Dinosaurs arrived hot on the tails of their more ancient relatives, reptiles called dinosauromorphs, a new study shows.
New dating of a dinosauromorph fossil layer in a geologic formation in Argentina reveal that it’s between 5 million and 10 million years younger than previously thought. That means dinosaurs and their dinosauromorph predecessors were separated by a gap of less than 5 million years, half the time previously thought. The timescale change suggests that dinosaurs evolved relatively rapidly, and that early dinosauromorphs probably weren’t part of life’s repopulation after a major mass extinction 252 million years ago, scientists report online December 7 in the Proceedings of the National Academy of Sciences.
“It’s a very exciting result,” says Paul Olsen, a paleontologist at Columbia University not involved with the study. “It’s changing our fundamental understanding of the origins of dinosaurs.”
Dinosaurs first appeared about 231 million years ago, and some of the oldest dinosaur fossils in the world can be found in Argentina. That’s where Randall Irmis and colleagues collected zircon crystals from a rock layer holding early dinosauromorph remains located just below a geologic layer with dinosaur fossils. The crystals, a product of volcanic explosions, contain the radioactive element uranium.
Over time, uranium decays into lead and the zircon begins to incorporate lead into its structure. The researchers measured the amounts of uranium and lead to date the dinosauromorph fossils to a 2-million-year span from 236 million to 234 million years ago. Previous dating methods pegged the time up to 10 million years earlier.
“We’ve shifted it into the Late Triassic” from the Middle Triassic Period, says Irmis, a paleontologist at the University of Utah in Salt Lake City.
Combined with ecological data, the new time tag indicates that dinosaurs had less of an impact on the ecosystem than researchers thought: While dinosaurs evolved quickly, fossil records from other formations indicate that dinosaurs came to dominate the landscape fairly gradually.
Irmis also notes that if the dinosauromorphs in the study had come from the Middle Triassic, they could have represented a sign of life’s recovery following the mass extinction at the end of the Permian Period, which preceded the Triassic. But because early dinosauromorphs appeared later in time and closer to the emergence of the dinosaurs, the new results show that the dinosauromorphs “really have nothing to do with that,” he says. Now, there’s “a gap in the fossil record that we need to fill” in order to understand the recovery of species after one of Earth’s most massive extinction events.
Editor’s Note: This story was updated December 15, 2015, to correct the impression that the dinosaur and dinosauromorph fossils were found in the same rock formation.