Around 250 million years ago, a large group of ancient reptiles named archosaurs diverged into two distinct lineages. One branch evolved into dinosaurs and birds and the other branch gave us the crocodiles and alligators we know today, plus a host of now-extinct relatives. A new fossil discovery, named Teleocrater rhadinus, is a species from the dinosaur/bird lineage that marks the oldest known cousin to the dinosaur and fills a large gap in our fossil history.

"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," says Ken Angielczyk, associate curator of fossil mammals at The Field Museum.

Stretching between six and 10 feet (1.8 and 3 m) long, the Teleocrater not only exhibits features later seen developed in dinosaurs, but also shares much in common with its distant crocodilian relatives. Significantly Teleocrater's ankle joints flex up and down, as well as rotate side to side. The later dinosaur and bird lineage shows this feature to have disappeared with both dinosaur and bird ankle joints only displaying hinge-like up and down motions.

Other previously discovered early examples of the dinosaur/bird archosaur lineage displayed very dinosaur-like characteristics, so this discovery fills a fascinating gap illustrating a compelling evolutionary step.

"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," explains Angielczyk.

The discovery also validates a nearly 80-year-old unpublished paleontological mystery after the first Teleocrater fossils were found in Tanzania in the 1930s. British paleontologist Alan Charig originally coined the name Teleocrater in his 1956 Ph.D thesis after examining the original fossils, but the unusual creature never really fit in with any evolutionary lineage and Charig didn't publish any further work on the matter.

This original fossil discovery lacked ankle bones, and for much of the 20th century remained a strange paleontological mystery until a new team returned to the original Tanzanian site and unearthed several new specimens in 2015. The new remains allowed scientists to finally formally confirm Charig's theory, as well as connect the species to its rightful place on the archosaur family tree.

The team is set to return to the dig site to try to find the final missing parts of the Teleocrater skeleton and ultimately shed further light on the earliest dinosaur relatives.

The finding was recently publish in the journal Nature.

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