Magazine article Science News

Early Mammal's Jaw Lost Its Groove

Magazine article Science News

Early Mammal's Jaw Lost Its Groove

Article excerpt

A tiny fossil skull found within 195-million-year-old Chinese sediments provides evidence that crucial features of mammal anatomy evolved more than 45 million years earlier than previously recognized.

The well-preserved fossil shows several characteristics of mammals, says Zhe-Xi Luo, a vertebrate paleontologist at the Carnegie Museum of Natural History in Pittsburgh. Most notably, there's no groove at the rear of the jawbone. This indicates that the three bones of the middle ear had separated from the ancient animal's mandible. This separation occurs in modern mammals but not in reptiles.

The jaw hinge of the skull also assumes an advanced form. A wide, mammal-style brain case gives the animal its genus name--Hadrocodium, which is Greek for full head--Luo says. The species name, wui, honors the paleontologist who discovered the specimen in 1985. Luo and his colleagues describe the new species in the May 25 SCIENCE.

Its discoverers almost mistook the corn-kernel-size skull for a bone fragment. A painstaking, grain-by-grain removal of the sediments encasing the fossil gradually revealed its distinct features. Six or seven lineages of ancient mammals lived about 195 million years ago. However, all of these had a grooved jawbone, one hallmark of ancient mammals that had only recently evolved from reptiles, Luo adds.

Although the Hadrocodium skull has several features of a mammal, Luo and his colleagues can't determine whether the animal's lineage evolved into one of the three known groups of modern mammals--placental mammals, marsupials, or the egg-laying platypus--or eventually went extinct. …

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