A GEOMETRIC MORPHOMETRIC APPROACH TO IDENTIFY PATTERNS OF TOOTH SHAPE IN EXTANT AND FOSSIL ELASMOBRANCHS
Taylor, Jordan Noelle
Isolated chondrichthyan teeth are among the most abundant vertebrate fossil represented in the fossil record, and are widely distributed along the southeastern United States. The coastal plain in South Carolina contains chondrichthyan remains from the Eocene epoch (55.8 to 33.9 Ma). The Eocene is distinguishable from other epochs by the presence of distinct, calcareous nannofossil assemblages and characteristic levels of carbon in the limestone. The Eocene is characterized by accelerated global cooling, changes in ocean circulation, and a drop in global temperature of about 2°C. Regional faunal diversity has been described for calcareous nannofossil assemblages from the Eocene, but has not yet been explored for elasmobranchs. The present study aims to identify the sources of tooth shape variation in extant elasmobranch fauna, as a means to identify individual teeth to species; this technique was used to identify isolated teeth to species found in the Late Eocene Parkers Ferry formation (33.239068° N, -80.425665°W) of South Carolina. Many extant sharks can be identified on the basis of tooth morphology, which has been typically qualitative in nature. Qualitative sorting is error prone and has led to taxonomic problems in the reporting of the fossil record, because there are several complicated patterns of heterodonty that influence tooth shape and confound accurate identification to species. A geometric morphometric technique was applied to 18 extant species to characterize sources of variation in tooth shape specific to: species (27.6%), jaw type (33%), and tooth position (28.3%). Eighteen extant species were successfully sorted using jaw type and species into a hierarchical classification tree (classification accuracy 83.6%). Variation in extant forms was used to build a model to interpret and classify fossil teeth; using a PCA, fossil forms exhibited monognathic and dignathic heterodonty, like their extant counterparts. The approach adopted in the present study provides a more objective means of sorting fossil sharks’ teeth, while also highlighting taxonomic identification problems in the fossil record.