Dinosaurs Evolved Rapidly After First Relatives Appeared

The first dinosaur relatives may have emerged up to 10 million years later than thought, then evolved rapidly to rule the world.

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.

The discovery, reported today in the Proceedings of the National Academy of Sciences, challenges the idea that the dinosaurs rose to dominance after the end-Permian extinction event - the largest mass extinction event in history 252 million years ago that wiped out so many of the large creatures that had existed before them.

"We thought that these rocks that included these early dinosauromorphs were recording the eventual recovery of ecosystems from this extinction, where ecologically things are finally getting back to normal and diversifying," Dr Irmis said.

"But now that we're moving those rocks much later in time, they're really too late to have anything to do with that recovery and now there's this gap in the record that we need to go out and fill."

Australian palaeontologist Dr Steven Salisbury from the University of Queensland said the finding does suggest the transition to dinosaur dominance happened relatively quickly, but the question remains as to why it happened so fast.

"The idea was the first dinosaurs didn't really get a stronghold until everything else had been taken out of the picture," he said.

"This indicates that dinosaurs getting a leg up so to speak didn't happen till quite a bit later."

The more precise dating technique is yet to be applied to other large fossil formations in other parts of the world, which may yield more answers to the question of how the dinosaurs evolved, Dr Irmis said.

"It's exciting to me to know that it's just a signal that there's so much more to figure out, and if we get a chance to date these other formations, think of what we'll learn then."

Shown is an artist's impression of animals that lived 235 million years ago in northwestern Argentina. Fossils found in this region include ancient mammal relatives Dinodontosaurus (left background) and early dinosaur relatives Lewisuchus (right background) and Lagerpeton (right foreground).