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.

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