Friday, May 20, 2011

Xilousuchus is An Archosaur

A sail-backed suchian from the Heshanggou Formation (Early Triassic: Olenekian) of China. 2011. S. Nesbitt, et al. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 101: 271-284.

Art by S. Nesbitt
The only known specimen of the 247 million year old Xilousuchus sapingensis has been reclassified as an archosaur. The specimen – a skull and 10 vertebrae – was found in the Heshanggou Formation in northern China.

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.

Art by S. Nesbitt
Among their findings was that bones at the tip of the jaw that bear the teeth likely were not downturned 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.

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. link
This article is from a special issue of Earth and Environmental Science Transactions of the Royal Society of Edinburgh on Late Triassic Terrestrial Biotas and the Rise of Dinosaurs.