Inspired by our friend Jack Tseng's "What is this?" post, I've decided to place here a mystery bone and you all have to guess what it is. The winner will receive a short on-site tour of the Project 23 excavation site, or, if you are a Page Museum volunteer and have no prior knowledge of what it is, you will win a hearty handshake from the LME (Lone Male Excavator).
I want a specific answer, should be two parts: what bone is it? & what species (or I'll take family) is it?
Ideally, you should also say whether it is a right or a left and what developmental stage it is in (roughly how old was the animal).