The first dinosaur relatives may have emerged up to 10 million years later than previously thought, then evolved rapidly into the animals that would take over the world, a new study suggests.
Researchers have used a relatively new dating technique to accurately determine the age of fossils of early dinosaur relatives - known as dinosauromorphs - found in a large collection in Argentina.
"If you met an early dinosauromorph in a dark alley, you'd think it was a dinosaur," said lead author and palaeontologist Dr Randall Irmis, curator of palaeontology at the Natural History Museum of Utah.
The Argentinian Chanares Formation includes fossils of dinosauromorphs such as the 70-centimetre-long Lagerpeton chanarensis that ran on its hind legs, and the even smaller Marasuchus dinosauromorph.
"Not only is this a classic fossil assemblage that's well known the world over for these early dinosaur relatives that are found in it, but it's also got these layers that are mixed with volcanic ash that we can date," Dr Irmis said.
By analysing the ratio of uranium to lead in zircon crystals in this volcanic ash, Dr Irmis and an international team of colleagues were able to precisely date when the zircon was formed and thereby establish an upper limit for the age of the fossils preserved within the ash-containing sediment.
"What we found was that this fossil layer was a lot younger geologically than we thought," Dr Irmis said.
"People previously thought it was somewhere between 240-245 million years old and we showed that it was about 235 million years old," said Dr Irmis, also an associate professor in the department of geology and geophysics at the University of Utah.
This places these early dinosauromorphs somewhere in the late Triassic period - much closer to when dinosaurs first appeared in the fossil record around 231 million years ago.