While the Pleistocene deposits from Diamond Valley Lake have lots of big impressive animals, small animals are actually much more numerous. The most common mammal in “The Valley of the Mastodons” is the pocket gopher!
While most of our pocket gopher (genus Thomomys) remains are isolated teeth, there are a few more substantial specimens including the partial cranium shown here. At top is the skull in dorsal (top) view, with the front of the skull to the left. Most of the back and left side of the skull is missing, but the front and part of the right side are in pretty good shape. The ring of bone on the right side of the skull (at the top of the image) is made up of mostly of the jugal and the zygomatic process of the squamosal. Together these bones form the outer margin of the orbit (eye socket) and the temporal fossa, an opening for the muscles that close the lower jaw.
Below is a lateral view, showing the right side of the cranium:
The long, curved shiny feature at the front of the skull (on the right side of the photo) are the incisor teeth. The presence of a single pair of enlarged, ever-growing incisors is a feature shared among all rodents. Unlike other gophers, in Thomomys the incisors are smooth and lack longitudinal ridges (one common name for Thomomys is the “smooth toothed pocket gopher”).
Here is the ventral (bottom) side of the skull:
While there’ some damage present here, and some cleaning is still needed, it looks like most of the tooth row is preserved. The complex tooth at the front of the row is the fourth premolar, while the large peg-like tooth that is partially sticking out of its socket is either the second or third molar.
Pocket gophers are still common in western North America. As burrowing animals, their preservation potential is pretty high; it’s a lot easier to become a fossil if you already live most of your life underground!