The Ancient Shark Falcatus falcatus
The foot-long extinct shark Falcatus falcatus lived during the early Carboniferous period around 325 million years ago. The species, known from fossils in the Bear Gulch formation of Montana, was very common in the ancient sea that once covered the area. In this fossil, you can see a strange sword-like appendage growing off the head of the lower shark, which is a male. The females do not have this spine, and it's thought it was used in the act of mating—perhaps by the female grabbing onto it, as preserved here.