phylogenetic relationships and classification of didelphid marsupials ...

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50 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 the tooth is rounded, and only the posterior cutting edge is well developed. Small anterior blades are variably present near the base of P3 in Chironectes and Philander, but the apex of the tooth is always rounded anteriorly as in the other taxa with single-bladed P3s. Most other plesiomorphic marsupials have three upper premolars, but some dasyurids (e.g., Dasyurus) have only two. In addition to the usual diastema between P1 and P2, some adult dasyurids (e.g., Murexia) and all adult peramelemorphians have a diastema between P2 and P3. The third upper premolar (P3) is taller than P2 in caenolestids, Dromiciops, and some dasyurids (e.g., Murexia, Sminthopsis), but P2 is taller than P3 in other dasyurids (e.g., Myoictis). An anterior cutting edge is apparently absent from P3 in all nondidelphid marsupials. UPPER MILK PREMOLAR: Although the deciduous upper premolar (dP3) of didelphids has consistently been described as large and molariform (Flower, 1867; Thomas, 1888; Bensley, 1903; Tate, 1948b; Archer, 1976b), there is noteworthy taxonomic variation in the morphology of this tooth. In most opossums dP3 is, indeed, a sizable tooth: its crown area ranges from about 42% to 96% of the crown area of M1, the tooth immediately behind it (Voss et al., 2001: table 5). An obviously functional element of the upper toothrow, dP3 occludes with both the deciduous lower premolar (dp3) and with m1. Hyladelphys, however, has a very small upper milk premolar (,10% of the crown area of M1; op. cit.) that appears to be functionally vestigial because it does not occlude with any lower tooth. Although molariform, the didelphid upper milk premolar does not always match the teeth behind it in all occlusal details. In most specimens, the paracone of dP3 is located on the labial margin of the crown, and the stylar shelf is correspondingly incomplete, whereas the paracone is lingual to a continuous stylar shelf on all didelphid molars (see below). Among those didelphids whose milk premolars we examined, only Caluromys has a dP3 in which the paracone is usually lingual to a continuous stylar shelf. The morphology of milk premolars remains to be widely surveyed in Marsupialia, but of those taxa that we examined, only Dromiciops has a large, molariform dP3 resembling the common didelphid condition (Marshall, 1982b: fig. 17; Hershkovitz, 1999: fig. 32). By contrast, dP3 is much smaller, more or less vestigial, and structurally simplified in caenolestids, dasyurids, and peramelemorphians (Tate, 1948a; Archer, 1976b; Luckett and Hong, 2000). UPPER MOLARS: Didelphid upper molars conform to the basic tribosphenic bauplan (Simpson, 1936) in having three principal cusps—paracone, protocone, and metacone—connected by the usual crests in a more or less triangular array (fig. 20). A broad stylar shelf and an anterolabial cingulum are invariably present; the centrocrista (postparacrista + premetacrista) and the ectoloph (preparacrista + centrocrista + postmetacrista) are uninterupted by gaps; the para- and metaconules are indistinct or absent; 15 and there is no posterolingual talon. In addition, most didelphids have several (usually five or six) small cusps on the stylar shelf, for which most authors employ alphabetical labels (after Bensley, 1906; Simpson, 1929; Clemens, 1966). Of these, stylar cusp B (labial to the paracone) is more consistently recognizable than the others, but a stylar cusp in the D position (labial to the metacone) is often subequal to it in size. Didelphids differ conspicuously in the relative width (transverse or labial-lingual dimension) of successive molars within toothrows. In some taxa, the anterior molars tend to be wide in proportion to more posterior teeth, but in others the posterior molars are relatively wider (fig. 21). The ratio obtained by dividing the width of M4 by the width of M1 (M4/M1; table 8) conveniently indexes this size-independent shape variation and ranges from a minimal value of 0.83 (in 15 The literature is inconsistent on this point, with some authors claiming to have observed significant taxonomic variation among Recent didelphids in the occurrence of conules (see Voss and Jansa, 2003: appendix 4). Indeed, careful examination of unworn teeth usually reveals a tiny enameled chevron on the postprotocrista that is presumably homologous with the metaconule of stem metatherians; corresponding structures on the preprotocrista, presumably vestigial paraconules, are much less frequently observed. The fact that conules have been scored as present and absent in the same taxon (e.g., Didelphis) by different authors (e.g., Reig et al. [1987] versus Wroe et al. [2000]) sufficiently illustrates the ambiguous interpretation of such indistinct features.
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 51 Fig. 20. Occlusal views of left upper and right lower didelphid molars illustrating features of crown morphology discussed in the text. Abbreviations: acid, anterior cingulid; alci, anterolabial cingulum; ecf, ectoflexus; encd, entoconid; encrd, entocristid; hycd, hypoconid; hycld, hypoconulid; hycld.n., hypoconulid notch; mec, metacone; mecd, metaconid; mecrd, metacristid; pac, paracone; pacd, paraconid; pacrd, paracristid; pomecr, postmetacrista; popacr, postparacrista; poprcr, postprotocrista; prc, protocone; prcd, protoconid; prmecr, premetacrista; prpacr, preparacrista; prprcr, preprotocrista; stB, stylar cusp B; stD, stylar cusp D. Caluromysiops) to a maximal value of 1.76 (in Lutreolina). Morphometric comparisons of dental measurements (Voss and Jansa, 2003: fig. 12) and visual inspections of toothrows suggest that this index is correlated with a pattern of molar occlusal transformation that has been described in the literature as ‘‘carnassialization’’ (Reig and Simpson, 1972: 534) or as ‘‘an emphasis on postvallum-prevalid shear’’ (Muizon and Lange- Badré, 1997). These tendencies are redundantly described by numerous dental ratios that have been coded as independent characters in previous phylogenetic studies (for an extended discussion, see Voss and Jansa, 2003). In effect, didelphids with relatively wide anterior molars have dentitions that can be described as not or only weakly carnassialized, whereas those with relatively wide posterior molars have strongly carnassialized teeth. Although the endpoints of this morphocline are strikingly different, the essentially continuous distribution of intermediate morphologies renders any attempt to make qualitative distinctions based on molar proportions an arbitrary exercise. We therefore employ ‘‘not carnassialized,’’ ‘‘weakly carnassialized,’’ ‘‘moderately carnassialized,’’ and ‘‘strongly carnassialized’’ as heuristic descriptors of a visually compelling but phylogenetically intractable aspect of didelphid dental variation. 16 The ectoflexus is a V-shaped labial indentation of the stylar shelf that is either present or absent on tribosphenic marsupial molars. Among didelphids, Caluromys and Caluromysiops are the only taxa that lack any trace of an ectoflexus (e.g., figs. 21A, B). In most other opossums, a distinct ectoflexus is present on M3, on M2 and M3, or (rarely) on M1–M3. When ectoflexi are present on multiple teeth, they invariably increase in depth from anterior to posterior. The shape of the centrocrista (postparacrista + premetacrista) and the descriptive nomenclature associated with its alternative states have been a source of some confusion among marsupial researchers. Traditionally, taxa with a ‘-shaped (labially inflected) centrocrista have been described as ‘‘dilambdodont’’ because the ectoloph (preparacrista + centrocrista + postmetacrista) is then Wshaped (like two inverted lambdas), whereas taxa with a straight (uninflected) centrocrista 16 We were not so circumspect in earlier phylogenetic analyses, where computed ratios were used to define qualitative states (e.g., Voss and Jansa, 2003: character 57), but measurements from additional taxa (including outgroups not analyzed by us in 2003) have blurred distinctions that once seemed clear.
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Page 5 and 6: ABSTRACT This report summarizes a d
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