Based on an American Association for the Advancement of Science press release

What is nearly 200 million years old, furry, weighed less than a paper clip and scurried beneath the feet of dinosaurs? A team of fossil-finders, led by researchers at Pittsburgh's Carnegie Museum of Natural History, suggest the answer may include one of your relatives - a distant cousin of modern mammals.

Classified as a new species, the newly discovered miniature mammal is the closest known relative to living mammals. It displays crucial mammalian features - a large brain and detached ear bones - yet it is forty million years older than any mammalian fossil previously found.

A team of U.S. and Chinese researchers led by Zhe-Xi Luo of the Carnegie Museum of Natural History identified and named the early Jurassic fossil as a new species, Hadrocodium wui - Hadro, meaning "full," and codium, meaning "head."

Compared even to today's living mammals with the same overall skull size, Hadrocodium had a large brain vault.

Anatomical features of Hadrocodium's skull also suggest that its middle ear bones were separated from its lower jaw, a key step in the evolutionary transition from reptiles to mammals. Luo and his colleagues suggest that expanded brain size may have pushed the ear bones away from the jaw, contributing to their detachment.

The fossil was originally uncovered 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 animal known from the time period.

Before the discovery of Hadrocodium, mammals were not known to have appeared on Earth until 160 million years ago.

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.

"Mammals have complex chewing mechanisms, as compared to non-mammals, including reptiles and dinosaurs. The features related to the mammalian feeding … are the most crucial and diagnostic features of all mammals because they are fundamental to the mammalian biology," noted Luo, describing the tiny skull and its matching teeth. For mammals, "these are considered some of the universally accepted identifiers."

"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," Luo adds.

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

Luo and his colleagues 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," says Luo. "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."

Luo and colleagues estimated Hadrocodium's mass (around two grams, or seven-tenths of an ounce) by comparing its skull size with the skull size of 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, says Luo.

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 Luo's co-investigator Alfred W. Crompton of Harvard University.

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