Scientists Find Bones of Possible Earliest Ancestor

May 24, 2001 -- A paper clip-sized furry animal that munched on insects and scurried in the shadow of dinosaurs 195 million years ago could well be the ancestor of all mammals, scientists say.

"Hadrocodium could be our distant cousin, it could be our great-great grand uncle, or Hadrocodium could be our direct ancestor," said Zhe-Xi Luo, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh who led a study about the creature that is appearing Friday in the journal Science.

Tiny Body, Big Brain

The tiny, fossilized skull of the shrew-like animal was found in southwest China in 1985 and, until now, had remained unidentified. After using advanced CT scanners and electron microscopes to analyze the animal's delicate bones, Luo and colleagues named the creature Hadrocodium, which means "full head" in Greek.

The animal's name refers to its brain cavity that appears to be unusually large (6 millimeters wide) for its tiny skull that measures a mere 8 millimeters in width. The researchers report the animal's big cranium was its most mammalian feature and suggest it may not only have been used for thinking, but also for keen smelling and hearing.

Luo and colleagues found the animal had large olfactory lobes that were likely useful for sniffing out insects. The large brain cavity might also explain the formation of another important feature in the animal's skeleton: its ear bones.

One way mammals are unique from reptiles is the way they have three ear bones that are separate from their jaw. In reptiles the lower jaw is made up of several bones, including the small bones that mammals use for hearing.

The single lower jawbone in mammals also makes for stronger, more powerful chewing. Luo suspects that Hadrobodium likely fed on insects and worms — but nothing tougher — since its teeth were sharp, but also fragile.

Luo suggests that an expanded brain cavity in Hadrobodium may have been what pushed the ear bones of Hadrocodium away from its lower jaw, helping their detachment and thereby making the animal more like modern mammals.

"It appears that some of the changes in the jaw and ear of the 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," said Luo in a statement.

Great Uncle or Grandpa?

Scientists believe that 280 million years ago a group of animals called "mammalian reptiles" broke from the group that became dinosaurs. About 80 million years later, another branch occurred, leading to a lineage of small mammals that likely darted warily around the feet of dinosaurs. It wasn't until dinosaurs died out 65 million years ago that mammals finally flourished.

Luo says Hadrocodium, estimated to weigh only 2 grams, could represent either a direct ancestor of today's mammals, or it could have belonged to a "cousin" or "sister" branch of animals that were related to the original mammals but eventually died out.

"No one signed any birth certificates in early Jurassic," said Luo. "So, scientifically, we can't be positive of its lineage."

Andre Wyss, a geologist at the University of California at Santa Barbara who reviewed Luo's study in Science, says the analysis of the tiny creature is "really important."

"It's the best-known fossil that could be an ancestral mammal," he said. "So it can help us figure out our lineage and how modern mammals evolved."