Early Dinosaur "Cousin" Surprisingly Crocodile-Like

If you asked paleontologists what the earliest ancestors of dinosaurs were like, most would put their money on the animals resembling miniature dinosaurs: small, meat-eating animals that walked on two legs. But in a paper published in Nature, scientists describe the earliest known dinosaur relative: a six-foot-long lizard-like carnivore called Teleocrater rhadinus.

“Teleocrater has unexpectedly crocodile-like features that are causing us to completely reassess what we thought about the earliest stages of dinosaur evolution,” says Ken Angielczyk, associate curator of fossil mammals and one of the paper’s authors. “Surprisingly, early dinosaur relatives were pretty profoundly not dinosaur-like.”

But Teleocrater didn’t look much like a dinosaur. In appearance, Teleocrater probably resembled the monitor lizards alive today. It was between six and 10 feet long, including its long neck and longer tail, and it probably weighed between 20 and 65 pounds. It likely stood around two feet tall at the hip. And while some of its features, like the jaw muscle attachments at the back of its skull, clearly put it in the bird-like archosaur camp with the dinos, it has a lot in common with its more distant crocodilian cousins. Most notably, its ankle joints could rotate from side to side as well as flexing up and down, while dinosaurs’ and birds’ ankle joints could only do a hinge-like up-and-down motion. That would have given Teleocrater a more splayed, crocodile-like gait rather than an ostrich-like trot. 

The fact that Teleocrater has lots of features in common with crocodilians gives us a new picture of how dinosaur-like features evolved. “We used to think that many of the distinctive features of bird-line archosaurs evolved very quickly after they diverged from the crocodile line because early bird-line archosaurs like Marasuchus, Dromoeron, and Lagerpeton were small and very dinosaur-like,” explains Angielczyk. “However, Teleocrater shows us that bird-line archosaurs initially inherited many crocodile-like features from the common ancestor of all archosaurs, and that the 'typical' bird-line features evolved in a step-wise fashion over a longer period of time. Scientists generally don’t love the term ‘missing link,’ but that’s kind of what Teleocrater is: a missing link between dinosaurs and the common ancestor they share with crocodiles.” 

“We found fossils that we thought might be from Teleocrater, but it wasn’t until we were back in the lab that we realized we’d found something really amazing,” says Angielczyk. The team’s paper officially gave the animal Charig’s name, Teleocrater rhadinus, meaning “slender complete basin,” a reference to the animal’s lean build and closed hip socket.

“The discovery of such an important new species is a once-in-a-lifetime discovery,” says Sterling Nesbitt of Virginia Polytechnic Institute and State University, the paper’s lead author. “Teleocrater fundamentally changes our ideas about the earliest history of dinosaur relatives.”