Cephalofoil dimorphism
Stingray mating



Hammerhead shark evolution
The evolution of the peculiar head shape of hammerhead sharks (Elasmobranchii, Carcharhiniformes, Sphyrnidae) has been the subject of much speculation. The dorso-ventrally compressed and laterally expanded pre-branchial head is an unmistakable diagnostic feature of the sphyrnid sharks. This unique head shape has been termed the cephalofoil in recognition of its wing-like appearance. The persistence of the sphyrnid cephalofoil over the past 20-25 million years and the accompanying radiation into several hammerhead shark species of diverse head morphologies tell of its evolutionary success.
Sphyrna tiburo
Sphyrna lewini
Eusphyra blochii
Various hypotheses have been advanced to explain the adaptive significance of this unusual head morphology but few have been empirically tested. It has been suggested that the cephalofoil evolved from the slightly flattened head typical of many carcharhinid sharks, and acts as a bowplane that provides hydrodynamic lift and increases maneuvering capabilities. Other hypotheses for the evolution of the cephalofoil involve potential advantages of spacing sensory structures at the lateral ends of the head (eyes, nostrils) or across the surface of the head (lateral line, electroreceptors). These hypotheses are not mutually exclusive, but each emphasizes a single factor that might have driven evolution of the cephalofoil. Our research has tested four of these hypotheses, the enhanced electroreception hypothesis, the olfaction hypothesis , the visual hypothesis and the hydrodynamic hypothesis.

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modified Dec 30 2003