A tiny Jurassic period mammal, whose specialized cutting and grinding teeth shed light on the evolution of the earliest mammals, has been discovered by a team of Chinese and American paleontologists that includes the acting co-director of the Carnegie Museum of Natural History.

The new mammal, Pseudotribos, just 5 inches long and weighing an estimated 20 to 30 grams, looks like a mole with prominent teeth. It was found in the 165 million-year-old lake beds of northern China in 2004 but was reported for the first time in the Nov. 1 issue of the international scientific journal Nature.

Dr. Zhe-Xi Luo, curator of vertebrate paleontology at the Carnegie and co-director of the Museum of Natural History, said the significance of the discovery lies in the mouth of the mammal.

"The story of the earliest mammals is a story of their teeth," Dr. Luo said. "By tracing their evolution in the rich fossil records of the Mesozoic, we get to understand how these cutting and grinding teeth evolved over and over again."

Many mammals have very specialized feeding adaptations because of their different teeth. Zebras eat plants, for example, and cats eat meat.

However, all are descendants of some ancient mammal that lived with the dinosaurs in the Mesozoic era, from 251 million to 66 million years ago, and have tribosphenic teeth -- capable of both cutting and grinding -- with the cutters or shearers in front of the grinders. That configuration allowed them to be more versatile in their feeding and is important for early mammal diversification.

The new Pseudotribos, an insectivore that also fed on worms, is a pseudo-tribosphenic mammal, meaning its teeth are reversed, with the grinder teeth in front of the cutter teeth, Dr. Luo said. Those teeth show that similar cutting and grinding teeth structures evolved several times in the Mesozoic era, and are considered by many paleontologists to be more advanced than the primitive mammal teeth that were limited to cutting.

"The pseudo-tribosphenic teeth and the true tribosphenic teeth are great examples of convergent evolution," Dr. Luo said, "and a great manifestation of how dental and feeding adaptation can be achieved by different lineages of mammals."

Pseudotribos lived on the ground and had strong limbs capable of powerful digging. It co-existed with freshwater arthropods, salamanders and other mammals, and several dinosaurs. The fossil is in the Institute of Geology, Chinese Academy of Geological Sciences in Beijing.