I've never seen a vertebra or bone like this. It's definitely bone because of the porosity on the sides. On the left of the picture there are two knobs poking out on the upper left and bottom left. I have no clue were these fit into and there's a depression between these knobs. I think vertebrae and bones are really neat because they all fit together like pieces in a puzzle and remind me of playing with legos when I was little. Props to anyone who can guess what this is but my bet would be some sort of whale.