Meeting Abstract

25.5  Monday, Jan. 5  Constructional Constraints In Sphyrnid Sharks: Shape Change And Space Utilization Through Phylogeny. MARA, Kyle R.*; MOTTA, Philip J.; PFEIFFENBERGER, Janne A.; University of South Florida, Tampa; University of South Florida, Tampa; University of South Florida, Tampa kmara@mail.usf.edu

Constructional constraints are particularly important within spatially limited structures such as the head. The head must contain all structures associated with feeding, respiration, neural integration, sensory reception, and musculoskeletal support. Sphyrnid sharks present an excellent study system for investigating the potential functional trade-offs within the head. Rhizoprionodon terraenovae, Eusphyra blochii, Sphyrna lewini, S. mokarran and S. tiburo were chosen to represent differences in head form through phylogeny. A combination of surface based geometric morphometrics and computed tomography volumetric analysis was utilized to investigate the implications of change in head form. The more basal, E. blochii, has relatively small anteriorly positioned eyes. Through phylogeny the relative size and position of the eyes changes, such that S. lewini and S. tiburo have larger more medially positioned eyes. Mouth size and position remain largely unchanged, however S. tiburo has relatively larger upper jaws. The position of the external nares is highly variable, but shows no phylogenetic trend. Interestingly, the volume of the brain is variable through phylogeny and the internal volume of the nasal capsule is increased in the most derived S. tiburo. These data indicate that the neurocranium and jaws are morphologically conserved whereas the laterally expanded cephalofoil and its consituent sensory components account for much of the morphological diversity within the clade. Therefore, there is little support for constructional constraints playing a role in the evolution of the sphyrnid cephalofoil.