Early dinosaur relative sported odd mix of bird, crocodile-like traits

While researching fossils in a museum in 2007, Sterling Nesbitt noticed one partial skeleton that was hard to place. Though the reptile — at the time, unofficially called Teleocrater rhadinus — was thought to be a dinosaur relative, it was an oddball. At about 2 meters long, it was larger than other close relatives, walked on four feet instead of two, and had an unusually long neck and tail. Since the skeleton was missing some key bones, it was hard to know where the creature, found in Tanzania in 1933, fit within Archosauria, the group that includes crocodiles, birds and dinosaurs.

Nesbitt, himself on the way to Tanzania for a dig, couldn’t shake thoughts of the strange fossil. “It would be nice if we found more,” the vertebrate paleontologist, now at Virginia Tech, remembers thinking.

Now, a decade later, he and colleagues have done just that, discovering three additional partial skeletons of T. rhadinus — including bones missing from the original specimen. The more complete picture of T. rhadinus provides the first good glimpse of a pivotal moment of dino history.

About 250 million years ago, Archosauria split into two branches: birdlike creatures (including dinosaurs) and crocodilians. Paleontologists have had a hard time finding the earliest members of the “bird” branch that lived within the first few million years after that split — and well before the origin of true dinosaurs (SN: 5/21/11, p. 22).

T. rhadinus, which lived about 245 million years ago, fills that gap, Nesbitt and colleagues report online April 12 in Nature. With an odd mix of mostly birdlike — but some surprisingly crocodilian — features, the reptile is revising researchers’ view of early dinosaur relatives.

When a femur was unearthed on a 2015 dig in Tanzania, Nesbitt saw that muscle-attachment marks on the bone were like the ones he saw in the museum specimen, confirming he had found the same species. Dinosaurs have similar markings on their femurs. Then, in the pile of bones Nesbitt’s team hauled back, he discovered ankle bones and parts of the skull that the original fossil lacked.

The skull had a depression thought to be unique to dinosaurs. T. rhadinus’ dent means that the feature evolved earlier than thought. The skull, as well as the femur, place the creature on the “bird” branch of Archosauria.

But the ankle bone’s hinge resembled a crocodile’s. So the common ancestor of birds and crocodilians must have had this type of ankle, Nesbitt says, with the bird group later evolving different ankle characteristics.

“We know very little about the early evolutionary history of dinosaurs and their closest relatives, but discoveries like Teleocrater are helping change that picture,” Nesbitt says. Much of what scientists thought they knew is not correct, he says, or is much more complicated.