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Early reptile fossil discovery gives clues to dinosaur evolution

Scientists have identified a new species of ancient reptile that fills a critical gap in the fossil record of dinosaur relatives.

The discovery means that current models of dinosaur origins will need to be revised, as some features that were believed to characterise dinosaurs evolved earlier than previously thought.

Teleocrater rhadinus was a carnivorous reptile about two to three metres long, with a long tail and neck, that walked on four legs held under its body. It lived about 245 million years ago, around the time that dinosaurs started to appear.

'Teleocrater is hugely exciting because it blows holes in many of our classic ideas of dinosaur origins,' says Dr Richard Butler of the University of Birmingham, co-author of the study published in Nature.

'Dinosaurs were amazingly successful animals. It's natural to want to know where they came from and how they became so dominant.'

The research was undertaken by palaeontologists from a number of institutions, including the Museum, which has housed the key specimen of this new animal since the 1950s.

Origins of dinosaurs, birds and crocodiles

Teleocrater is an example of an archosaur - a successful group of animals that appeared about 251 million years ago, at the start of the Triassic Period.

The group includes a wide range of species - some extinct and some still living - including all dinosaurs, pterosaurs, birds and crocodilians. Despite their many differences, all archosaurs descend from a common ancestor and share some common features.

Teleocrater appears in the fossil record just after the archosaurs split into two main lineages - a bird branch (leading to pterosaurs, dinosaurs and eventually birds) and a crocodile branch (eventually leading to today's alligators and crocodiles).

A simplified depiction of relationships within the archosaur group, including Teleocrater rhadinus 
© Richard Butler, University of Birmingham

Clues to dinosaur ancestors

Teleocrater appears soon after this split, in the earliest part of the Triassic, and sits on the bird branch of the evolutionary tree. By examining its anatomical features, palaeontologists have gained a better idea of what dinosaur ancestors looked like.

Teleocrater holds a number of surprises for palaeontologists, who until now have had to rely on other, slightly later, dinosaur relatives. It now looks as if those species were highly specialised with atypical features, and therefore might not be good starting points for tracing the general line of evolution that led to dinosaurs.

For example, the early dinosaur relatives found so far were all small animals that were adapted to run on their hind legs only. By contrast, Teleocrater was bigger, at two to three metres in length and 10 to 30 kilogrammes in weight, and was a sturdy four-footer.

Reconstruction of the skeleton of Teleocrater rhadinus. The total length is two metres. 'Teleocrater' is derived from Greek and refers to its closed hip socket ('Teleos', finished or complete and 'krater', bowl or basin), and 'rhadinos' (slender) refers to the slender body plan. © Scott Hartman

Teleocrater possesses a number of features only previously seen in archosaurs on the bird branch of the family tree or observed in animals with a crocodile lineage.

It had a flexible, crocodile-like ankle joint, unlike the more fixed and hinge-like ankle found in later bird-branch species. Cross-sections of its bones indicate that it grew more like a dinosaur, with growth early in life that was faster than a crocodilian relative. It also had depressions on the roof of the skull for enlarged, dinosaur-like jaw musculature.

This combination of traits places some of the features previously thought to characterise dinosaurs earlier on the bird branch of the archosaur evolutionary tree. This shows that the features have more ancient origins.

New Teleocrater discoveries

Although a partial Teleocrater skeleton has been part of the Museum collections since the 1950s, recent discoveries of new fossils have improved scientists' picture of the reptile. This allows researchers to fill in some of the details not available from the Museum's specimen.

Sterling Nesbitt (left) and Christian Sidor (right), co-authors of the study, excavating the remains of Teleocrater rhadinus and other animals in southern Tanzania in 2015 © Roger Smith

The type specimen of Teleocrater - on which the species name is based - was found in Tanzania in 1933 during an expedition led by palaeontologist Francis Rex Parrington from the University of Cambridge.

The fossils were then studied by Alan Charig, one of Parrington's former students, during the 1950s. Charig went on to become a curator at the Museum, bringing Teleocrater with him.

Charig went to Tanzania in search of more fossils in the 1960s, but he was unable to find other Teleocrater material. Some critical bones were particularly elusive - they would have helped him determine whether Teleocrater was more closely related to crocodilians or to dinosaurs.

Charig died before he was able to complete his studies.

The discovery of additional fossils by a US-led team in Tanzania in 2015, combined with a re-examination of the original fossils at the Museum, finally helped reveal the surprising relationship between Teleocrater and its dinosaur cousins.

Alan Charig (1927-1997) studied Teleocrater rhadinus for decades and named the species

'My colleague Alan Charig would have been thrilled to see one of "his" animals finally being named and occupying such an interesting position in the tree of life,' says Museum palaeontologist Prof Paul Barrett.

'Our discovery shows the value of maintaining and reassessing historical collections. Many new discoveries like this one can be made by looking through museum collections with fresh eyes.'

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