About 20 million years ago during the Miocene epoch, the ancestor of all hammerhead shark species appeared in the ocean. DNA sequenced from hammerheads allover the world was used to establish a phylogenic tree. Ancestral hammerheads were as big as modern hammerheads, and constitute a monoyphyletic group derived from the Carhcarhinids. One of the 12 genera of the Requiem shark, and one of the largest families of shark. This family of shark underwent divergent evolution, with some growing larger, some reducing in size, and the cephlafoil changing in both size and shape. Winghead was the first shark to diverge from the groups common ancestor, and the Bonnethead the most recent. All genera Sphyrna (9 species) and Eusphyrna (1 species) belonging to the family Spyhrnidae, and characterized by a shared ancestral character, a unique lateral  expansion called a cephlafoil.