Curious fossil could rewrite early days of the dinosaurs

[vIs^♚]

                                                                     

    In this artist’s rendition of life in the middle Triassic, carnivorous Teleocrater rhadinus, newly identified as a very early ancestor of dinosaurs and pterosaurs, feasts on mammallike reptile Cynognathus.

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.

Indeed, Teleocrater isn’t a completely new discovery. A specimen was first unearthed in what is now Tanzania in the 1930s and sat in London’s Natural History Museum until 1956, when Ph.D. candidate Alan Charig (later a paleontologist at the museum) dubbed it T. rhadinus (referring to the shape of the animal’s hip and its slender body). Charig, who died in 1997 but is included as an author on the new paper, speculated that it was some sort of early dinosaur relative. But the fossil was in pieces, just bits of vertebrae and pelvis and limb, and difficult to place on the family tree.

Then in 2015, Sterling Nesbitt, a paleontologist at Virginia Polytechnic Institute and State University in Blacksburg, and a team of researchers headed back to southern Tanzania to take another look at the middle Triassic rocks where Teleocrater was first discovered. This time, the rocks—estimated to be about 247 million to 242 million years old—yielded several individuals of the same species. With that new bounty, the researchers were able to catalog many more of the creature’s features—enough to place it on the vertebrate family tree.

Teleocrater, Nesbitt and his co-authors report online today in Nature, belongs at the very base of the avian lineage that later gave rise to dinosaurs. It has a characteristic muscle scar on the upper leg bone that is found only in the avian lineage of birds and dinosaurs and is missing in crocodiles and their relatives. But Teleocrater also had a crocodilelike ankle, with a ball-and-socket joint. That suggests that the crocodile ankle came first, and the bird ankle evolved later.

That’s key, because the ankle joint has been used for decades as an indicator of avian versus crocodilian lineage, so Teleocrater must be close to the split between them. And in several respects, Nesbitt says, “Teleocrater looks more like the relatives of the crocodiles than the relatives of dinosaurs.” The carnivorous animal, which was roughly the size of a small lion, walked on all fours, its forelimbs and hindlimbs are similar in proportion, and the limbs themselves are pretty short relative to the length of the body.

Seemingly small details like these can produce ripples throughout paleontology collections, because they can help researchers properly classify fossils that had seemed to be walking contradictions. “These fossils exist in museums around the world, but until you find a keystone—something that helps you understand the full anatomy [of a species]—you won’t understand where these animals go on the tree of life,” Nesbitt says.

After identifying Teleocrater as an ancestor along the avian lineage, the authors could group it with several other difficult-to-place animals, including Dongusuchus and Spondylosoma, naming a new group of long-necked, carnivorous quadrupeds dubbed Aphanosauria (hidden or obscure lizards, in Greek). Aphanosaurs, they suggest, are the earliest group in the avian stem lineage to diverge from the crocodile lineage. And that suggests that these bird and dinosaur ancestors were far more diverse and widely distributed across the globe during the middle Triassic than once thought.

The find may also alter what paleontologists hunt for in the field, as well as how they understand existing collections, says Max Langer, a paleontologist at the University of São Paulo in Rio Claro, Brazil. “Now that we know the diagnostic features of this group of archosaurs, everybody working on middle Triassic rocks will be looking for something similar.”

Edited April 13, 2017 by vIs^♚