The Early Jurassic fossil was identified and named as a new species, Hadrocodium wui , by a team of U.S. and Chinese researchers led by Zhe-Xi Luo of the Carnegie Museum of Natural History. Anatomical features of Hadrocodium 's skull suggest that its middle ear bones were separated from its lower jaw, a key step in the evolutionary transition from reptiles to mammals.
Hadrocodium 's well-preserved skull shows that the unique skeletal features of mammals probably evolved step by step, and were in place long before the appearance of the living mammals, say Luo and colleagues.
The well-known transition from mammal-like reptiles to mammals includes changes in the jaw, teeth, hearing structures, and brain size. In reptiles, for instance, the lower jaw consists of several bones, and the three bones homologous to the mammalian middle ear are attached to both the jaw and the cranium. In mammals, the lower jaw consists of a single bone, and three of the "reptilian" jaw elements lose their attachment to the jaw to become the mammalian middle ear bones.
All these features add up to an animal with sensitive hearing and a strong jaw hinge for more elaborate and powerful chewing, says Luo. But did all these features evolve at once?
Some of Hadrocodium 's contemporaries, early mammal-like species such as Sinoconodon and Morganucodon, possess the characteristic single jaw hinge of living mammals. But unlike Hadrocodium, these
Contact: Cherita Gonzales
American Association for the Advancement of Science