He did find this, however, buried in the arena sand:
|Right hind foot and tail of some large rodent|
It was pretty creepy, and it didn't want to come out. Originally, I thought it was a full carcass, some burrowing animal that had burrowed in the not-frozen arena sometime recently and died there.
After our work was done, I went back with a shovel and discovered it was only the back half of a carcass. (Muskrat? Possum? Without a head and discolored from the stone dust, it was hard to identify...note: further research reveals it to have been, almost certainly, a muskrat.) So I suspect Mr. or Ms. Rodent of Uncomfortable Size was deposited in our arena for safe-keeping by some clever canid (Coyote? Fox? Wild dog?) sometime fairly recently. Sorry, Fido, your snack has been relocated.