phylogenetic relationships and classification of didelphid marsupials ...

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48 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 Fig. 18. Unworn premaxillary dentitions of Marmosops pinheiroi (A, AMNH 267341) and Lutreolina crassicaudata (B, AMNH 210422) illustrating taxonomic differences in the shape of the incisor crowns. In Marmosops and many other small didelphines, the crowns of I2–I4 are symmetrically rhomboidal, with subequal anterior (mesial) and posterior (distal) cutting edges that converge to form a sharp central apex; a distinct posterior corner (distostyle) is always present on I5. By contrast, in Lutreolina (and certain other taxa), the crowns of I2–I4 are conspicuously asymmetrical, with longer anterior than posterior cutting edges; a distinct distostyle is usually absent on I5. Scale bars 5 1mm. in most of these taxa, but I1 is a laterally compressed, distinctly chisellike tooth in caenolestids, and it is a labiolingually compressed, flat-crowned tooth in peramelemorphians. The unworn posterior incisors have approximately rhomboidal crowns with subequal anterior and posterior cutting edges that converge to a sharp apex in most dasyurids and in Dromiciops, but I2–4 in caenolestids have strikingly asymmetrical crowns (appropriately described as ‘‘hatchet-shaped’’ by Osgood, 1921: 117), the posterior margins of which are deeply notched in Rhyncholestes (see Osgood, 1924: figs. 3b, c; Bublitz, 1987: fig. 13). In peramelemorphians, I2–4 are flat crowned (like I1), but I5 (when present) is caniniform. All of the upper incisors are single rooted in dasyurids, caenolestids, Dromiciops, and many peramelemorphians, but I5 is double rooted in Perameles gunnii. UPPER CANINE: The didelphid upper canine (C1) is invariably separated from the posteriormost upper incisor by a wide diastema that may or may not contain a distinct paracanine fossa for the reception of the lower canine. In most didelphids, C1 occupies the premaxillary-maxillary suture, despite the fact that the maxilla always extends anterolaterally beyond this tooth, sometimes reaching the alveolus of I5; therefore, the premaxilla only contacts the C1 alveolus medially (fig. 14). In Caluromys and Caluromysiops, however, the upper canine alveolus is entirely contained within the maxilla (figs. 38, 39). The upper canine is a simple unicuspid tooth in most didelphids, but some species (e.g., Gracilinanus emiliae; Voss et al., 2001: fig. 12A) have a small posterior accessory cusp, and others (e.g., Marmosops pinheiroi; fig. 53) have both anterior and posterior accessory cusps. Some care is needed in determining canine crown morphology, however, because accessory cusps are sometimes obliterated by wear (as inferred from their absence in old adults of species that uniformly exhibit such structures as juveniles and subadults). Alternatively, a false posterior accessory cusp is sometimes formed when the trailing edge of C1 is notched by occlusion with p1. Unlike other teeth whose size does not increase ontogenetically after the crown is fully erupted, didelphid canines often appear much larger in older adults than in younger animals because the root is continuously extruded from the alveolus throughout life. Most other marsupials have single-rooted unicuspid upper canines like those of didelphids, but accessory cusps are present in some peramelids (e.g., Echymipera) and dasyurids (e.g., Antechinomys; Archer, 1976b). Additionally, the canine is double rooted and provided with accessory cusps in female Caenolestes and Rhyncholestes and in
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 49 Fig. 19. Lateral views of left C1–P3 in Hyladelphys kalinowskii (A, AMNH 267338), Gracilinanus agilis (B, AMNH 134005), and Thylamys pusillus (C, AMNH 275445) illustrating taxonomic differences in the relative heights of P2 and P3 (toothrows are not drawn to the same scale). both sexes of Lestoros, whereas male Caenolestes and Rhyncholestes have single-rooted unicuspid teeth (Osgood, 1924; Bublitz, 1987). The position of C1 with respect to the maxillary-premaxillary suture has not been widely surveyed in Marsupialia, but the upper canine alveolus is entirely contained by the maxilla in Dromiciops, some peramelemorphians (e.g., Echymipera, Perameles), and some dasyuromorphians (Murexia, Sminthopsis). In several diprotodontian clades, C1 is absent. UPPER PREMOLARS: The didelphid first upper premolar (P1) is usually in contact with or very close to C1, but a diastema is often present between P1 and P2. The gap between P1 and P2 tends to be individually and ontogenetically variable, however, and a continuous range of intermediates exist between taxa in which it is usually absent (e.g., Lestodelphys; fig. 51) and those in which it is normally present (e.g., Philander; fig. 48). By contrast, no diastema is ever present between P2 and P3, the crowns of which are in contact or closely approximated in all examined opossum dentitions. Although consistently smaller than the posterior premolars, P1 in most didelphids is at least half the height or width of P2, which it essentially resembles in having a large, laterally compressed, more or less triangular central cusp that is usually flanked by smaller anterior and posterior accessory cusps. By contrast, P1 is a much smaller (apparently vestigial) tooth that lacks distinct occlusal features in Caluromys (fig. 38) and Caluromysiops (fig. 39). In a few specimens of the latter genus (e.g., AMNH 244364), P1 is missing. The second and third upper premolars are always large and well-developed teeth. Each is surrounded by a broad basal cingulum in Caluromys and Caluromysiops, but the basal cingulum of P2 and/or P3 is often incomplete or indistinct in other didelphids. The second upper premolar is distinctly taller than P3 in Caluromys, Caluromysiops, andHyladelphys (fig. 19A), but P3 is distinctly taller than P2 in Chironectes, Cryptonanus, Didelphis, Lestodelphys, Lutreolina, Monodelphis, Philander, and Thylamys (fig. 19C). By contrast, P2 and P3 are about the same height in Chacodelphys, 14 Glironia, Gracilinanus (fig. 19B), Marmosa, Marmosops,andMetachirus. When unworn, P3 is provided with sharp anterior and posterior cutting edges—each of which extends from the cingulum to the apex of the tooth—in Caluromys, Caluromysiops, Glironia, and Hyladelphys. In all other didelphids, however, the anterior margin of 14 Although we (Voss et al., 2004a) previously reported that P2 is taller than P3 in Chacodelphys based on our examination of the holotype of C. formosa (USNM 236330), subsequently collected material (Teta et al., 2006) includes five individuals in which P2 and P3 are subequal in height. We reexamined USNM 236330 and concluded that, although the crown of P3 was completely exposed above the alveolus, it had not yet attained its definitive adult position in the toothrow of this young adult specimen.
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