A Morphometric Approach for Addressing Tooth-Based Species Delimitation in Fossil Mako Sharks, Isurus (Elasmobranchii: Lamniformes)

Morphological studies of fossil and extant shark teeth have typically been qualitative in nature, with resulting taxonomic problems due to the complicated forms of heterodonty exhibited by many sharks. This is apparent in the designation of fossil species assigned to Isurus (Lamniformes), where the status of the putative Neogene fossil species Isurus xiphodon and I. hastalis is solely based upon supposed differences in tooth morphology. Here we apply the geometric morphometric techniques of Procrustes superimposition, resampling-based Goodall's F-test, and canonical variates analysis to extant Isurus teeth, with the goal of assessing whether these quantitative analytical techniques provide a more objective basis for taxonomic decisions than do traditional qualitative morphological descriptions. These quantitative techniques are then applied to I. hastalis and I. xiphodon to examine whether I. xiphodon should be considered a junior synonym of I. hastalis or a separate species. Results show that geometric morphometric techniques can differentiate between the two extant species as well as the two extinct species, supporting I. xiphodon as a valid taxon. We suggest that this type of analysis is applicable for examining tooth-based species delimitations of sharks with both extant and extinct members, and has the potential to be applied to other fossil shark species as well.