This week's Fossil Friday specimen is a femur (thigh bone) from a mastodon, collected from the West Dam of Diamond Valley Lake.Like many of the Western Science Center specimens, this femur is only partially prepared, and still sits in its field jacket. The posterior side of the bone is shown above, with the back of the knee joint visible on the left. As this is the right femur, there should be a large ball joint (the femoral head) in the upper right, that would articulate with the hip socket. Unfortunately the femoral head was broken off and not preserved.The preserved part of this bone is roughly 70 cm long. That's actually pretty small for a mastodon, large specimens of which can have femora more than a meter in length. This was probably a sub-adult mastodon, although the partial fusion of the knee joint to the rest of the bone suggests that it wasn't too young.