One of the central problems of palaeontology is that hard parts of organisms are often very well preserved, soft parts not so well. Thus, shells and bones are common in the fossil record; skin, muscles and other soft tissues are not. Shark teeth, for instance, are really common. They’re very hard, and sharks continuously replace their teeth during their lives, so the fossil record is littered with them. Unfortunately, shark skeletons are made of cartilage, not bone. Cartilage does not fossilise wel, so the great, great majority of shark fossils are nothing but teeth.
A particularly intriguing set of shark teeth is that of Helicoprion, a type of shark that lived about 280 million years ago. The teeth are arranged in a spiral to form what looks like a kind of circular saw. This seems like a somewhat impractical way of carrying your teeth around, so the question of how the tooth spiral might fit into that shark’s jaw has led to quite some disagreement. Early reconstructions gave Helicoprion a long, spiral-shaped lower jaw to accommodate the tooth whorl, like so:
While this looks very fearsome and intimidating, it doesn’t seem very practical, to say the least. I, for one, am not sure how the animal could actually eat anything with a toothy fiddlehead like that. More recent reconstructions, helped by fragmentary bits of preserved Helicoprion cartilage skeleton, have given it a more realistic jaw with a single whorl in the lower jaw that continuously generates new teeth that grow outwards to replace those that have fallen off.
Apparently the teeth show almost no wear, indicating that Helicoprion ate soft prey, like squid, which were squished between the tooth whorl and the upper jaw. Annoyingly, squid fossils are also rather tricky, as they are usually represented by little more than their bullet-shaped internal shells. What did the rest of the animal look like? Let the puzzling commence.