The Mystery Solved.

The mystery bone introduced in this post has been identified by an anonymous commenter as the pelvic bone of an ostrich. All the images of skeletal ostriches I could find seem to confirm this. I compared it with an emu, and it's definitely a better match for the ostrich.

The complete, unbroken bone is actually even more fantastical than what we saw:
Check out that crazy curly bit on the lower left: it actually connects in the middle, just like your pelvis. But different. (Image from this site, where you can buy an ostrich pelvis of your very own.)

There used to be ostrich farms in the area our Mystery Bone was discovered, so this find isn't as bizarre as it may seem.

Nice work, Anonymous (you know who you are).