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Monday, June 27, 2011

Early archosaur reclassified

A 250 million year old fossil that was thought to belong to a creature that lived before birds and crocodiles branched off from each other has now been found to belong securely to the crocodile branch of the tree, according to Sterling Nesbitt of the University of Washington and his colleagues Jun Liu of the American Museum of Natural History in New York and Chun Li of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. The researchers examined the fossil remains of Xilousuchus sapingensis and found that it has more in common with crocodiles than with birds.

As you can see from the cladogram below, birds and reptiles shared a common ancestor.  Somewhat more surprisingly, this diagram also shows that crocodiles are more evolutionarily similar to birds than they are to lizards, and that birds form a subgroup within reptiles. Creatures within the entire group of reptiles, including birds, are classified as archosaurs.

File:Tuatara cladogram.svg
1. Tuatara
2. Lizards
3. Snakes
4. Crocodiles
5. Birds
By Benchill, Nov. 8, 2007

The most interesting thing about this reinterpretation of the X. sapingensis fossil, is that it places the crocodile/bird divide further back in history. The largest extinction in Earth’s history (in which 95% of marine life and 70% of land dwellers perished) occurred at the end of the Permian period 252 million years ago, or just before X. sapingensis lived. Does this mean that archosaurs had diverged into bird and crocodile lines before that extinction event, or afterwards?  If before, it means that each of the subgroups shown above had to have survived the extinction event.  If after, the subgroups would have had to have separated from each other within a few million years.  And thus, scientific debate goes on.

This is a reconstruction of X. sapingensis, based on the fossil.
Credit: Sterling Nesbitt

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