A comparative study on the pre-branchial cranial anatomy of the scalloped hammerhead sharks [Sphyrna lewini (Griffith & Smith, 1834)] and the Brazilian sharpnose shark [Rhizoprionodon lalandii (Valenciennes, 1839)] was carried out to check the modification in the musculature, inervation and optic stalk related to the appearance of the cephalofoil in Sphyrna Rafinesque, 1810. A total of seven adults and one juvenile of R. lalandii and eight juveniles of S. lewini were examined. In S. lewini the levator palaliquadrati and the levator labii superioris were the most modified cephalic muscles, as they became dorsalventrally attached and laterally developed. Among the oculomotor muscles, the recti followed the lateral expansion of the head constituting the rectal stalk associated with the nerves II, III, IV and VI and the optic stalk. It was observed that the oculomolorius branch "a" does not inervate the adductor mandibulae as it was mentioned in a previous paper. The myological structures and the inervation pattern presented diagnostic characters. Despite the shared characters between Carcharhinidae and the Sphyrnidae, the cephalofoil represents an autapomorphy which includes all the hammerhead sharks in the family Sphyrnidae.
Carcharhiniformes; Sphyrna; Rhizoprionodon; muscles; inervation