Berkeley Hills - Cenozoic (mammal-age) fossil (beaver)

A drawing of a Miocene beaver skull found in the Siesta Formation in the Berkeley Hills.

The earliest land-animal fossils common in the San Francisco Bay region are Miocene mammals, including ancient horses, hippos, and beavers.

Fossils form when the remains of plants and animals, or traces of their lives such as footprints, are buried in sediments and preserved when the sediments are converted to sedimentary rocks. Sometimes the original animal or plant material is preserved, other times only an imprint or mold is preserved, and sometimes the original material is replaced by new material in the same shape as the original.

The Miocene beaver provides a good example of how ancient Cenozoic animals were similar to, but not quite the same as, modern animals. Look at the teeth in the drawing. Instead of the broad, flat teeth of a modern beaver, the Miocene beaver had curved, pointed teeth. Perhaps it should be called "saber-toothed beaver"?

Learn more about fossils at the USGS or by visiting the Paleontology Portal Web site.