phylogenetic relationships and classification of didelphid marsupials ...

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58 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 Fig. 24. Occlusal views of right dp3 and m1 illustrating taxonomic differences in trigonid morphology of the deciduous tooth. Top, Marmosops impavidus (MUSM 13286) with a complete (tricuspid) dp3 trigonid. Bottom, Marmosa murina (MUSM 15297) with an incomplete (bicuspid) dp3 trigonid. LOWER MILK PREMOLAR: Taxonomically significant variation among didelphids in the size and shape of dp3 was reported by Voss et al. (2001: table 5). Many didelphids (Caluromys lanatus, Caluromysiops, Chironectes, Cryptonanus chacoensis, Didelphis, Lutreolina, Marmosops, Metachirus, Monodelphis emiliae, Philander) have a fully molariform dp3 in which the trigonid is represented by its normal complement of three cusps (paraconid, protoconid, metaconid) in a more or less triangular configuration (fig. 24, top). A fully molariform dp3 is also the modal condition for Monodelphis brevicaudata, in which seven of nine specimens examined with intact milk dentitions had complete trigonids. By contrast, dp3 is only partially molariform in Cryptonanus unduaviensis, Gracilinanus agilis, Lestodelphys, Marmosa, and Thylamys, which have incomplete, blade-like trigonids bearing only one or two distinct cusps (fig. 24, bottom). Although just the protoconid of dp3 is usually distinct in our material of Caluromys philander, the trigonid of this taxon is triangular in occlusal outline and more closely resembles the fully molariform condition than the blade-like uni- or bicuspid alternative. Among other plesiomorphic marsupials that we examined, only Dromiciops has a large and fully molariform dp3, whereas dasyurids and peramelemorphians have small, structurally simplified and obviously vestigial lower milk premolars. LOWER MOLARS: Didelphid lower molars conform to the usual tribosphenic pattern and are readily described using standard nomenclature (fig. 20). The trigonid moiety of each tooth invariably consists of three distinct cusps (paraconid, protoconid, and metaconid) that are connected by two tall and deeply notched crests (paracristid and metacristid); three additional cusps (hypoconid, entoconid, and hypoconulid) surround the talonid basin. A well-developed anterior
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 59 cingulid (precingulid) is always present, but a posterior cingulid (postcingulid) is just as consistently absent. 20 The anterior cingulid never extends all the way to the base of the paraconid because it abuts on the hypoconulid of the preceding tooth (dp3 in the case of m1), resulting in a prominent concavity in the anterolingual occlusal outline (the ‘‘hypoconulid notch’’ of Archer, 1976b). The entocristid is a short crest that approximately parallels the lingual margin of the tooth. Didelphid lower molars exhibit taxonomic shape variation that is correlated with carnassialization as previously described for the upper teeth. Thus, taxa with uncarnassialized molars have relatively small trigonids and large talonids, whereas opossums with highly carnassialized molars have relatively large trigonids and small talonids. Trigonid enlargement is invariably accompanied by an increase in occlusal relief, the protoconid and metacristid in particular tending to become very tall in the most carnassialized forms (e.g., Lestodelphys and Lutreolina). The width gradient previously noted for the upper molars is also seen in the lowers, m2 being the widest tooth in uncarnassialized dentitions whereas m3 or m4 is widest in progressively more carnassialized forms. As for the upper teeth, the highly divergent endpoints of this conspicuous gradient of dental shape variation are connected by an almost continuous taxonomic series of morphological intermediates. Only a few lower molar traits that vary among didelphids seem useful for the purposes of taxon diagnoses and phylogenetic analysis. Among these, the labial prominence of the hypoconid tends to vary along the toothrow, so we standardized comparisons at the third molar locus. In most opossums, the hypoconid is labially salient (projecting beyond the protoconid or level with it) on m3, but the hypoconid is lingual to the protoconid in Chacodelphys, Lestodelphys, Lutreolina, and Monodelphis. Most didelphids have a well-developed entoconid that is about as tall as the 20 A posterior cingulid (‘‘postcingulum’’) was illustrated by Goin and Candela (2004: fig. 2B) on a diagram representing the occlusal features of a ‘‘didelphimorphian opossum,’’ but no living didelphimorphian has such a structure. hypoconid and much exceeds the adjacent hypoconulid in height and breadth on m1–m3 (fig. 25, top). In Chacodelphys and Monodelphis, however, the entoconid is very small, never more than subequal to the hypoconulid, and often smaller than that cusp (fig. 25, bottom); it becomes indistinct or is obliterated by wear in most adult specimens. Discrepant observations in the literature about didelphid entoconid size variation were discussed by Voss and Jansa (2003). In most didelphids, the hypoconulid is much closer to the entoconid than it is to the hypoconid, and is said to be ‘‘twinned’’ with the former cusp. Although twinning of the hypoconulid and entoconid is often stated (or implied) to be universally present among tribosphenic marsupials (e.g., by Clemens, 1979; Kielan-Jaworowska et al., 2004), it is not. The hypoconulid and entoconid are not closely approximated in Caluromysiops, and in some specimens (e.g., FMNH 121522) the hypoconulid is at the center of the posterior talonid margin, equidistant to the hypoconid and the entoconid. Most other plesiomorphic marsupials have lower molars that resemble those of didelphids in common tribosphenic features, but several differences are noteworthy. Dasyurids, for example, have a well-developed posterior cingulid on m1–m3, the anterior cingulid of peramelemorphian lower molars lacks a hypoconulid notch, the anterior cingulid is vestigial in Dromiciops (which lacks a hypoconulid notch on m1), and an entocristid is absent in some dasyurids (e.g., Sminthopsis) and peramelemorphians (e.g., Perameles). As they are in so many other character complexes, caenolestids are the most divergent basal marsupials in lower molar morphology. In both Caenolestes and Rhyncholestes the paracristid is unnotched and decreases gradually in height from the apex of the protoconid to the anterolingual corner of the tooth; the paraconid is therefore indistinct. Whereas the paracristid runs in an uninterrupted arc from the protoconid to the anterolingual corner of the tooth on m1, the paracristid on m2 and m3 abuts the hypoconulid of the preceding tooth (m1 and m2, respectively), at which point it is abruptly deflected posterolingually. The only distinct trigonid cusp on m4 is the metaconid, from
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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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