Fossil Indicates Bird, Crocodile Family Trees Split Earlier Than Previously Thought

Posted on May 18, 2011

Xilousuchus sapingensis

The only known specimen of Xilousuchus sapingensis has been reexamined and is now classified as an archosaur. Archosaurs, characterized by skulls with long, narrow snouts and teeth set in sockets, include dinosaurs as well as crocodiles and birds.

The Xilousuchus sapingensis fossil was unearthed in China in the 1970s. The creature, which died about 247 million years ago. was originally thought to be a distant relative of both birds and crocodiles. Researchers now think the creature comes from the crocodile family tree after it had already split from the bird family tree.

The new examination dates the X. sapingensis specimen to the early Triassic period, 247 million to 252 million years ago, said Sterling Nesbitt, a UW postdoctoral researcher in biology. That means the creature lived just a short geological time after the largest mass extinction in Earth's history, 252 million years ago at the end of the Permian period. The evidence, he said, places X. sapingensis on the crocodile side of the archosaur family tree. This indicates that the early members of the crocodile and bird family trees evolved earlier than previously thought.

Nesbitt says, "We're marching closer and closer to the Permian-Triassic boundary with the origin of archosaurs. And today the archosaurs are still the dominant land vertebrate, when you look at the diversity of birds. Archosaurs might have survived the extinction or they might have been a product of the recovery from the extinction."

Nesbitt also says, "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."

Image: Sterling Nesbitt

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