Patterns of Differentiation and Disparity in Cranial Morphology in Rodent Species of the genus Megadontomys (Rodentia: Cricetidae).

Rachel M. Vallejo, José Antonio Guerrero, and Francisco X. González-Cózatl (2017) The genus Megadontomys is a Mexican endemic group of rodents with allopatric populations occurring in fragmented patches of cool-humid forest. In this study we used geometric morphometrics methods to assess patterns of morphological variation and differentiation in skull and mandible among and within species of the genus. ANOVA showed that sexual dimorphism was significant for skulls size (P < 0.01) but not for mandibles, and MANOVA indicated that both structures did not differ in shape between sexes. ANOVA reveled a significant di erence among the three species (P < 0.01), M. nelsoni exhibit the largest skull. Canonical variate analyses and Goodall's test found differences in both skulls and mandibles shape among species, being M. cryophilus and M. thomasi the most divergent. The comparison between phylogroups within M. thomasi also revealed significant differences in shape for both structures. Disparity assessment showed that M. thomasi is the species that contributed the most to the overall shape disparity (51.80% for skull and 38.29% for mandible). The permutation test of phylogenetic signal in morphometric data was signi cant for the skull but not for the mandible. Morphometric data support the recognition of three morphotypes whitin the genus. The sister species M. nelsoni and M. thomasi displayed a grater shape similarity in the skull and mandible shape between them. In contrast, M. cryophilus exhibited the greatest shape divergence relative to the other species. The morphological evidence supports the existence of the two different phylogroups within M. thomasi, supporting their recognition as Evolutionary Significant Units previously suggested on molecular data. The lack of phylogenetic signal in the mandible corresponds with the environmental plasticity of this structure as compared with the skull.