USING GEOMETRIC MORPHOMETRICS TO DISTINGUISH CHAETODIPUS AND PEROGNATHUS (RODENTIA, HETEROMYIDAE)

The Heteromyidae are a group of extant small rodents abundant in North American Cenozoic fossil assemblages. Fossil heteromyids are excellent biostratigraphic and paleoecological indicators and understanding their diversity, speciation and extinction in deep time will aide in our understanding of North American landscape evolution. Two genera of heteromyids, Chaetodipus and Perognathus, share very similar tooth morphology and their fossil record consists of mostly isolated molars and jaws. Previous genetic studies show these extant genera likely diverged in the early Miocene (~20 million years ago), however, the Chaetodipus fossil record starts in the Pleistocene (~2 million years ago) while the Perognathus fossil record starts in the middle Miocene, near the time suggested by molecular divergence. Our current understanding is that these two genera are indistinguishable based on linear tooth measurements and descriptive dental morphology alone. In this study, we tested if 2D geometric morphometrics could distinguish Chaetodipus and Perognathus dentition. We landmarked the occlusal surface of the molars using characters found in both fossil and modern specimens. This study landmarked the upper and lower tooth row of 164 modern Chaetodipus and Perognathus specimens. We used the R packages “geomorph” and “Morpho” to run a canonical variates analysis to see if principal component variation could predict known taxonomic identifications. Starting with only known modern specimens, this study built a training set to see how accurately this landmarking scheme could identify molars to genus. Using all four molars in the tooth row, there is an 84% to 90% classification accuracy when comparing only the Chaetodipus and Perognathus specimens. An isolated upper fourth premolar has a 78% classification accuracy while the isolated lower fourth premolar has a 69% classification accuracy. Confirming that 2D geometric morphometrics can successfully distinguish these two genera, we can move forward and landmark fossil molars of Chaetodipus and Perognathus specimens to better understand heteromyid diversity in deep time.