Phylogeny of hammerhead sharks (Family Sphyrnidae) inferred from mitochondrial and nuclear genes.

Hammerhead sharks (Family Sphyrnidae) get their name from their laterally expanded, dorsal-ventrally compressed head, a structure referred to as the cephalofoil. Species within the family vary for head size and shape and for body size in ways that are functionally significant. Here we infer the phylogeny for all species within the family based on analysis of mitochondrial and nuclear genes amounting to 6292 base pairs. Mixed model Bayesian analysis of the concatenated data and Bayesian estimation of the species tree (BEST) converged on the same topology of the relationships. Shimodaira-Hasegawa tests revealed that all previously proposed hypotheses could be refuted by the data. The new hypothesis for the group suggests that the ancestor of all extant sharks was large (>200 cms) and that small body size probably evolved twice at different times and places. Moreover, the results suggest that once the cephalofoil evolved, it underwent divergent evolution in different lineages presumably in response to unique selective regimes. Copyright 2010 Elsevier Inc. All rights reserved.