Oct. 6, 2010 -- It was 250 million years ago, and the world had just been through hell. Something -- perhaps an asteroid impact, perhaps a spate of volcanic eruptions darkening the skies worldwide -- had wiped out 70 percent of the life on earth.
But scientists now report they have found the footprints of small dinosaurs, preserved in fossilized soil, made less than 2 million years later -- a blink of an eye in geological terms. They are the oldest evidence of dinosaurs ever found.
"This would have been a post-apocalyptic world," said Stephen Brusatte, a young palentologist from Columbia University and New York's American Museum of Natural History, who made the find along with two colleagues. "The landscape would have been mostly barren, so there was a great opportunity for new species to emerge."
Brusatte and his fellow scientists, Grzegorz Niedzwiedzki and Richard Butler, say they found the footprints a year ago in what is now a limestone quarry in central Poland. There are no fossilized bones to go with them, but they can tell from their size and shape that the animal was about the size of a modern house cat -- fleet of foot, with larger legs in back than in front, foraging for food on a vast, muddy flood plain.
The scientists have named it Prorotodactylus, and they published their description of it in a British scientific journal, Proceedings of the Royal Society B.
"The neat thing is that the site is just after the extinction," Brusatte said, "raising the tantalizing possibility that the dinosaurs arose as a direct response to the opening created after this greatest mass extinction of all time. Ten years ago, we thought that the rise of the dinosaurs was completely unconnected."
Prorotodactylus would have been a rare creature on that flood plain, the remains of which are pockmarked with hundreds of footprints of other, larger species -- ancestors of crocodiles and various amphibians. "They still were not the dominant species you think of when you picture T-rex and later dinosaurs like that," said Brusatte in an interview with ABC News.
Oldest Dinosaur Footprints Ever Found
But 4 million years later, the researchers say they see footprints of much larger dinosaurs, 6 or 7 feet long, standing on their hind legs. The dinosaurs gradually became the world's dominant animals, surviving two more extinctions before finally being wiped out 65 million years ago, perhaps by another asteroid hit.
Scientists have learned to extrapolate about the dinosaurs from limited evidence -- the size of a footprint, the length of the animal's stride, the distance between the left and right feet, how much the feet sank into the soil.
Brusatte said they can tell Prorotodactylus was a dinosaur because "it had three parallel toes, not splayed out like a crocodile's or a lizard's, and its ankle joint is a simple hinge, not a ball joint like we have."
They can tell its date by layering in the ground; deeper fossils are older fossils. The mass extiction of 252 million years ago, known as the Permian-Triassic Boundary, left a dark, distinctive layer of soil.
"You're always learning new things," said Brusatte. "We're right on the leading edge of dinosaur lineage."