It looks like a fern made of teeth. It looks like the fossilized remains of a circular saw. It looks old, nasty, and sharp, but most of all it looks bizarre.
Alexander Petrovich Karpinsky first found this fossil over one hundred years ago east of the Russian Urals. Clues surrounding the fossil helped him identify the fossil as a species of shark, which he dubbed Helicoprion. But reconstructing the fossil proved problematic. The rest of the animal was missing.
Sharks and their relatives don’t have bony skeletons, like most fish and tetrapods. Instead, their body is built on a frame of cartilage. Cartilage is softer, spongy and, most importantly, does not fossilize well. Hoping to find a fossil made of cartilage is about the same as hoping to find a fossil with the skin still attached. Not likely. The tooth whorl was the only hard, bony thing left to fossilize after the animal died and so it sat, alone, with no clues left to place it on the shark’s body.
Normally, you can figure out where a bone sat based on what the bone looked like. Leg bones are long and skinny, ribs are flat and curved. Skulls are instantly recognizable. But the whirling buzzsaw was too bizarre to place definitively. Karpinsky suggested that it could have risen out of the animal’s top jaw like an elephant’s trunk. Others thought it belonged on the back, or trailing behind the tail. Then there were other questions. Was it flexible like a whip or was it hard like a sawblade? Was it a weapon? A display? The next hundred years were a long slog of careful analysis, discovery, and arguments.
Over time, the general consensus focused on the whorl as a feeding structure, firmly set in the creature’s lower jaw. Models still varied back and forth, however, on where in the jaw the structure lay. The most recent model was published only this year.
Whether this model solidifies as Helicoprion‘s true form remains to be seen, but the evidence seems solid. CT-analysis of other closely related species supports the idea that the toothy whorl was a solid, fused mass that replaced the fish’s more traditional lower jaw. Teeth, continually produced at the back of the jaw, pushed the entire saw forward – the older, blunter teeth eventually disappearing underneath the jaw entirely. As the animal grew the saw would rotate again and again until a massive spiral had built up. The team that developed this model also suggests that Helicoprion wasn’t a true shark, but a member of a closely related group known as ghost sharks.
If this model holds water, then scientists may have finally figured out the story behind one of the weirdest fossils ever found.
Links: Scientific American