Mammal-like reptile skull from Mexico.

Mammal-like reptile skull from Mexico

One of the paleontologist's best weapons in the creation-versus-evolution debate is a family of mammal-like reptiles called Tritylodonts, which clearly show the movement from bones in the lower jaw of reptiles to the ears of mammals. A new fossil--representing the most advanced Tritylodont--found in northeastern Mexico doesn't add that much to the understanding of the reptile-mammal transition per se, but it does help scientists sort out the relationships between the seven or so Tritylodont family members.

James Clark and James Hopson of the University of Chicago discovered the skull of what they call a new species, Bocatherium mexicanum (named after the La Boca rock formation where it was found), which they believe dates to the middle Jurassic, roughly 180 million years ago. This would make the creature Mexico's oldest terrestrial vertebrate and the first mammal-like reptile to be found there. The researchers think that Bocatherium was a rodent-like herbivore that coexisted with early carnivorous mammals for 50 million years before becoming extinct in the late Jurassic some 150 million years ago. As reported in the May 30 NATURE, the Bocatherium skull has a snout structure--in which the bone holding the upper teeth in more primitive species has become cylindrical in shape--similar to two other species found in China and Great Britain, enabling the researchers to diagram how they various Tritylodonts are related to one another.

According to Clark, scientists are divided as to whether Tritylodonts or another reptilian family--the Ictieosaurs--are the true ancestors of mammals. Ictieosaurs, like mammals, are meat eaters, but Tritylodonts share a more similiar skeletal structure with mammals. Clark hopes that the Mexico site where Bocatherium was found will bear more fossils that will help explain how mammals came to be.