Tiny fossil teeth discovered in Inner Mongolia are a new species of birch mouse, indicating that ancestors of the small rodent are much older than previously reported, according to paleontologist Yuri Kimura at Southern Methodist University in Dallas.
Fossils of the new species were discovered in sediments that are 17 million years old, said Kimura, who identified the new species and named it Sicista primus to include the Latin word for "first."
Previously the oldest prehistoric ancestor of the modern-day birch mouse was one that inhabited Inner Mongolia 8 million years ago.
Adding 9 million years to the ancestry of the rodent family that includes birch mice and jumping mice distinguishes this genus, Sicista, as a "living fossil," Kimura said. That places the genus among some of the most unique rodents on earth those whose ancestry spans 2 to 3 times the average, she said.
Kimura identified Sicista primus from 17 tiny teeth, whose size makes them difficult to find. A single molar is about the size of half a grain of rice. The teeth, however, are distinctive among the various genera of rodents known as Dipodidae. Cusps, valleys, ridges and other distinguishing characteristics on the surface of the teeth are identifiable through a microscope.
"We are very lucky to have these," Kimura said. "Paleontologists usually look for bones, but a mouse is very tiny and its bones are very thin and fragile. The teeth, however, are preserved by enamel. Interestingly, small mammal teeth are very diverse in terms of their structure, so from that we can identify a species."
Kimura reported the new species in the article "The earliest record of birch mice from the Early Miocene Nei Mongol, China" in the scientific journal Naturwissenschaften. Images of the expedition and fossil teeth are posted on the SMU Research flickr page. Kimura discusses the research in "Inner Mongolia yields 'living f
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Southern Methodist University