I got this at a place that sells bulk rocks. It was in a batch of rocks meant to be stepping stones, and I have no idea where in the world it's from. I know it's too messed up to ID well at all, I'm just wondering what general category of animal this is. My best guess is it's the outside of a large fish skull. There's nothing inside the slab, and the area in the right side of the pic looks like a shattered gill plate to me. It's about 8" across, and the rock it's in feels harder than most limestone I've come across. I didn't see any other fossils in the other slabs, and I doubt the stepping-stone-store is selling rocks from a rich fossil site.
Any idea? I'd be happy with just "yeah, that's definitely a fish" or something similar. It's staying outside as a stepping stone, as I'm of the opinion that all stepping stone paths should have a large and interesting stone to stop on, and it's definitely not in good enough shape to be worth displaying. It's neat, though.