New Mammal Fossil Sheds Light on Teeth Evolution

Anne Casselman
for National Geographic News
October 31, 2007
A newly discovered species of fossilized mammal from the Jurassic era shows that the basic tooth template shared by all mammals today evolved independently at least twice in the past.

The find also adds to a growing body of evidence suggesting that early mammals were much more diverse than previously thought.

Fossilized skeletal remains of the new species, Pseudotribos robustus, were found recently in 165-million-year-old lakebeds in the Inner Mongolia region of northern China. (See a map of the find location.)

From the creature's build and makeup, paleontologists believe that the 4.7-inch-long (12-centimeter-long) creature was a very strong digger that ate insects and plants.

But the biggest news is its choppers.

"This thing is very advanced in terms of its tooth structure," said Richard Cifelli, a paleontologist at the University of Oklahoma in Norman, who was not part of the study.

"It has departed considerably from the ancestral pattern where it could only cut up things; now it can grind things up."

This is the same dental adaptation that is believed to have blossomed in today's mammal lineages. The advent of the cut-and-grind tooth is generally considered the driver for the vast diversity of mammals alive today.

But since Pseudotribos robustus belongs to a different and long-lost lineage, it must have evolved the cut-and-grind tooth independently. (Related: "'Popeye' Jurassic Mammal Found, Had 'Peculiar Teeth'" [March 31, 2005].)

This is an example of a process known as convergent evolution.

"It shows that this key feature ... evolved in two completely different ways but with the same functional outcome," said Hans-Dieter Sues, associate director for research and collections at the Smithsonian National Museum of Natural History in Washington, D.C. He was also not involved in this work.

The study appears in this week's issue of Nature and was partly funded by the National Geographic Society, which owns National Geographic News.

You Are What You Eat ... With

Today two types of mammals dominate the world: marsupials and placentals. Together they account for the vast majority of species diversity seen in mammals.

While they differ vastly in body shape and lifestyle, the niche they fill in nature boils down to their teeth.

"It has long been supposed that the adaptation that allowed them to be so successful was a multifunction molar that can both cut and grind," Cifelli explained.

"For mammals you are what you eat with," added Zhe-Xi Luo, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, and one of the study's authors.

"Essentially all mammalian diversity is broken down pretty much by the different dental design that we each have."

Whether it be hyenas that can grind bone or horses that eat hay, all teeth in today's mammal species are derived from this basic dental template.

Colorful Crew

The finding also suggests that early mammals were beginning to diversify much earlier than previously thought.

The first two-thirds of mammalian history takes part in the age of the dinosaurs. During that time it has been thought that mammals remained in much the same form: small, furry, nocturnal, insect-eating animals that skirted around dinosaurs. (Related: "Dinosaur Extinction Spurred Rise of Modern Mammals, Study Says" [June 20, 2007].)

"Our general view of what happened is that they didn't really go into any extravagant ecological niches until dinosaurs became extinct," Cifelli said. "Now we're finding that wasn't the case."

Previous fossil finds at the same lakebed show that mammals were already gliding around and swimming as far back as about 165 million years ago.

Along with the new find, the discoveries suggest that mammals underwent tremendous diversification during the middle of the Jurassic period, Cifelli said.

"We're seeing a host of skeletal adaptations that say, hey, mammals were doing these wild and crazy things—they weren't just lying around in their little hidey holes," he added.

"[Pseudotribos robustus] helps to show that the earliest mammals coexisted with the dinosaurs are far more diverse than we ever have imagined," study author Luo said.

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