Stethacanthus was one of the earliest sharks ever to roam the planet, which is really saying something for a group as ancient as sharks. Its heyday was the early Carboniferous epoch, about 360 million years ago — long before the dinosaurs took over the Earth. It's distinct not only as one of the oldest members of a super-old family, but because of its odd anvil-shaped dorsal fin. Scientists have suggested the idiosyncratic feature may have been used to intimidate predators or attract mates, but our favorite theory is that it was a docking mechanism that allowed Stethacanthus to hitch rides on larger marine animals. What a mooch!
Artwork by Dmitry Bogdanov.