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