Symmoriiforme

The Symmoriiformes had bodies with the same general shape as the Cladoselachidae. The caudal fin was externally symmetrical, as in Cladoselache, but the dorsal fins lacked associated spines. Unique to Symmoriiforms was the presence of a long, pointed metapterygial axis (and extension from the pectoral fin) whose function is unknown. The Stethacanthids are a family of Symmoriiforms from the Carboniferous. They exhibit a unique "shoulder spine" that originates dorsally at the base of the head. This large brush-like spine is present in sexually mature males, and may have been used as a form of sexual display.