Evolutionary Relationships of Hammerhead sharks (Sphyrnidae): Reconciling signals across anatomical and molecular data

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Graham, Jasmin Rae
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Two conflicting hypotheses have been forwarded to account for the evolution of hammerhead sharks. The first, based on phenetic assessment of overall form, suggests that the hammer-like structure on the head (the "cephalofoil") has become exaggerated over the course of its evolution. The second, based on comparisons of DNA sequence data, suggests that the early forms likely had a well-developed cephalofoil that became reduced over the course of evolution. These two hypotheses invoke opposing directions of natural selection. In the current study, skeletal anatomical features of seven species of hammerhead sharks (Sphyrnidae) and two species of requiem shark (Carcharhinidae) were used to estimate a phylogenetic hypothesis for the group. Specimens were CT scanned and segmented to create virtual 3-D models of the sharks. A character matrix was derived from the segmented CT scans and subjected to phylogenetic analysis. The phylogenetic tree estimated from these data was contrasted with the two prior conflicting hypotheses. The phylogenies created using the anatomical data gathered from the CT scans were consistent with the former hypothesis. They showed that the hammerhead shark most closely related to the outgroup sharks from the Carcharhinidae family was <i>Sphyrna tiburo</i>, the “bonnethead” shark, which has one of the least developed cephalofoils among all of the hammerhead family. The data suggested that cephalofoil size has increased over the course of evolution, suggesting that the laterally expanded head shape may indeed confer some evolutionary advantage. In contrast, however, an analysis of both nuclear and mitochondrial DNA sequence data using a newly developed cross species gene capture approach yielded a tree that was concordant with previous hypotheses based on molecular data but strongly discordant with the tree created based on the anatomical data. We attempted to reconcile these two conflicting hypotheses and pinpoint the causes underlying the conflict between the analyses. Initial indications suggest that the way the trees are rooted may account for the apparent conflict.
Sphyrnidae, Hammerhead, evolution, gene capture, CT Scan