that later appeared in the Jurassic period, which started around 200 million years ago.

In fact, they were mere pipsqueaks next to some of their nasty neighbors. The scientists found remains of crocodile-like phytosaurs up to 25 feet long, and a relative of the equally long and vicious four-legged predator Postosuchus.

The New Mexico site at the time these animals lived was a lush environment with a vast river system, flood plains and forests with towering large conifer trees.