Abstract
Morphological analyses are critical to quantify phenotypic variation, identify taxa, inform phylogenetic relationships, and shed light on evolutionary patterns. This work is particularly important in groups that display great morphological disparity. Such is the case in geomyoid rodents, a group that includes 2 of the most species-rich families of rodents in North America: the Geomyidae (pocket gophers) and the Heteromyidae (kangaroo rats, pocket mice, and their relatives). We assessed variation in skull morphology (including both shape and size) among geomyoids to test the hypothesis that there are statistically significant differences in skull measurements at the family, genus, and species levels. Our sample includes 886 specimens representing all geomyoid genera and 39 species. We used the geometric mean to compare size across taxa. We used 14 measurements of the cranium and lower jaw normalized for size to compare shape among and within taxa. Our results show that skull measurements enable the distinction of geomyoids at the family, genus, and species levels. There is a larger amount of size variation within Geomyidae than within Heteromyidae. Our phylomorphospace analysis shows that the skull shape of the common ancestor of all geomyoids was more similar to the common ancestor of heteromyids than that of geomyids. Geomyid skulls display negative allometry whereas heteromyid skulls display positive allometry. Within heteromyids, dipodomyines, and non-dipodomyines show significantly different allometric patterns. Future analyses including fossils will be necessary to test our evolutionary hypotheses.