Xenacanthus was a genus of prehistoric freshwater shark that lived during the mid-Paleozoic era through to around the end of the Triassic period, so approximately 200 million years of this weird little eel shark swimming around the various rivers and lakes of the world. Amazingly, this meant the Xenacanthus actually survived one extinction event, the Permian–Triassic transition, only to be taken out by another, the Triassic–Jurassic transition. Typical.

It takes its name from its unique dorsal spine, which projected out from the back of its head, earning the name from the Ancient Greek Xénos meaning “foreign” or “alien” and akanthos meaning “spine”. It’s also a common belief that this spine was venomous, similar to that of a stingray.

Though Xenacanthus fossils have been found all over the world, few of them have been complete, so most of the information scientists have about their diet and behaviour comes from samples of their teeth and spines.