What it means to be a mammal: New clues from tiny fossil described in Science

May 24, 2001

195 million years ago, a tiny furry animal weighing no more than a large paper clip scurried in the shadow of the dinosaurs. A painstaking examination of its 12 millimeter-long fossil skull in the 25 May issue of the journal Science suggests that this miniature mammal relative is the closest known relative to the living mammals, and possessed crucial mammalian features forty million years earlier than previously recognized in the fossil record.

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 species don't show complete separation of the middle ear bones from the jaw.

What makes Hadrocodium different in this respect? A clue lies in the name that Luo and colleagues bestowed on this animal: Hadro, meaning "full," and codium, meaning "head." Hadrocodium has a large brain vault compared to other living mammals with its same skull size. Since the bones of the mammalian middle ear stop growing in size earlier than the brain, the Science authors suggest that expanded brain size may have pushed the ear bones away from the jaw, contributing to their detachment.

"It appears that some of the changes in the jaw and ear of mammals were completed before the appearance of Hadrocodium, and that Hadrocodium represents the final step in the separation between the middle ear and the mandible," says Luo.

Hadrocodium 's big brain isn't simply a matter of overall increased volume, notes Luo. He says that a recent CT scan of the fossil shows enlargement in specific areas of the brain, such as the olfactory lobes.

The Science researchers also compared 90 skeletal features in Hadrocodium, other early mammal fossils, and the living mammals, and their analysis suggests that the new species is the closest known relative, or sister-taxon, to living mammals.

"Hadrocodium could be our distant cousin, an early mammal that existed alongside the ancestor of living mammals. Or it could be our great-great grand uncle, closely related to living mammals but not in our direct lineage. Or Hadrocodium could be the direct ancestor of living mammals. The fossil evidence can't distinguish between these three possibilities. But we are satisfied to know that Hadrocodium is the sister taxon to all living mammals," says Luo.

Luo and colleagues estimated Hadrocodium 's mass (around two grams) by comparing its skull size with skull size in other mammals. The two-gram estimate ranks Hadrocodium as one of the smallest mammals, and the smallest discovered from this early period of mammal evolution. Its tiny size and distinctive teeth suggest that it was an insectivore, say the Science authors.

There are large size differences among the insectivorous, mammal-like species from the early Jurassic, indicating that the earliest-known mammals occupied a wide range of ecological niches.

"Recent discoveries around the world suggest that there's more diversity among early mammals than we thought," notes Science co-author Alfred W. Crompton of Harvard University.

The fossil was discovered in 1985 in the fossil rich beds of the Lower Lufeng Formation in China, but its significance was hidden under the sediments encasing the skull. After meticulous preparation to remove the sediments, the researchers finally realized that its anatomy was completely different from any other mammal known from the time period.

The 25 May issue of Science includes a related Perspective article, "New Clues About Mammal Origins," by André Wyss at the University of California, Santa Barbara.
The other members of the research team include Ai-Lin Sun at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Science, Beijing. This research was supported in part by NSF, NGS, the Carnegie Museum, NIH, and Harvard University.

American Association for the Advancement of Science

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