Before dinosaurs roamed the Earth, a group of reptiles sometimes called pre-dinosaurs were present. It was originally thought that these creatures lived about 245 million years ago, but new research suggests that they may have lived about 10 million years later. This would put them much closer in time to the first dinosaurs.
Using radiometric dating, scientists have now estimated that these pre-dinosaurs were present about 236 million years ago, just 5 million years before the first dinosaurs evolved. This means that dinosaurs evolved much quicker than the 10-15 million years previously thought and inhabited many of the environments previously held by their ancestors.
“They appear and they start to diversify quite rapidly,” said Claudia Marsicano, head author of the study. “They evolved quite successfully.”
These reptiles were not much different than the early dinosaurs. They would have walked upright, been similar in size and probably had a similar diet. The key difference though is in the hip joint.
“The dinosaurs themselves have a hole in their hip-sockets, so where the leg-bone fits into the hip, there’s a complete hole through the bone, whereas in the dinosaur precursors and the other reptiles, there’s just a shallow depression as opposed to a hole,” said Randal Irmis, who participated in the study.
Early dinosaurs are thought to have even co-existed with some of the pre-dinosaurs. Fossil evidence suggests that these reptiles may have lived as late as 201 million years ago, well into the time of dinosaurs.
This discovery comes from a group of pre-dinosaur fossils found in the Chañares Formation in Argentina. Scientists were able to use a mineral known as zircon to estimate the age of the sediment layer that the fossils were found in.
When zircon minerals form, usually in volcanic eruptions, they trap small amounts of uranium. A certain portion of the trapped uranium is radioactive and decays over time to lead at a rate known to scientists.
Because of this, the approximate time that the zircons were formed can be determined, and since these zircons were deposited at about the same time as the fossils were, they’ll be about the same age. This is how the estimate of the age of these dinosaur ancestors was found.
Little is known about how the first dinosaurs evolved. Learning that dinosaurs evolved much faster than previously thought, in the space of just 5 million years, will be valuable in determining just how dinosaurs evolved from earlier reptiles. Mapping out events such as this is important in understating the important stages of dinosaur emergence and evolution.