Great Basin Naturalist Memoirs


The rodent superfamily Geomyoidea is an old, undoubtedly monophyletic lineage having only obscure affinities with other rodent groups. Geomyoid rodents, autochthonous in North America, experienced major evolutionary diversification in the Mio-Pliocene coincident with the development of the Madro-Tertiary Geoflora and the climatic trend toward increasing aridity and coolness. Extant geomyoids are divisible into two groups: (1) the Geomyidae, all members of which are fossorial, and (2) the Heteromyidae, whose members display an adaptive continuum from bipedal, xeric-adapted forms to scansorial, mesic-adapted forms. These moieties, although recognizable on biochemical criteria, become particularly difficult to distinguish when paleontological data are considered. Nevertheless, most lines of evidence indicate that the families Heteromyidae and Geomyidae are distinct, monophyletic lineages.
The extant heteromyids comprise three main lineages (including six genera) that diverged during the Eocene: (1) subfamily Perognathinae (Chaetodipus and Perognathus); (2) subfamily Dipodomyinae (Dipodomys and Microdipodops); and (3) subfamily Heteromyinae (Liomys and Heteromys). Protein differentiation has occured at heterogeneous rates among these major lineages. Based on available karyotypic data, the main direction of chromosomal evolution in the Heteromyuidae appears to be toward increasing chromosome number. Cladistic analysis of morphological characters used in previous studies aupports biochemical evidence allying Microdipodops with Dipodomys. A model is introduced to describe how heterochronic changes in ontogeny may explain the great breadth of morphological diversification within the superfamily. Taxonomic recommendations at the subfamilial, generic, and sub-generic levels are provided.