phylogenetic relationships and classification of didelphid marsupials ...

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54 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 near the base of the paracone; the anterolabial cingulum does not converge toward the preprotocrista in these taxa, but passes obliquely dorsally such that the two crests are discontinuous on the anterior surface of the tooth crown (fig. 21E, F). Voss and Jansa (2003: 39) discussed several inconsistencies in the published literature concerning the distribution of these traits among didelphids. The postprotocrista also exhibits noteworthy variation among didelphids. In most genera, this crest decreases in height and width as it passes posterolabially around the base of the metacone before merging with the posterior surface of the tooth crown, leaving a distinct groove or gap in the posterior wall of the trigon basin. By contrast, the postprotocrista connects directly with the base of the metacone, such that the posterior wall of the trigon basin has no gap or groove, in Chironectes, Didelphis, Lutreolina, and Philander; instead, the unworn postprotocrista of these taxa exhibits a distinct carnassial notch near the base of the metacone. Dasyurids and Dromiciops have tribosphenic upper molars that differ from those of didelphids in only minor details. Indeed, some dasyurids and didelphids are indistinguishable in published matrices of upper molar characters (e.g., Archer, 1976b: table 1), although there is a tendency for stylar cusp D to be larger than stylar cusp B when both structures are present in the former group (Wroe, 1997), and a few dasyurids (e.g., Sarcophilus) are conspicuously divergent in other upper molar traits. The upper molars of Dromiciops differ from those of most didelphids by having a linear centrocrista, indistinct stylar cusps, and a stylar shelf that is conspicuously reduced in width labial to the paracone on M1. 17 17 These distinctive features are not apparent in some published illustrations of the dentition of Dromiciops, notably Marshall’s (1982b) figure 16b, where an occlusal view of the upper teeth of FMNH 22671 shows ‘-shaped centrocristae and an unreduced stylar shelf on M1. However, in another illustration (based on FMNH 22673; op. cit.: fig. 17b), M1 is correctly shown with a reduced stylar shelf labial to the paracone, and with a linear centrocrista. We borrowed FMNH 22671 to determine whether this specimen is unusual in any way and found that it is not: the paracone of M1 is almost on the labial margin of the tooth, and the centrocristae of M1–M3 are linear; therefore, Marshall’s figure 16b is inaccurate. The upper molars of Recent peramelemorphians differ from those of didelphids in several respects, most notably by having a discontinuous centrocrista: instead of forming a ‘-shaped (labially inflected) crest with an apex that is lingual to the stylar shelf, the postparacrista and the premetacrista of modern peramelemorphians terminate on the labial margin of the tooth, where they are separated by a small gap (Muirhead and Filan [1995] referred to this trait as the result of the centrocrista ‘‘breaching’’ the ectoloph). Additionally, all modern peramelemophians have a large posterolingual cusp on M1–M3; in most genera (e.g., Echymipera, Perameles) this is a hypertrophied metaconule, but in Macrotis it is the metacone (Archer, 1976b). Another peramelemorphian trait is that the preparacrista of M1 is a tall crest that passes posterolabially to a large stylar cusp at or near the C position (Archer, 1976b). The anterolabial cingulum is present in some peramelemorphians (e.g., Echymipera) but not in others (e.g., Perameles). On caenolestid molars the principal labial cusps are hypertrophied stylar elements in the B and D positions (Osgood, 1921; Marshall, 1987; Goin and Candela, 2004; Goin et al., 2007). In both Caenolestes and Rhyncholestes, the paracone is absent (or indistinguishably fused with stylar cusp B). The metacone is also absent in Rhyncholestes but, although fused to the lingual aspect of stylar cusp D, the metacone is still clearly recognizable in Caenolestes. Given this interpretation of caenolestid cusp homologies, it follows that a low crest along the labial base of the tooth crowns in both genera is a neomorphic cingulum rather than a vestigial stylar shelf. The first two caenolestid upper molars also have a well-developed posterolingual cusp that has been interpreted either as a hypertrophied metaconule (Marshall, 1987; Goin and Candela, 2004; Goin et al., 2007) or as a neomorphic outgrowth of the postprotocingulum (Hunter and Jernvall, 1995). Another distinctive trait of the upper molars of Recent caenolestids is the complete absence of an anterolabial cingulum. UPPER DENTAL ERUPTION SEQUENCES: As described by Tribe (1990), P3 is the last upper tooth to erupt in most didelphids. By contrast, P3 erupts before M4, which is the
