The bonnethead (Sphyrna tiburo), also called a bonnet shark or shovelhead, is a small member of the hammerhead shark genus Sphyrna, and part of the family Sphyrnidae. It is an abundant species in the littoral zone of the North Atlantic and the Gulf of Mexico, is the only shark species known to display sexual dimorphism in the morphology of the head, and is the only shark species known to be omnivorous. Description The shark is characterized by a broad, smooth, spade-like head: it has the smallest cephalofoil (hammerhead) of all Sphyrna species. The body is grey-brown above and lighter on the underside. Typically, bonnethead sharks are about long, with a maximum size of about . The generic name Sphyrna derives from the Greek word for "hammer", referring to the shape of this shark's head; the specific name tiburo derives from the Spanish word "tiburón", meaning "shark". Morphology Sexual dimorphism Bonnethead sharks are the only sharks known to exhibit sexual dimorphism in the morphology of the head. Adult females have a broadly rounded head, whereas males possess a distinct bulge along the anterior margin of the cephalofoil. This bulge is formed by the elongation of the rostral cartilages of the males at the onset of sexual maturity and corresponds temporally with the elongation of the clasper cartilages. Pectoral fins and swimming The pectoral fins on most fish control pitching (up-and-down motion of the body), yawing (the side-to-side motion), and rolling. Most hammerhead sharks do not yaw or roll and achieve pitch using their cephalofoils. The smaller cephalofoil of a bonnethead shark is not as successful, so they have to rely on the combination of cephalofoils and their large pectoral fins for most of their motility. Compared to other hammerheads, bonnethead sharks have larger and more developed pectoral fins and are the only species of hammerhead to actively use pectoral fins for swimming. Evolution Using data from mtDNA analysis, a scientist has found that the evolution of hammerhead sharks probably began with a taxon that had a highly pronounced cephalofoil (most likely that similar to the winghead shark, Eusphyra blochii), and was later modified through selective pressures. Thus, judging by their smaller cephalofoil, bonnethead sharks are the more recent developments of a 25-million-year evolutionary process. Distribution and habitat This species occurs on both sides of the American coast, in regions where the water is usually warmer than . In the Atlantic, it