Systematic Studies on Opossums (Didelphidae) and Rodents (Cricetidae).

The analyses presented in Chapter I provide a test and refinement of previously presented hypotheses concerning the genealogical relationships among contemporary didelphid marsupials. Within the Didelphinae two well-characterized natural groups are delineated, the large-boiled, 22N karyotype forms, and the small-bodied genera with Metachirus as a relatively early offshoot of the latter group. I regard the lack of a pouch, reduced claws, details of the foot structure and architecture of the auditory bullae as strong evidence for the affinity of Metachirus to the small-bodied didelphines, thus resolving the ambiguity in the placement of this genus implicit in Kirsch's serological comparisons. Thylamys Gray, formerly treated as a subgenus within Marmosa, is restricted to the elegans-group of Tate, including tatei Hardley and karimii Petter and elevated to generic rank. Thylamys shows closer affinity, based on derived similarities, to Monodelphis and Lestodelphys than to any of the taxa in Marmosa. Tate's microtarsus-group, formerly included in Thylamys shows closer affinity to the other species-groups of Marmosa than to the other genera of small didelphines. With the removal of the elegans-group the remaining taxa in Marmosa form a diagnosable, possibly monophyletic group. Recognition of the species-groups within Marmosa with more formal taxonomic epithets does not seem warranted based on the evidence currently available. In Chapter II, the quantitative description of growth curves for several morphometric traits provides a basis for assessing the ontogenetic patterns underlying differences in morphological structure among neotomine-peromyscine rodents. Morphometric differences among contemporary rodent species may result from relatively simple changes in relative growth rates and timing. Quantitative ontogenetic studies add a dynamic component to the assessment of morphological similarity, thus providing a more robust procedure for detecting homoplasy than static comparison of adult morphology. Applying the principles of phylogenetic systematics to studies of developmental timing among closely related taxa may be a useful and informative complement to studies based on molecular similarity or static comparison of adult morphology.Ph.D.BiologyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/160111/1/8422213.pd