WASHINGTON (REUTERS).- Tiny footprints from Poland show that the first dinosaurs were extremely small animals that walked on four legs -- and probably only came to rule the world after a mass extinction knocked out many big reptiles, scientists said on Tuesday.

The 250-million-year-old footprints are the oldest evidence of dinosaurs, Stephen Brusatte of the American Museum of Natural History in New York and colleagues said.

The animal was about the size of a small domestic cat, they reported, and would have lived near rivers where larger crocodilians thrived.

"We describe the indisputably oldest fossils of the dinosaur lineage: footprints from the Early Triassic (around 250 million years old) from Poland," Brusatte's team wrote in Britain's Royal Society journal Proceedings of the Royal Society B.

"These footprints, along with those from two other slightly younger sites, provide important insight on the origin and early evolutionary history of dinosaurs. They indicate that the earliest dinosaur relatives were extremely small animals, with feet only a few centimeters (inches) long, that walked on four legs and were remarkably rare compared to contemporary reptiles."

Ironically, the fossils indicate that a mass extinction allowed the small and rare dinosaurs to proliferate -- an event mirrored 65 million years ago when an asteroid and volcanic events drove the dinosaurs into extinction and allowed mammals to thrive.

"Most radically, the new footprints only slightly postdate the greatest mass extinction of all time (Permo-Triassic event), suggesting that the origin of dinosaurs occurred in the immediate aftermath of this catastrophe," the researchers wrote.

They also found footprints dating back 246 million years made by two-legged dinosaurs called Sphingopus -- the oldest evidence of bipedal dinosaurs.

"For the first 20-50 million years of dinosaur history, dinosaurs and their closest relatives were living in the shadow of their much more diverse, successful, and abundant crocodile-like cousins," Brusatte said in a statement. "The oldest dinosaurs were small and rare."

(Reporting by Maggie Fox, Editing by Sandra Maler)