Last week, I got to visit the Museum of Osteology in Oklahoma City. It's an amazing collection — well worth driving out of your way to see. I was expecting just a selection of different animal skeletons. The actual collection was a lot bigger and more awesome than I'd guessed it would be, and included some really nice exhibits on evolutionary adaptation, convergent evolution, deformed skeletons of both humans and animals, and the process of stripping a body down to a clean and shiny bone structure.
One of the things I found really fascinating was the skeletal features that you can't see just by looking at the outside of an animal. Take this Indian Rhinoceros, for instance. You'll notice that his horn is not a part of the skull. That's because the horn isn't really bone. The "horn" isn't a horn, at all.
Horns are made of bone. They're hard on the outside thanks to a thin layer of keratin — the stuff that makes up your fingernails and hair. But the majority of that material is living bone. Rhinos, on the other hand, have "horns" that are almost 100% keratin. They're really thick bundles of protein fibers.
That's a pretty well-known fact. But it's one thing to know it intellectually, and another thing entirely to see the place where that keratin horn attaches to the animal's actual bone structure. The intricate, lacy network of spongy bone was absolutely fascinating to me. It reminded me of the way ceramic artists will attach one piece of clay to another by scoring little cuts into both pieces and then applying a layer of thin, goopy clay that cements the cuts together as it dries. Seeing the rhino skull really drove home the idea that the "horn" was something else entirely. The horn was attached to the bone. It wasn't part of the bone.