Armoured archosaur opens new chapter on dinosaur evolution

Palaeontologists from the American Museum of Natural History have described a new species of archosaur, the dinosaur-like reptile which predated dinosaurs.

Publishing in the Zoological Journal of the Linnean Society, researchers describe the species Mambachiton fiandohana from a 235 million-year-old fossil found in Madagascar.

The newly-described Mambachiton is significant as the earliest known member of the bird line of archosaur evolution and because it has armour, a rare feature in bird-line archosaurs.

Archosaurs are dinosaur-like creatures that predate dinosaurs. These reptiles evolved into two main branches: the bird line (including pterosaurs and dinosaurs) and the crocodilian line (including crocodiles, alligators, caimans and gharials).

“Dinosaurs were latecomers to the Triassic reptile party,” says lead author Dr Sterling Nesbitt, an associate professor of geosciences at Virginia Tech and research associate in the American Museum of Natural History’s Division of Palaeontology.

“They showed up well after many dinosaur-looking reptiles were established across our planet.” 

The Mambachiton was around 1.5 to 2 metres long, weighing 10 to 20 kilograms.

The species had an extensive series of bony plates, or armour, covering its backbone.

Reconstruction of the newly-described archosaur species
Reconstruction of Mambachiton fiandohana. Scale bar = 25 cm / Credit: Nesbitt et al

This kind of armour is common in crocodilians, but rare in bird line archosaurs except for dinosaurs like stegosaurs, anklyosaurs and titanosaur sauropods.

The newly described species shows armour was an ancestral characteristic of bird line archosaurs that was lost in evolution for a time, later reappearing in dinosaurs.

The finding demonstrates that features like armour in dinosaurs and their ancestor archosaurs depends on interactions with the ecosystem. Features are retained when needed, and disappear when no longer required.

Previous studies have identified other features such as skull structure and feathers in archosaurs, that have reappeared much later in dinosaurs and pterosaurs.

The museum’s Frick Curator of Fossil Mammals, Dr John Flynn, led the team of researchers that discovered the fossil. 

He says “this discovery documents the importance of the southern hemisphere fossil record in understanding this important period of the Triassic, when dinosaurs were first appearing”.

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