Shark-tooth pattern

The Shark-Tooth Pattern is a stand-alone fossil that can be donated to the museum in.

In New Horizons
"This shark-tooth pattern comes from the lower jaw of an ancient shark of the genus Helicoprion. Its teeth seem to have grown in a distinctive arrangement rather disturbingly termed a "tooth-whorl." I say "seem" because shark skeletons are made not of bone, but cartilage, except for their teeth. Consequently, their bodies are never preserved as fossils, and questions about their jaws remain unanswered. The size and placement in the stone of the shark's teeth are actually the only things we have to work with. Sometimes in research we must maintain a stiff upper lip, even in the absence of a stiff lower jaw!" –Blathers

In Real Life
Helicoprion was a shark-like fish that lived off the southwestern coast of Gondwana in the Early to Middle Permian. While it is more closely related to sharks and other cartilaginous fishes than the bony fishes, its closest living relatives are actually the chimaeras, or rat fish. Like most cartilaginous fish, Helicoprion's body would have decayed quickly. As such, the only fossils found so far have been those of its tooth whorl. First described in 1899, the tooth whorl had baffled paleontologists for over a century, with ideas for what part of the body it was on ranging from the snout to the dorsal fin, to (possibly the most famous early idea) the outside of the lower jaw. Finally, in 2013, researchers working with related species discovered that the tooth whorl in fact sits inside the lower jaw. The whorl grows as Helicoprion ages, with newer teeth growing on the outside while the older teeth get pushed towards the middle of the spiral.