Relationships and Divergence times among the Orders and Families of Marsupialia

-The approximately 330 living species of marsupials are currently divided into three American (Didelphimorphia, Microbiotheria, and Paucituberculata) and four Australasian (Dasyuromorphia, Diprotodontia, Notoryctemorphia, and Peramelemorphia) orders. Studies of interordinal relationships generally support the monophyly of Australidelphia, which includes the four Australian orders and the South American Microbiotheria. Within Australidelphia, monophyly of the Australasian orders (Eomarsupialia), relationships between the Australasian orders, and diprotodontian interfamilial relationships are still disputed. We analyzed protein-coding portions of five nuclear genes (ApoB, BRCA1, IRBP, Rag1, and vWF) from representatives of all extant marsupial families using maximum parsimony, maximum likelihood, and Bayesian methods. Two relaxed molecular clock methods (Multidivtime, IRDIVTIME) were employed to estimate divergence times. Likelihood and Bayesian analyses favored rooting the tree between Didelphimorphia and all other marsupials, but alternate positions for the root (Paucituberculata versus other marsupials, Ameridelphia versus Australidelphia) could not be rejected. Australidelphia was supported in all analyses, but interordinal relationships within this clade were not strongly supported. Diprotodontian monophyly was recovered in all analyses. Within this order there was a basal split between Vombatiformes (koalas, wombats) and Phalangerida (kangaroos, possums). Within Phalangerida, Macropodiformes grouped with Phalangeroidea to the exclusion of Petauroidea. Within Petauroidea, there was a basal split between Acrobatidae and all other petauroids. Among the remaining petauroids, Pseudocheiridae grouped with Petauridae to the exclusion of Tarsipedidae. Multidivtime divergence time estimates suggest a Late Cretaceous common ancestor for Marsupialia (80.4-78.1 Ma), interordinal divergences that range into the early Paleocene (60.7-59.5 Ma for Dasyuromorphia to Peramelemorphia), and mostly Paleogene interfamilial diversification. IRDIVTIME analyses resulted in slightly older dates for the most recent common ancestor of Marsupialia (83.9-80.6 Ma), but also showed increased stemminess (i.e., proportion of overall tree length comprised of internal branches) and a longer time window (~ 36 million years) for interordinal cladogenesis than Multidivtime analyses. Multidivtime dates for the last common ancestor of Australidelphia (65.0-64.8 Ma) allow for overland dispersal to Australia prior to the submergence of the South Tasman Rise at 64 Ma (Woodburne and Case, 1996). By contrast, IRDIVTIME dates for the last common ancestor of Australidelphia (62.2-58.2 Ma) are slightly younger than dates for the submergence of the South Tasman Rise and imply over water dispersal. INTRODUCTION Marsupialia is a diverse group of mammals that includes more than 330 extant species that occur in North America, South America, and Australasia (Wilson and Reeder, 2005). Simpson’s (1945) classification of mammals lumped all marsupials into a single order with six superfamilies. Recent classifications typically recognize seven orders and 18–22 families of living marsupials (Marshall et al., 1990; Aplin and Archer, 1987; Kirsch et al., 1997; Springer et al., 1997). Among the seven orders, Didelphimorphia (New World opossums), Microbiotheria (the Chilean monito del monte), and Paucituberculata (South American caenolestids) are American whereas Dasyuromorphia (dasyurids, numbats, thylacines), Diprotodontia (wombats, koalas, kangaroos, Old World possums),