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Teeny-weeny creature pushes back timeline on earliest mammals

Friday, May 25, 2001

By Byron Spice, Science Editor, Post-Gazette

Paleontologists say a tiny, furry creature that scurried amidst Jurrasic-era dinosaurs more than 195 million years ago is a surprisingly close relative to living mammals and could be a common ancestor to all mammals.

The newly discovered species, called Hadrocodium wui, likely weighed less than a dime. But Zhe-Xi Luo, a paleontologist at the Carnegie Museum of Natural History who led the study of its fossil skull, said the extinct creature has a number of modern features not previously seen in an animal this old.

Though Hadrocodium was no more than a tenth of the size of a mouse, its brain case was large for its body size, compared with other mammal-like creatures of the time. Hadrocodium means "full head."

Also, its hinged jaw was a single bone, unlike the jaws of its reptile ancestors, and it apparently had a three-bone middle ear that was separate from the jaw.

"We were very puzzled about how to interpret such advanced features in such an old fossil," Luo said, explaining why he and colleagues at Harvard University and the Chinese Academy of Sciences took more than 10 years to analyze the fossil. They are reporting their findings in today's issue of the journal Science.

Andre Wyss, a mammalian paleontologist at the University of California at Santa Barbara who wrote an accompanying commentary, said the discovery will provide a better understanding of the timing of changes that occurred as mammals evolved from reptiles.

Mammals flourished only after the dinosaurs became extinct 65 million years ago. But all three major groups of living mammals -- the egg-laying monotremes (platypus), and the live-bearing marsupials (kangaroos) and placentals (mice) -- had diverged long before that time. Though Hadrocodium is technically not a mammal, its existence 195 million years ago suggests that mammals are at least that old, Wyss said.

Paleontologists always used to search for the common ancestor of all mammals, Wyss said, but it has become obvious that there's no way of proving what animal might be that ancestor. So now the tendency is to identify creatures such as Hadrocodium as close relatives, he said, and not worry about whether it is a direct ancestor.

"It could be our ancestor," Luo said. "It could be our great-granduncle, or our distant cousin." There's no real way to know. "But it's a really close relative to living mammals."

Xiao-Chun Wu, a reptile paleontologist, had no such suspicions when he unearthed the fossil in 1985 from the famed Lufeng Basin in southwestern China. That formation has produced prodigious numbers of fossils of reptiles, of mammal-like reptiles and early mammals. The tiny fossil didn't excite Wu -- it simply looked like a fragment of a lizard-like fossil that was likely to be of minor, if any, importance.

Wu didn't study the fossil until three years later. But as he began to chip rock away, he realized the minuscule fossil wasn't a fragment, but a complete skull. Recognizing it wasn't a reptile skull, he took it to his mentor at the Chinese Academy, vertebrate paleontologist Ai-Lin Sun.

She, in turn, contacted Luo, who set up a collaboration with Alfred Compton of Harvard's Museum of Comparative Zoology. They particularly needed the help of William Amaral, a preparator at the Harvard museum who is recognized as an expert in removing rock from tiny fossils.

With a fossil the size of the skull -- less than a half-inch long and about one-third of an inch wide -- "one minor error and an anatomical feature might get knocked off," Luo explained. So work progressed slowly. As Amaral exposed part of the fossil, the scientists would study it, then more fossil would be exposed, followed by more study.

One of the first surprises was what was not found. In reptiles and some mammal-like animals, the ear is encased in the jaw. But as rock was removed from Hadrocodium's jaw, the jaw groove associated with the reptilian ear was missing. Instead, the jaw was smooth, like a modern mammal's.

This suggests that the middle ear bones were separate from the jaw, though the tiny ear bones weren't found. It could be that the evolution of the creature's large brain case had forced the ear to separate from the jaw, Luo said.

Luo has since taken the skull to Penn State University, where researchers will perform intense CT scans of the skull. Eventually, those scans should make it possible to reconstruct the external contours of the brain. Already, it appears that Hadrocodium's brain wasn't simply big, but that particular structures, such as the olfactory lobes, were enlarged.

Like mammals, Hadrocodium had teeth that allowed it to chew, rather than just grab and swallow food like a reptile. Its teeth were too delicate to chew vegetation, Luo said, so it would have been an insect eater.

Hadrocodium is the smallest of any mammal-like creature from the era of the dinosaurs and would be one of the smallest mammals ever. The smallest living mammal today is the hog-nosed, or bumblebee, bat of Thailand, which at 2 grams is roughly the same weight as Hadrocodium.

"It's amazing that something so small and delicate could survive all this time," Wyss said.

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