I’m still on the experiment.com-funded Mastodons of Unusual Size tour, and will be doing an additional post on that tomorrow. My inability to type and drive at the same time means I can only do a short Fossil Friday post.
The bone shown above is a mastodon astragalus, the most proximal of the ankle bones. The upper end of the astragalus articulates with the lower ends of the shin bones (the tibia and fibula), although in elephants the amount of movement at that joint is limited. Proboscidean ankle bones are much less complex than in most other animals, and are optimized for weight-bearing. Interestingly, it seems that some dwarfed elephants evolved a rather different ankle structure for walking in hilly terrain.
This astragalus was found at the East Dam of Diamond Valley Lake.