Using 2D Dental Geometric Morphometrics to Identify Modern Chaetodipus and Perognathus Specimens (Rodentia, Heteromyidae)

Abstract

The Heteromyidae (pocket mice and kangaroo rats) are a group of extant small rodents abundant in North American Cenozoic fossil assemblages. Two genera of heteromyids, Chaetodipus and Perognathus, share similar tooth morphology whose fossils are distinguished using skull shape and body size estimates. 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 begins in the middle Miocene, near the time suggested by molecular divergence. Other studies found these two genera are not distinguishable from each other using descriptive dental morphology alone. In this study, I asked whether two-dimensional geometric morphometrics on complete dentition and isolated premolars can accurately identify Chaetodipus and Perognathus specimens at the genus and species-level. I developed a landmarking scheme based on features that are consistent through wear, are recognizable in the fossil record, and could be subset for analyses on individual molars. I landmarked the occlusal surface of the upper and lower tooth rows of modern Chaetodipus (n=83) and Perognathus specimens (n=80), including 12 of the 26 extant species across the two genera. We used the R packages “geomorph” and “Morpho” to run a canonical variates analysis to investigate whether principal component variation could predict known taxonomic identifications. The morphospace using complete dentition can identify specimens to genus with 90-92% accuracy and to species with more variable accuracy. Similarities in body size and biogeographic ranges explained genus-level misidentification while phylogenic relationships explained species-level misidentification. Specifically, Perognathus parvus, the largest Perognathus species in the analysis, was most frequently misidentified as Chaetodipus. I found an isolated premolar provides sufficient information for genus-level identification (69%-84% accuracy), but not for species-level identification (26%-56% accuracy). The morphospace suggests the anterior-posterior length and transverse width ratios of the premolars are diagnostic for genus identification. This morphospace of modern specimens can be used to identify the fossil dentition of Chaetodipus and Perognathus specimens already in museum collections and refine our existing knowledge of heteromyid evolutionary history.