phylogenetic relationships and classification of didelphid marsupials ...

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56 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 Fig. 22. Lingual views of lower incisors. Top, Metachirus nudicaudatus (AMNH 266452) with distinct posterior accessory cusps on i1–i4. Bottom, Lutreolina crassicaudata (AMNH 210424) without distinct posterior accessory cusps on i1–i4. Lestodelphys, Monodelphis, Tlacuatzin). In all of these taxa, the unworn crown of c1 tapers to a sharp apical point (fig. 23A). By contrast, the lower canine is procumbent and tends to become premolariform in other small didelphids. Premolarization of c1 includes the development of a flattened anterior blade (rather than a sharp tip) and a posterior cingulum that sometimes produces a distinct accessory cusp. Whereas these
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 57 Fig. 23. Labial views of unworn anterior mandibular dentitions illustrating taxonomic differences in lower canine (c1) morphology. A, Lestodelphys halli (MMNH 17171) with erect, acutely pointed, and simple c1. B, Marmosa mexicana (AMNH 17136) with semiprocumbent and flat-crowned but simple c1. C, Marmosops pinheiroi (AMNH 267342) with fully premolariform c1 (procumbent and flat crowned with posterior accessory cusp). tendencies are minimally indicated on the unworn teeth of Chacodelphys and Metachirus (which are flattened along their anterior margin but have no posterior cingulum), they are increasingly apparent in Cryptonanus, Gracilinanus, Marmosa (fig. 23B), and Thylamys. Premolarization of the lower canine is maximally developed in the smaller species of Marmosops (e.g., M. parvidens and M. pinheiroi), where c1 and p1–3 form a series of four strikingly similar teeth (fig. 23C). Generally speaking, taxa with welldeveloped accessory cusps on the upper canine also have a well-developed posterior accessory cusp on c1, so variation in these occluding teeth is at least partially correlated. Large dasyurids (e.g., Dasyurus) have erect, unicuspid lower canines, but many small dasyurids (e.g., Sminthopsis) have procumbent, premolariform teeth. The lower canines are small and blunt in peramelemorphians. In Dromiciops, c1 is procumbent and more or less premolariform (Marshall, 1982b: fig. 14). LOWER PREMOLARS: Didelphid lower premolars are unremarkable teeth, each having a single dominant cusp and a well-developed distal cingulum that often produces a posterior accessory cusp. Small diastemata are sometimes present between p1 and c1, between p1 and p2, and (rarely) between p2 and p3. The lower first premolar (p1) is vestigial and occlusally featureless in Caluromys and Caluromysiops, but it is well developed in all of the other genera. The second lower premolar is distinctly taller than p3 in most didelphids, but p2 and p3 are subequal in Marmosops incanus and most examined species of Monodelphis and Thylamys. In Lestodelphys and in certain other species of Monodelphis (e.g., M. emiliae), however, p3 is distinctly taller than p2. Most other plesiomorphic marsupials have three lower premolars that are essentially similar to those of didelphids, although some dasyurids (e.g., Dasyurus) have only two. Among the taxa we examined, p3 is taller than p2 in caenolestids, Dromiciops (see Marshall, 1982b: fig. 14), some dasyurids (e.g., Murexia), and some peramelemorphians (e.g., Echymipera), but p2 and p3 are subequal in height or p2 is the taller tooth in other dasyurids (e.g., Sminthopsis) and peramelemorphians (e.g., Macrotis).
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PHYLOGENETIC RELATIONSHIPS AND CLAS
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CONTENTS Abstract .................
Page 5 and 6: ABSTRACT This report summarizes a d
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