Crocodile/Bird Split Pushed Back in Time due to New Study

A re-examination of a early Triassic vertebrate fossil found in China during the 1970s originally thought to be an ancestor of both crocodiles and Aves (birds)  has led to scientists concluding that this fossil represents an ancestor of crocodiles only.  This suggests that the crocodile/bird evolutionary split took place earlier than previously thought – perhaps as far back as in the Palaeozoic more than 250 million years ago.

The only known specimen of Xilousuchus sapingensis has been re-examined by a Washington University based researcher and it has been re-classified as a member of the Archosauria, characterised by skulls with long, narrow snouts and teeth set in sockets.  After the Permian mass extinction event, it was the Archosaurs that quickly diversified to become one of the dominant types of land vertebrate on the planet.  The Archosaurs gave rise to the crocodiles, birds and to the dinosaurs.

The new examination dates the fossil of . sapingensis to the very early Triassic period (247 million to 252 million years) commented Sterling Nesbitt, the University of Washington postdoctoral researcher responsible for this new study.  The new study places this particular reptile on the crocodile side of the Archosaur family tree.

An Illustration of the Remains of Xilousuchus sapingensis

Picture Credit: Sterling Nesbitt

The drawing shows the skull and neck vertebrae from the fossil Archosaur. The areas coloured “white” in the diagram show the portions of the illustration that represent actual fossils.  The extended neural spines from the neck vertebrae numbers six onwards, suggest a sail-like structure on the back, similar in form and function to the sail on a Pelycosaur such as Dimetrodon.

The researcher Nesbitt stated:

“We are marching closer and closer to the Permian-Triassic boundary with the origin of the Archosaurs, and today the Archosaurs are still the dominant land vertebrate, when you look at the diversity of birds.”

This new study could sharpen the debate amongst palaeontologists as to the origins of the Archosauria.  Did they exist before the Permian mass extinction, surviving the extinction event or were only Archosaur precursors around in the Late Permian geological period?

Nesbitt added:

“Archosaurs might have survived the extinction or they might have been a product of the recovery from the extinction.”

The research is published this week in the online journal “Earth and Environmental Science Transactions of the Royal Society of Edinburgh.

Co-authors are Jun Liu of the American Museum of Natural History (New York) and Chun Li of the Institute of Vertebrate Paleontology and Palaeoanthropology (Beijing, China). Nesbitt did most of his work on the project while a postdoctoral researcher at the University of Texas at Austin.

The X. sapingensisspecimen consisting of a skull and ten vertebrae, was found in the Heshanggou Formation in northern China, an area with deposits that date from the early and mid-Triassic period, from 252 million to 230 million years ago, and further back, before the mass extinction event that brought the Permian geological period to an end.

The fossil was originally classified as an Archosauriform, a “cousin” of Archosaurs, rather than a true Archosaur, but that was before the discovery of more complete early Archosaur specimens from other parts of the Triassic period. The researchers examined bones from the specimen in detail, comparing them to those from the closest relatives of Archosaurs, and discovered that X. sapingensis differed from virtually every Archosauriform.

Among their findings was that bones at the tip of the jaw that bear the teeth likely were not down-turned as much as originally thought when the specimen was first described in the 1980s. They also found that neural spines of the neck formed the forward part of a sail similar to that found on another ancient Archosaur called Arizonasaurus, a very close relative of Xilousuchus found in Arizona.

An Picture of Arizonasaurus

Picture Credit: Everything Dinosaur

The family trees of birds and crocodiles meet somewhere in the early Triassic and Archosauriforms are the closest cousin to those Archosaurs, Nesbitt said. But the new research places X. sapingensis firmly within the Archosaur family tree, providing evidence that the early members of the crocodile and bird family trees evolved earlier than previously thought.

Commenting on the appearance of this ancestral crocodile, Sterling said:

“This animal is closer to a crocodile, but it’s not a crocodile. If you saw it today you wouldn’t think it was a crocodile, especially not with a sail on its back.”

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