phylogenetic relationships and classification of didelphid marsupials ...

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84 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 subfamily Didelphinae, and he did not use tribal nomenclature to indicate relationships among them. In effect, his treatment of Recent didelphids amounted to no more than an alphabetic list of generic names, conveying even less information about inferred relationships than the classifications of his predecessors. Cabrera’s (1958) checklist of South American mammals was one of the last attempts to classify Recent opossum diversity by traditional (prephylogenetic) criteria, and it remained more or less unchallenged until the advent of molecular systematics in the mid- 1970s. Despite the fact that at least some contemporaneous researchers (e.g., Reig, 1955) recognized suprageneric groups among living opossumlike marsupials, Cabrera did not. Instead, only his taxonomic sequence and his use of subgenera for Monodelphis and Marmosa hint at inferred patterns of relationships. Most of the generic names that Cabrera recognized as valid are still used in the same sense today, with the conspicuous exception of Marmosa: whereas the nominotypical subgenus contained (in addition to Marmosa sensu stricto) species now referred to Marmosops, the subgenus Thylamys contained (in addition to Thylamys sensu stricto) species now referred to Chacodelphys, Cryptonanus, and Gracilinanus. Reig et al. (1985) provided the first classification of opossumlike marsupials based on an explicitly phylogenetic analysis (subsequently published by Reig et al., 1987), and their classification was also the first to incorporate results from molecular and cytogenetic research (Kirsch, 1977b; Reig et al., 1977). Not surprisingly, this classification (table 17) departed from its predecessors in several respects. Among them, Dromiciops was once and for all removed to a separate family, ‘‘caluromyines’’ and didelphines were distinguished as separate subfamilies, and several tribes of the latter were recognized: all of the large opossums with a diploid number (2n) of 22 chromosomes were referred to the Didelphini, all of the smaller taxa with 2n 5 14–18 chromosomes were referred to the Marmosini (thus implying a close relationship between Monodelphis and the ‘‘murine’’ opossums), and a monotypic tribe was created for Metachirus. The classification proposed by Hershkovitz (1992b) was not supported by any phylogenetic analysis, nor were phylogenetic criteria cited to support any of its novel features, including the recognition of four groups of Recent opossums as separate families (Marmosidae, Caluromyidae, Glironiidae, Didelphidae). Taxonomic rank aside, these taxa represent significant departures from previous classifications by grouping Metachirus with Monodelphis and the ‘‘murine’’ opossums, by the absence of any formal relationship between this group (Marmosidae) and the large opossums with 22 chromosomes (Didelphidae), and by the separation of Glironia on the one hand from Caluromys and Caluromysiops on the other. Also unlike other classifications, four ‘‘murine’’ genera (Gracilinanus, Marmosops, Marmosa, Micoureus) were associated in a single taxon (Marmosinae) that excluded Thylamys and Lestodelphys. Kirsch and Palma (1995) formalized the results of their DNA-DNA hybridization experiments in a classification that, for the first time, associated Metachirus (as a monotypic tribe) with the large 2n 5 22 opossums (Didelphini), associated Monodelphis with Marmosa and Micoureus, and recognized the other ‘‘murine’’ opossums (Gracilinanus, Lestodelphys, Marmosops, Thylamys) as forming a separate clade. All of these taxa were referred to the family Didelphidae, whereas Glironia, Caluromys, andCaluromysiops were referred to the Caluromyidae. McKenna and Bell’s (1997) classification of Recent opossums followed Reig et al. (1985), but they used the name Monodelphini for the group that those authors called Marmosini. This was the system subsequently adopted by Gardner (2005, 2008) who lumped all of the old ‘‘murine’’ genera together with several new taxa in the tribe Monodelphini. No subordinate taxa (within Monodelphini) were recognized by these authors to indicate relationships among the many genera assigned to this group. A Revised Phylogenetic System As reviewed above, all previous classifications of Recent opossums contain nonmonophyletic groups. Examples include ‘‘Marmo-
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 85 Reig et al. (1985) a sini’’ sensu Reig et al. (1985), ‘‘Marmosidae’’ sensu Hershkovitz (1992b); ‘‘Marmosopsini’’ sensu Kirsh and Palma (1995), and ‘‘Monodelphini’’ sensu Gardner (2005, 2008). Therefore, a revised phylogenetic system is needed. The classification suggested below is based on the following principles: (1) taxa should be demonstrably monophyletic; (2) binomial nomenclature should remain as stable as possible consistent with the requirement of monophyly; (3) it is not necessary to name every node in a cladogram; (4) if future discoveries can be reasonably anticipated, taxa should be named so that these can be incorporated with minimal disruption; and (5) usage should be consistent with the International Code of Zoological Nomencla- TABLE 17 Classifications of Recent Opossums, 1985–2008 Hershkovitz (1992b) b Kirsch and Palma (1995) Gardner (2008) c Didelphidae Marmosidae Didelphidae Didelphidae Didelphinae Marmosinae Didelphinae Caluromyinae Didelphini Gracilinanus Didelphini Caluromys Chironectes Marmosops Didelphis Caluromysiops Didelphis Marmosa Philander Glironia Lutreolina Micoureus Lutreolina Didelphinae Philander Thylamyinae Chironectes Didelphini Marmosini Thylamys Metachirini Chironectes Lestodelphys Lestodelphyinae Metachirus Didelphis Marmosa Lestodelphys Thylamyinae Lutreolina Micoureus Metachirinae Thylamyini Philander Monodelphis Metachirus Thylamys Metachirini Thylamys Monodelphinae Lestodelphys Metachirus Metachirini Monodelphis Marmosopsini Monodelphini Metachirus Caluromyidae Marmosops Chacodelphys Caluromyinae Caluromyinae Gracilinanus Cryptonanus Caluromys Caluromys Marmosinae Gracilinanus Caluromysiops Caluromysiopsinae Marmosini Hyladelphys Glironia Caluromysiops Marmosa Lestodelphys Glironiidae Micoureus Marmosa Glironia Monodelphini Marmosops Didelphidae Monodelphis Micoureus Philander Caluromyidae Monodelphis Didelphis Caluromyinae Thylamys Chironectes Caluromys Lutreolina Caluromysiops Glironiinae Glironia a This is also the classification of Reig et al. (1987) and Marshall et al. (1990). b The classification of Hershkovitz (1992a) differs from this one in minor details. Hershkovitz (1997) proposed that Lutreolina and Chironectes each be placed in monotypic subfamilies of Didelphidae. c Gardner’s (2008) suprageneric classification and nomenclature followed McKenna and Bell (1997) but included subsequently described genera. ture. Points (1) and (2) should be familiar to most readers and, we trust, are not controversial. Point (3) might be disputed by those who feel that cladograms should be completely described by classifications, but the mnemonic value of names is lost if they proliferate beyond reasonable need, the hierarchical ordering implied by the consistent use of familiar suffixes (e.g., -idae, -inae, -ini) is compromised if many clade names with unfamiliar endings are interpolated, and naming weakly supported clades that might disappear when new character data or taxa are added to future analyses is not in the interest of nomenclatural stability. Point (4) refers to long branches, which can be interpreted as artifacts of extinction or
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PHYLOGENETIC RELATIONSHIPS AND CLAS
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CONTENTS Abstract .................
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ABSTRACT This report summarizes a d
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2009 VOSS AND JANSA: DIDELPHID MARS
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