PARSING POPULATIONS USING MORPHOLOGY: 3D MANDIBLE LANDMARKS SEPARATE SISTER SPECIES OF POCKET MICE (MAMMALIA: RODENTIA)

Genetically and geographically disparate, but nearly morphologically indistinguishable, several populations of Perognathus flavus and P. merriami inhabit the arid southwestern U.S. Recent molecular work has identified four major allopatric lineages that diverged in the Late Miocene during the rise of interior grasslands and complex basin topographies. We µCT scanned 69 specimens for which locality, species, and genetic data were available to determine whether 3D geometric morphometrics can delineate similar populations. Specimens were µCT-scanned at 5-17µm resolution and reconstructed volumes were thresholded to produce isosurfaces of right hemimandibles. We placed 12 landmarks on the 3D jaw models using Geomagic DesignX, employing standard reference planes, best-fit splines, and other semi-automated tools to improve replicability of placed points. Using student t-tests we robustly separated the two species using the standardized distances between (1) the ventral most point of the angular process and posteriormost point of the condylar process (p=0.036) and (2) the ventral most point of the angular process and the inflection point along the curving ascending ramus (p=0.036). P. merriami was shown to have a laterally inflected coronoid process and a more dorsal-ventrally pronounced condylar process relative to P. flavus. Discriminant function analysis (DFA) and comparison of standardized linear measures failed to reliably classify populations. However, when we used specific combinations of measurements identified using stepwise DFA, we correctly matched 100% of processed specimens to their populations using jackknife cross-validation. Both species of Perognathus are identified as granivorous in published diet data, but our results indicate that the primary morphological differences between them relate to moment arms in the levers associated with mastication. Continuing work is implementing semi-landmark outlines to characterize subtle morphological variation between congeneric. Most importantly, this example demonstrates that 3D geometric morphometrics and uCT can separate morphologically similar specimens into populations, expanding application of ecological and evolutionary studies to the fossil record.