This was found in Mineral Wells fossil park in North Texas. It's a huge borrow pit that's easily 20 feet deep in some spaces, with everything gathered down at the bottom. Most of the things inside are crinoids and brachiopods, with the rare trilobite. I wouldn't expect to find a bone from anything in there, but this really looks like something to me.
It's about 4cm long, and it feels like a pretty standard rock. The reddish-brown areas are smoother and slightly concave, and it looks to me like this was broken off of something. There's also an area on the back side where the same reddish-brown color is showing through just slightly, in a small ring about 1cm across that more or less lines up with the center space on the other side. I'd try to clear off some of what looks like matrix, but it's very hard, and I don't have the tools to deal with it.
Any ideas?