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 55 last upper tooth to emerge in Chironectes, Didelphis, Lutreolina, and Philander. In a few didelphids (e.g., Metachirus nudicaudatus, Monodelphis brevicaudata), M4 and P3 appear to erupt simultaneously. Molar eruption sequences have not been widely surveyed among other marsupials, but P3 seems to be the last upper tooth to erupt in most nondidelphid clades, with the exception of Dromiciops and caenolestids, in which M4 is usually last (Hershkovitz, 1999; Luckett and Hong, 2000). LOWER INCISORS: The four lower incisors of didelphids are subequal in size and essentially similar in coronal shape, but the alveolus of i2 is always ‘‘staggered’’ (displaced lingually and dorsally relative to the alveoli of i1, i3, and i4), and the root of this tooth is correspondingly supported by a prominent labial buttress of alveolar bone (Hershkovitz, 1982, 1995). The unworn lower incisor crowns of most didelphids have a distinct lingual cusp or heel (fig. 22, top) that is indistinct or absent in Chironectes, Didelphis, Lutreolina (fig. 22, bottom), and Philander. This character has the same taxonomic distribution among didelphids as Takahashi’s (1974) distinction between the ‘‘subrectangular’’ teeth of Chironectes, Didelphis, Lutreolina, and Philander on the one hand, and the ‘‘suboval’’ lower incisors of Caluromys, Marmosa, Metachirus, and Monodelphis on the other, but we are unable to appreciate the basis for her shape descriptors. No didelphid has a lobed or indented cutting edge on any of the lower incisor crowns. Among other marsupials, Dromiciops resembles didelphids in having four lower incisors, whereas dasyurids and peramelemorphians have only three. The missing tooth in dasyurids and peramelemorphians is presumably i4, because there is only one tooth behind staggered-and-buttressed i2, and because it seems more likely that teeth should be lost from the end of the incisor series than from intermediate loci. In all of these polyprotodont groups, the lower incisors are short-crowned nonprocumbent teeth that differ from those of didelphids in only minor details. The unworn lower incisors of most dasyurids (e.g., Murexia, Sminthopsis) have a distinct lingual cusp or heel, but this structure is lacking in at least some of the larger carnivorous forms (e.g., Dasyurus). A distinctive feature of unworn i3 in dasyurids is the consistent presence of a small posterior lobe on the cutting edge of the tooth. 18 As implied by Bensley (1903: 113), this structure appears to be homologous with the much larger posterior lobe of i3 seen in peramelemorphians. All peramelemorphian lower incisors lack a lingual cusp or heel, but i1 and i2 are otherwise unremarkable in this group. The lower incisor dentition of Dromiciops is similar to that of didelphids except for the fact that i2 is neither staggered nor buttressed in microbiotherians (Hershkovitz, 1999: figs. 5A, 29E). The anteriormost lower incisor of caenolestids is a long-crowned procumbent (gliriform) tooth that most authors (e.g., Ride, 1962; Marshall, 1980; Sánchez-Villagra, 2001b) assume to be either i1 or i2. Whichever it is, at least two of the four singlerooted unicuspids that normally occur between the gliriform tooth and the anteriormost undisputed premolar must also be incisors. 19 None of the lower incisors of Caenolestes or Rhyncholestes has a staggered alveolus. LOWER CANINE: The lower canine is an erect unicuspid tooth that is conspicuously differentiated from the incisors and premolars in all of the larger didelphids (Caluromys, Caluromysiops, Chironectes, Didelphis, Lutreolina, Philander), and in some of the smaller forms as well (Glironia, Hyladelphys, 18 This is the ‘‘posterobuccal’’ lobe of Archer (1976b: 6) who stated that it is present in only a few dasyurids. However, it is immediately recognizable in juvenile specimens of all the dasyurids we examined. Ontogenetically persistent in some taxa, it is soon obliterated with wear in others and may be indistinguishable in adults. 19 Like others who have pondered caenolestid antemolar tooth homologies (e.g., Thomas, 1895; Ride, 1962; Marshall, 1980), we assume that no more than one lower canine and three lower premolars are normally present. Because the presence of two lower premolars (each with two roots; Osgood, 1921) is undisputed, only one of the problematic unicuspids could belong to this series. Hershkovitz (1982) implied that none of the lower incisor alveoli of caenolestids are staggered, but he later (Hershkovitz, 1995) stated that Recent caenolestids have a staggered i2 (‘‘i3’’). In all of the specimens of Caenolestes and Rhyncholestes that we examined, however, the four unicuspids behind the gliriform tooth have colinear alveoli; there is no suggestion of lingual displacement at the second locus.
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PHYLOGENETIC RELATIONSHIPS AND CLAS
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