Early Croc-Like Cousin of Dinosaurs Discovered

The new species Teleocrater rhadinus hunting a cynodont, a close relative of mammals. (Credit: Museo Argentino de Ciencias Naturales)

(CN) – The discovery of an ancient crocodile-like reptile might fill a critical gap in the fossil record of dinosaur cousins.

Roaming Earth more than 245 million years ago, Teleocrater rhadinus had a long neck and tail and walked on four crocodile-like legs, suggesting the new species appears in the fossil record just after a large group of reptiles known as archosaurs branched off into two distinct evolutionary lines – one of which eventually became dinosaurs.

Writing in the journal Nature, a team of researchers details the new species, which was discovered in Tanzania in the 1930s but never formally classified until more fossils were found in 2015.

“We found fossils that we thought might be from T. rhadinus, but it wasn’t until we were back in the lab that we realized we’d found something really amazing,” said co-author Ken Angielczyk, a curator at the Field Museum in Chicago.

Paleontologist F. Rex Parrington discovered the first T. rhadinus fossils, and gave them to Alan J. Charig – a former curator at the Natural History Museum of London – to study the specimens in the 1950s. However, the fossils lacked crucial bones like the ankle bones, so Charig was unable to determine the exact lineage of T. rhadinus before his death in 1997.

During his studies, Charig tried to establish whether the fossils were more closely related to dinosaurs or crocodiles – the two groups that split off from archosaurs about 250 million years ago during the Triassic Period.

With both bird-like and crocodile-like features, T. rhadinus shows up right after archosaurs split into a bird-like branch – which lead to dinosaurs – and a crocodilian branch.

Scientists excavate the remains of Teleocrater rhadinus and other animals in southern Tanzania in 2015. Christian Sidor (left), Sterling Nesbitt (middle left), Kenneth Angielczyk (upper right), Michelle Stocker (lower right). (Photo by Roger Smith)

“The discovery of T. rhadinus fundamentally changes our ideas about the earliest history of dinosaur relatives,” said the paper’s lead author Sterling Nesbitt, a paleontologist from Virginia Tech. “It also raises far more questions than it answers.”

T. rhadinus was between 6 and 10 feet long including its long neck and tail, and probably weighed between 20 and 65 pounds. The authors believe the species probably looked similar to today’s monitor lizards, such as the Komodo dragon with its elongated neck.

“Surprisingly, early dinosaur relatives were pretty profoundly not dinosaur-like,” Angielczyk said.

The ankle joints of the new species are of particular importance. The new species could rotate its ankle from side to side as well as flex it up and down, while the ankle joints of birds and dinosaurs could only do a hinge-like up-and-down motion. The ankle bones of the T. rhadinus would have given it a more crocodile-like walk.

“T. rhadinus 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,” Angielczyk said.

While not a direct ancestor of dinosaurs, the authors say the new species is the oldest known dinosaur cousin.

“Scientists generally don’t love the term ‘missing link,’ but that’s kind of what T. rhadinus is: a missing link between dinosaurs and the common ancestor they share with crocodiles,” Angielczyk said.

 

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