phylogenetic relationships and classification of didelphid marsupials ...

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6 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 TABLE 1 Higher Classification and Geographic Distribution of Recent Marsupials a Genera Species Distribution b DASYUROMORPHIA Dasyuridae 20 69 OW Myrmecobiidae 1 1 OW Thylacinidae DIDELPHIMORPHIA 1 1 OW Didelphidae 18 c 91 c DIPROTODONTIA NW Acrobatidae 2 2 OW Burramyidae 2 5 OW Hypsiprymnodontidae 1 1 OW Macropodidae 11 65 OW Petauridea 3 11 OW Phalangeridae 6 27 OW Phascolarctidae 1 1 OW Potoroidae 4 10 OW Pseudocheiridae 6 17 OW Tarsipedidae 1 1 OW Vombatidae MICROBIOTHERIA 2 3 OW Microbiotheriidae NOTORYCTEMORPHIA 1 1 NW Notoryctidae PERAMELEMORPHIA 1 2 OW Chaeropodidae 1 1 OW Peramelidae 6 18 OW Thylacomyidae PAUCITUBERCULATA 1 2 OW Caenolestidae 3 6 NW a Numbers of genera and species after Wilson and Reeder (2005) except as noted. b NW 5 New World; OW 5 Old World (Sahul). c After Gardner (2008). are represented by a mere handful of living forms: Microbiotheria by a single genus and species (Dromiciops gliroides), and Paucituberculata by six species in three genera (Caenolestes, Lestoros, Rhyncholestes). Both groups inhabit wet-temperate Patagonian forests, but paucituberculates also occur in climatically similar montane habitats of the tropical Andes (Bublitz, 1987; Patterson and Gallardo, 1987; Albuja and Patterson, 1996; Hershkovitz, 1999; Lunde and Pacheco, 2003). Because microbiotherians and paucituberculates were more diverse and widely distributed in the Tertiary than they are today (Marshall, 1980, 1982b; Bown and Fleagle, 1993; Goin, 1997; Goin and Candela, 2004), these orders are appropriately regarded as relictual elements in modern faunas. Instead, the only substantially intact radiation of New World marsupials is represented by the family Didelphidae, commonly known as opossums, of which 91 Recent species in 18 genera are currently recognized (Gardner, 2008). Although didelphids were the first metatherians to be encountered by European explorers (Eden, 1555), the first to be described scientifically (Tyson, 1698), and the first to be classified by taxonomists (Linnaeus, 1758), zoological interest in opossums was soon eclipsed by the discovery of Australasian marsupials. Nevertheless, steady advances in didelphid taxonomy (mostly involving the description of new species) were made throughout the 19th and 20th centuries. Although this progress was periodically summarized by authors (e.g., Thomas, 1888; Winge, 1893; Cabrera, 1919, 1958), there is no modern monographic treatment of the family, and the current suprageneric classification (McKenna and Bell, 1997; Gardner, 2005, 2008) does not reflect current knowledge about opossum evolutionary relationships. The history of didelphid phylogenetic research was reviewed by Jansa and Voss (2000), who analyzed DNA sequence data from the nuclear Interphotoreceptor Retinoid Binding Protein (IRBP) gene for 21 species. Subsequent reports were focused on analyses of morphological and karyotypic character data in combination with IRBP sequences for larger sets of species (Voss and Jansa, 2003; Voss et al., 2005), on phylogenetic problems associated with individual taxa (Voss et al., 2004a; Jansa and Voss, 2005), or on the evolutionary dynamics of newly sequenced nuclear loci (Jansa et al., 2006; Gruber et al., 2007). However, no comprehensive phylogenetic synthesis has yet been attempted, and much new morphological and molecular character data remain unanalyzed. This monograph summarizes our collaborative research to date on didelphid phylogenetic relationships and concludes the first
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 7 phase of our ongoing evolutionary study of the family. Although we take this opportunity to review information that has previously been published elsewhere, many new results are reported herein. Among these, we describe morphological comparisons of didelphids with other plesiomorphic (polyprotodont) marsupial clades; we describe and illustrate integumental and craniodental traits not previously discussed in the literature; we report new sequence data from the Breast Cancer Activating 1 gene (BRCA1) and the von Willebrand Factor gene (vWF); we analyze didelphid phylogenetic relationships based on a new set of morphological and karyotypic characters together with sequence data from five protein-coding nuclear loci; we propose a new suprageneric classification consistent with our analytic results; and we provide morphological descriptions and cranial illustrations of all currently recognized genera to facilitate taxonomic identifications. Materials and Methods PHYLOGENETIC ASSUMPTIONS AND TAXON SAMPLING: Our phylogenetic analyses are based on the assumption of didelphid monophyly, which is strongly supported by a variety of nuclear-gene sequence datasets (e.g., Jansa and Voss, 2000; Amrine-Madsen et al., 2003; Meredith et al., 2008). Our ingroup terminals (table 2) consist of 44 species representing every currently recognized didelphid genus and subgenus, including all of the species previously analyzed by Jansa and Voss (2000, 2005), Voss and Jansa (2003), Voss et al. (2004a, 2005), Jansa et al. (2006), and Gruber et al. (2007). Altogether, this is by far the most taxon-dense and character-rich didelphid dataset ever analyzed. The outgroups for this study include representatives of every extant ordinal-level clade that has ever been recovered within one or two nodes of Didelphidae in previous analyses of marsupial relationships, including caenolestids (Paucituberculata), dasyurids (Dasyuromorphia), Dromiciops (Microbiotheria), and peramelids (Peramelemorphia). We did not include Notoryctes (Notoryctemorphia) or diprotodontians because these taxa have never been considered close to didelphids, and because their highly derived morphologies raise many unresolved issues of homology that are beyond the scope of this study. COMPARATIVE MORPHOLOGY: Our morphological comparisons are based on skins, skulls, and fluid-preserved specimens (appendix 1), which we examined for discretely varying traits that could be scored for most of the terminal taxa in our analysis. Additionally, we summarize information about other phenotypic characters that, although not suitable for phylogenetic analysis due to continuous variation (e.g., body size), provide taxonomically useful descriptors. Information about morphological character variation among nondidelphid marsupials is largely based on our scoring of outgroup terminal taxa, but we also include remarks on the morphology of other Old World marsupials where these seem relevant or interesting. To provide a maximally user-friendly reference, our morphological observations are summarized in two different formats. First, we describe taxonomic comparisons using normal (nontelegraphic) prose in the body of the text. These accounts, organized organ-system-by-organ system, are accompanied as necessary by illustrations of representative specimens. Second, we summarize morphological descriptors taxon-by-taxon using telegraphic prose in the generic accounts of our classification. Although largely redundant with our data matrix (appendix 4), these generic descriptions are easier to use for taxonomic identifications, and they provide an opportunity to summarize relevant observations that were not encoded as characters. Formal character descriptions (in appendix 3) are accompanied by technical details about coding procedures and ordering criteria. Our criteria for character choice were described at length by Voss and Jansa (2003), so they are not repeated here. The only noteworthy methodological change in this study concerns our coding of polymorphisms. As in our previous study, we ignored rare variants (on the assumption that all characters are polymorphic given sufficiently large samples) and coded only those polymorphisms represented by nearly equal frequencies of alternative states (e.g., when
Page 1 and 2: PHYLOGENETIC RELATIONSHIPS AND CLAS
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