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.
By Benchill, Nov. 8, 2007
This is a reconstruction of X. sapingensis, based on the fossil.Credit: Sterling Nesbitt