80 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 Fig. 36. The 50% majority-rule consensus <strong>of</strong> post-burnin trees resulting from a mixed-model Bayesian analysis <strong>of</strong> the combined (nonmolecular + molecular) dataset including Chacodelphys. Conventions for indicating nodal support are described in the caption to figure 28. The broken arrow indicates the alternative position <strong>of</strong> Glironia recovered by parsimony analysis.
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 81 <strong>of</strong> our knowledge, no strongly conflicting valid results have been obtained in any other published <strong>phylogenetic</strong> analysis <strong>of</strong> molecular or morphological character data. 22 Although this tree is fully resolved with high support values at most nodes, several outst<strong>and</strong>ing problems merit comment. As we have previously noted elsewhere, Chacodelphys exhibits conflicting patterns <strong>of</strong> derived morphological similarities with Thylamys + Lestodelphys on the one h<strong>and</strong> <strong>and</strong> with Monodelphis on the other (Voss et al., 2004a). Weak parsimony support for nodes along the <strong>phylogenetic</strong> path between Thylamys + Lestodelphys <strong>and</strong> Monodelphis in the combined-data analysis that includes Chacodelphys (fig. 36) presumably reflects such character conflict, although it is noteworthy that Bayesian support for the same nodes is unaffected by taxon addition. Although we are convinced that Chacodelphys is closely related to Lestodelphys <strong>and</strong> Thylamys by a host <strong>of</strong> phenotypic resemblances too indefinite to code as characters but too numerous to ignore, a compelling analytic solution to this vexing problem is unlikely to be forthcoming until at least some <strong>of</strong> the missing molecular data for Chacodelphys can be obtained from fresh material. However, even substantial amounts <strong>of</strong> sequence data are clearly not enough to resolve other <strong>phylogenetic</strong> uncertainties. Indeed, it is not a little frustrating that, with .7000 bp <strong>of</strong> protein-coding nuclear sequence in h<strong>and</strong>, the <strong>relationships</strong> <strong>of</strong> Glironia, Cryptonanus, <strong>and</strong> Tlacuatzin should still be problematic. Glironia is <strong>of</strong> special concern, because the two plausible <strong>phylogenetic</strong> resolutions for this genus (fig. 34A, B) determine the root <strong>of</strong> the <strong>didelphid</strong> radiation. The alternative <strong>phylogenetic</strong> resolutions <strong>of</strong> Cryptonanus <strong>and</strong> Tlacuatzin within their respective groups do not seem like comparably weighty issues, but each could affect ecobehavioral character optimizations <strong>of</strong> significant evolu- 22 DNA-DNA hybridization results showing Gracilinanus nested within Marmosops (Kirsch <strong>and</strong> Palma, 1995; Kirsch et al., 1997) were based on taxonomic misidentifications (Voss <strong>and</strong> Jansa, 2003: 57). Analyses <strong>of</strong> 12S gene sequences by Palma <strong>and</strong> Spotorno (1999) recovered Metachirus <strong>and</strong> Marmosops as sister taxa, but this grouping was not found in subsequent analyses <strong>of</strong> 12S sequence data (Steiner et al., 2005) for reasons that remain unexplained. tionary interest. Phenotypically, Cryptonanus is certainly more similar to Gracilinanus than it is to Thylamys or Lestodelphys, whereas Tlacuatzin is undeniably more similar to Marmosa than it is to Monodelphis. The positions <strong>of</strong> these genera in our combineddata trees are therefore plausible despite the absence <strong>of</strong> compelling parsimony or Bayesian support. Although it might seem best to wait until these few remaining issues are convincingly resolved before proposing a formal <strong>classification</strong>, there is no guarantee that additional data will soon be forthcoming or useful. In the meantime, other <strong>relationships</strong> that are strongly supported by our results merit nomenclatural recognition, <strong>and</strong> the contents <strong>of</strong> some taxa currently recognized as valid need to be revised on the basis <strong>of</strong> our analytic results. As documented below, no previous <strong>classification</strong> is consistent with what is now confidently known about <strong>didelphid</strong> <strong>phylogenetic</strong> <strong>relationships</strong>. CLASSIFICATION Most essays on marsupial <strong>classification</strong> (e.g., Simpson, 1945; Kirsch, 1977a; Marshall, 1981; Aplin <strong>and</strong> Archer, 1987; Archer <strong>and</strong> Kirsch, 2006) are uninformative about the historical development <strong>of</strong> opossum systematics. To be sure, the chronological <strong>and</strong> bibliographic details <strong>of</strong> <strong>didelphid</strong> taxonomy are <strong>of</strong> limited interest, so the following paragraphs mention only a few milestones on the long road to the currently accepted <strong>classification</strong>. Because most <strong>of</strong> the technical information about how <strong>and</strong> why names were formerly applied to taxa was recently summarized by Gardner (2008), this brief narrative is primarily intended to serveasanintroductiontotherevised<strong>phylogenetic</strong>systemthatwepropose. Linnaeus (1758) described only five species <strong>of</strong> <strong>marsupials</strong>, all <strong>of</strong> which were <strong>didelphid</strong>s. Although the four Linnaean species currently recognized as valid (marsupialis, phil<strong>and</strong>er, opossum, <strong>and</strong> murina) are now referred to different genera, the great Swede placed all <strong>of</strong> them in the genus Didelphis. New generic names for opossums proliferated over subsequent decades <strong>of</strong> the 18th <strong>and</strong> 19th centuries (table 15), but no consistent binomial usage had emerged prior to Thomas’s (1888) l<strong>and</strong>-
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