Spotting

That right angle should give it away, you'd think. The only thing I've seen so far to approach it is the pectoral girdle (modified scapula or shoulder blade) of a turtle http://campus.murraystate.edu/academic/f... . But I haven't really looked at other vertebrates...yet.

If so it would be among the more bizarre advertisements ever. :) Anyway it does give a sense of scale... The size and shape of the bone make me think it might come from a turtle.

Maybe it's undergone some abrasion. I'll think about it. While it is good to put in an object to give a sense of scale probably better to use a coin, pocket knife ballpoint pen. This might be mistaken for an advert.