How did the dinosaur become the dinosaur? Somewhere along the line, the ancestor of dinosaurs diverged from the ancestor of crocodiles, a momentous split in the evolution of vertebrates that ultimately set the stage for the age of dinos. But the details of that split remain mysterious, thanks to a dearth of fossils of early dinosaur relatives. Enter the newly identified 247-million- to 242-million-year-old Teleocrater rhadinus, a close relative of dinosaurs that also happened to walk on all fours and share some key features with the ancestors of crocodiles. These shared features, the authors say, suggest that it’s time to rethink what we thought we knew about dinosaurs’ earliest ancestors.
“We’ve been waiting a long time to find fossils like this that fit in this part of the family tree,” says Randall Irmis, a paleontologist at the Natural History Museum of Utah in Salt Lake City, who was not involved in the work. “This has pretty big implications for how we understand the early evolution of dinosaurs.”
Some 251 million years ago, at the end of the Permian period, a mass extinction wiped out most of life on Earth. In its wake arose a group of egg-laying reptile precursors called archosaurs, the common ancestors of dinosaurs, flying reptiles known as pterosaurs, and crocodiles. At some point during the next period, the Triassic, pterosaurs and dinosaurs split off from the crocodile lineage.
Those two different lineages, avian versus crocodilian, have long been identified by their types of ankle joints. Dinosaurs and pterosaurs all have a version of a hinged, birdlike ankle, rather than the crocodilelike ankle with ball-and-socket joint.
But exactly what early dinosaurs and their closest relatives looked like has been something of a mystery, because few fossils exist from the dawn of the dinosaurs. And many of the fossils that do exist, collected perhaps decades to a century ago, languish unidentified in museum drawers.