Early Mammal’s Jaw Lost Its Groove

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.

An artist’s conception of Hadrocodium Wui and its skull, with a paperclip to provide a sense of scale for the 2-gram insectivore. Mark A. Klinger/CMNH

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. “We just know that it’s more closely related to living mammals than anything else living at that time,” says Luo.

Dental wear on the teeth, among other evidence, indicates that the lilliputian skull belonged to a nearly full-grown or adult Hadrocodium. The animal would have been no longer than a paper clip and weighed only about 2 grams, making it the smallest so-called mammaliaform known from its time and rivaling the tiniest mammals of any period.

Hadrocodium‘s small proportions also reveal that there was more evolutionary diversity among early mammals than had been previously known. Its diminutive stature and sharp teeth almost guarantee that the animal’s diet consisted of insects, worms, or other small invertebrates, Luo notes. Even 195 million years ago, “in the early Jurassic period, mammals were occupying all available ecological niches,” he adds.

The volume of the ancient skull, although minuscule, is on par with skull-body proportions seen in modern mammals, says André R. Wyss, a paleontologist at the University of California, Santa Barbara. The tiny fossil might help scientists work out how today’s major groups of mammals are related and provide insight into what ancestral mammals looked like, he notes. The presence of advanced features in a skull from 195 million years ago indicates that the earliest stages of mammalian evolution unfolded even further back in the past.

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