An international team of researchers has been uncovering fossils during archaeological digs in Inner Mongolia and has discovered some tiny fossilised mouse teeth which they believe are from an ancient ancestor of the birch mouse.

On the expedition team members Palaeontologist Yuri Kimura, from the South Methodist University (SMU) in Dallas, identified the new species of mouse from 17 tiny fossilised teeth. Discussing the discovery on the SMU Research's webpage she said, ''We are very lucky to have these. Palaeontologists 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''.

The prehistoric ancestor to today's birch mouse genus was previously thought to be around 8 million years old. This latest research indicates that it was potentially 17 million years old. In addition to proving the ancestors of this small rodent are much older than first thought, Kimura also challenges the belief that these ancestors migrated to Asia from North America. Her report on the investigations concludes that the ancestors actually dispersed from prehistoric Asia's grasslands and forests into North America using the Bering Land Bridge.

Rodents are some of the most abundant mammals found on earth and make up around 42 percent of the world's living mammals. Following the downfall of the dinosaurs over 65 million years ago, rodents dispersed around the world and evolved into many of the species we know today. However, only 1.5 percent of these modern rodent species can be traced back to the Early Miocene period (23 to 16 million years ago) or older.

This new species of birch mouse, named Sicista primus, has now been reported by Kimura in the scientific journal Naturwissenschaften (The Science of Nature). You can see images of Kimura's research and expedition on the SMU Research flickr site.

Top image: Birch mouse Credit: Dodoni.