Here's a couple bones I saw in a private collection over winter break, and these left the collector and myself stumped. I have no idea what on earth this thing is, but it looks mammalian.
Things to note: The concave part does look like an articulation, and the bones appear to be symmetrical. The larger of the two has some mud pellets cemented to it, so ignore those.
The part of the rock unit this is from is latest Miocene-Pliocene, and is generally devoid of sirenians. The usual suspects are pinnipeds and cetaceans.
I have seen a similar element at UCMP associated with an odontocete ulna, but nothing was said about the weird element.
Does this look obvious to anyone? I apologize for the colorful bedspread that served as my backdrop: