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 e