Apparently this is the fossil of the serpent in Genesis ... before God cursed it to crawl on its belly.
It reminds me of that bit in
The Name Of The Rose:
"Some time ago, in a German cathedral, I saw the skull of John the Baptist at the age of twelve."
"Really?" I exclaimed, amazed. Then, seized by doubt, I added, "But the Baptist was executed at a more advanced age!"
"The other skull must be in another treasury."