shark fin spine Ctenacanthus mayi
Fossils from sharks (cartilaginous fish) are the most common fish fossils in the Mississippian rocks of Illinois. In some shark species, the males and females look different. Males have projections on the undersides of their pelvic fins. They are used during mating. Males also have unique fin spines, like the curved spike of [i]Falcatus[/i] or the brush-like structure on the head of [i]Stethacanthus[/i]. These structures probably played a role during courtship and mating.