Date of Degree


Document Type


Degree Name





Robert S. Voss

Committee Members

Ana C. Carnaval

Nancy Simmons

Frank Burbrink

Sharon A. Jansa

Subject Categories

Biodiversity | Evolution | Genetics | Zoology


Mammal, Marsupial, DNA, Neotropical, South America, taxonomy


Short-tailed opossums (genus Monodelphis) comprise the most species-rich genus of New World marsupials, with 25 currently recognized species. Monodelphis comprise small, terrestrial species collectively widespread in South America, which makes the group potentially informative about biogeographic processes that have shaped the continental fauna. The genus exhibits striking variation in several phenotypic characters, notably pelage coloration, behavior, and reproductive strategies. This diversity is unique among Neotropical marsupials, and makes the group particularly interesting to investigate the evolution and the adaptive significance of phenotypic trait variation. Despite this potential, missing knowledge on phylogeny and basic taxonomy precludes broader studies on evolution and biogeography on this group. To remedy this situation this thesis is centered on undertaking a comprehensive phylogenetic analysis and taxonomic revision of Monodelphis.

In chapter one (published in collaboration with Dr. Sharon A. Jansa and Dr. Robert S. Voss) we present the first phylogenetic analyses of Monodelphis based on dense taxonomic sampling—23 of the 25 currently recognized species—and multiple genes—one mitochondrial gene (CYTB), two autosomal exons (IRBP, BRCA1), one autosomal intron (SLC38), and one X-linked intron (OGT). Parsimony, maximum-likelihood and Bayesian analyses of this dataset (110 terminals, 4983 aligned sites) strongly support the monophyly of Monodelphis and recover six major clades within the genus. Additionally, the analyses suggest that several nominal taxa are synonyms of other species in the genus (M. “sorex” of M. dimidiata, M. “theresa” of M. scalops, M. “rubida” and M. “umbristriata” of M. americana, and M. maraxina” of M. glirina). By contrast, four unnamed genetic lineages recovered by the analyses may represent new species. Reconstructions of ancestral states of two discrete characters—dorsal pelage color pattern and habitat—suggest that the most recent common ancestor of Monodelphis was uniformly colored (with unpatterned dorsal pelage) and inhabited moist forest. Whereas most dorsal pelage patterns (e.g., reddish sides and reddish head-and-rump) appear to have evolved homoplastically in Monodelphis, dorsal stripes may have had a unique historical origin in this genus.

In chapter two (also published in collaboration with Dr. Sharon A. Jansa and Dr. Robert S. Voss) we infer historical-biogeographic patterns and processes in South America based on estimated divergence times and ancestral distributions for this diverse clade of marsupials. The dating analysis suggests that the diversification of Monodelphis was not restricted to a narrow time interval, such that speciation cannot be attributed to a single causal historical factor. In particular, speciation within the Atlantic Forest biome appears to have occurred in the Neogene, whereas most Amazonian speciation events are Pleistocenic. Amazonia and the Atlantic Forest hosted most of the diversification events in Monodelphis and were estimated as the historical sources of lineages that subsequently colonized other areas (e.g., Tepuis, Andes, Cerrado) during the Pliocene and Pleistocene. Model selection suggests an important role for founder-event speciation, a process seldom accounted for in previous biogeographic analyses of continental clades. These results, together with others recently reported from analyses of South American datasets suggest that dispersal-mediated cladogenesis has been an important process in the evolution of Neotropical vertebrate faunas. Possible mechanisms for founder-event speciation in low-vagility terrestrial vertebrates such as Monodelphis include historically transient connections between currently disjunct biomes and rapid reproductive isolation of populations colonizing adjacent but ecologically disparate biomes.

In the following chapter (to be published in collaboration with Dr. Robert S. Voss), an integrative approach—phylogenetic information together with phenotypic data from external and craniodental morphology— is used to provide a revised subgeneric classification of Monodelphis, intended to serve as the basis for future revisionary research at the species level. Over 2000 specimens of Monodelphis were analyzed for morphology, including representative material of all species currently recognized as valid, together with all undescribed forms discussed in the previous chapters. Discrete characters found to be useful for diagnosing monophyletic groups of species previously established by molecular research are defined and illustrated. Five subgenera are formally recognized, for which morphological diagnosis, comparisons, and patterns of geographical distribution and sympatry are provided.

In chapter four, a paper (already published) describing a new species of Monodelphis from Atlantic Forest in Brazil is presented.