phylogenetic relationships and classification of didelphid marsupials ...

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46 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 er, has only a single mental foramen, which is very large and occupies the same position as the anterior foramen in those taxa with two lateral mandibular perforations. Alternative states of the form and orientation of the marsupial angular process were defined by Sánchez-Villagra and Smith (1997), but we are unable to consistently recognize the distinction between the ‘‘rodlike’’ and ‘‘intermediate’’ conditions they scored for didelphid exemplars. Instead, the common didelphid condition seems more appropriately characterized as ‘‘acute and strongly inflected’’ (Voss and Jansa, 2003: 34). Caluromys and Caluromysiops are the only didelphids with bluntly rounded and weakly inflected mandibular angles. Most other plesiomorphic marsupials have mandibles that grossly resemble the common didelphid morphotype, but Dromiciops has only one mental foramen and some peramelemorphians (e.g., Perameles gunnii) often have three mental foramina. Additionally, Caenolestes has a small foramen in the retromolar space that Sánchez-Villagra et al. (2000) called the retrodental canal (after Hoffstetter and Villaroel, 1974), and it has a small masseteric foramen near the ventral margin of the masseteric fossa (Osgood, 1921), the lumen of which communicates with that of the much larger mandibular foramen on the medial surface of the jaw. Dentition All didelphids normally have 50 teeth that most authors now recognize as including I1– 5, C1, P1–3, and M1–4 in the upper dentition and i1–4, c1, p1–3, and m1–4 in the lower dentition. As in other metatherians, only the third upper and lower premolars have deciduous precursors (Flower, 1867; Luckett, 1993; Luo et al., 2004; van Nievelt and Smith, 2005a, 2005b). Despite this prevailing consensus, marsupial tooth homologies have long been controversial, with the result that several alternative systems have been used to identify didelphid dental loci (table 7). Asfaraswehavebeenabletodetermine without dissections or X-rays (i.e., from external inspection of teeth in situ, and from the alveoli of specimens with loose teeth), all didelphids have single-rooted incisors and canines, double-rooted premolars and lower molars, and three-rooted upper molars; apparently, dP3 always has three roots and dp3 has two. Most of the following remarks therefore concern crown morphologies, which afford greater scope for taxonomic comparisons. UPPER INCISORS: The didelphid first upper incisor (I1) is a more or less styliform, somewhat procumbent, hypsodont tooth that is conspicuously unlike I2–I5, from which it is always separated by a small diastema. Although taxonomic variation among opossums in I1 hypsodonty and diastema formation has been suggested by authors (e.g., Takahashi, 1974; Archer, 1976b; Creighton, 1984; Wroe et al., 2000), diagnostically useful differences are more easily recognized among the posterior teeth of this series. The crowns of I2–I5 in most didelphids are approximately symmetrical, labiolingually compressed rhomboids with subequal anterior (mesial) and posterior (distal) cutting edges that converge to form a sharp central apex; usually, a distinct anterior angle (mesiostyle) and a posterior angle (distostyle) can be seen on unworn teeth (fig. 18A). This is the morphology that Takahashi (1974: 414) termed ‘‘premolariform’’, as exemplified by Monodelphis, Marmosa, and Metachirus in her study. In many taxa with posterior upper incisors of this type, the tooth crowns increase in length (anteroposterior or mesiodistal dimension) from front to back, such that I2 appears visibly smaller than I5 in labial view. This tendency is very pronounced in some taxa with premolariform incisors (e.g., Marmosops, Metachirus) but seems to grade imperceptibly (via intermediate morphologies) to the essentially uniform toothrows of other taxa (e.g., Lestodelphys, Tlacuatzin) in which I2 and I5 are subequal in crown length. An alternative morphology, in which the crowns of I2–I5 are conspicuously asymmetrical, with much longer anterior (mesial) than posterior (distal) cutting edges, occurs in Caluromys, Caluromysiops, Didelphis, Glironia, Hyladelphys, Lutreolina, andPhilander. On unworn teeth of these taxa, the anterior angle (mesiostyle) is more frequently distinct than is the posterior angle (distostyle), which is often entirely lacking on I5 (fig. 18B). This is the morphology that Takahashi (1974) labeled ‘‘incisiform,’’ which she recorded for
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 47 This study a Thomas (1887) b Chironectes in addition to Caluromys and other taxa in her study. However, the unworn juvenile dentitions of Chironectes that we examined have approximately symmetrical, rhomboidal crowns. There is a tendency (more marked in Glironia than in other genera of this category) for I2–I5 to decrease in crown length from front to back, such that I2 is sometimes visibly larger than I5 in labial view. Aspects of didelphid upper incisor morphology reported by authors whose observations we have not been able to replicate were discussed by Voss and Jansa (2003: 35). TABLE 7 Alternative Hypotheses of Marsupial Tooth Homologies Archer (1984) c Hershkovitz (1992b) d Luckett (1993) e Goin (1997) f P1 P1 P1 P1 dP1 P1 P2 P3 P2 P2 dP2 P2 dP3 dP4 M1 M1 dP3 M0 P3 P4 P3 P3 P3 P3 M1 M1 M2 M2 M1 M1 M2 M2 M3 M3 M2 M2 M3 M3 M4 M4 M3 M3 M4 M4 M5 M5 M4 M4 i1 i1 i1 i2 i1 i2 i2 i2 i2 i3 i2 i3 i3 i3 i3 i4 i3 i4 i4 — — i5 i4 i5 p1 p1 p1 p1 dp1 p1 p2 p3 p2 p2 dp2 p2 dp3 dp4 m1 m1 dp3 m0 p3 p4 p3 p3 p3 p3 m1 m1 m2 m2 m1 m1 m2 m2 m3 m3 m2 m2 m3 m3 m4 m4 m3 m3 m4 m4 m5 m5 m4 m4 a The system adopted here was used by Tate (1933) and is currently followed by most metatherian researchers (e.g., Marshall et al., 1995; Rougier et al., 1998; Wroe et al., 2000; Voss and Jansa, 2003). Note that there is no substantive inconsistency in the literature regarding the identification of I1–I5, C1, or c1. In order to focus these comparisons on real differences in assumptions about dental homologies, semantically equivalent notations used by authors (e.g., I 2 for i2, pm 3 for P3) have been modified as necessary to conform with our usage. b Also Thomas (1888), who assumed that marsupials primitively had four premolars, of which the second was lost. c Based on the assumption that the replaced teeth are M1/m1 (Archer, 1978), this is the nomenclature followed throughout most of the marsupial literature for the next 10 years (e.g., Reig et al., 1987). d Based on the assumption that marsupials primitively had five lower incisors, of which the first was lost (Hershkovitz, 1982). Note that whereas Hershkovitz (1992b) accepted Archer’s (1984) system of postcanine homologies, Hershkovitz (1997, 1999) did not. e The only difference between Luckett’s (1993) notation and ours concerns his designation of the anterior premolars as first-generation (formerly deciduous) teeth. Although we do not dispute his interpretation of the developmental data at hand, we think it confusing to use deciduous notation for unreplaced teeth. f Goin’s (1997) system was based on Hershkovitz’s (1982) theory that the ancestral first lower incisor of marsupials is missing in living taxa, and on Archer’s (1978) conjecture that replaced teeth in the upper and lower dentition are molars; by designating these as M0/m0, Goin intended to preserved traditional notation for the permanent molariform teeth. Most peramelemorphians and Dromiciops resemble didelphids in having five upper incisors, but I5 is missing in dasyurids, caenolestids, Echymipera, and Rhynchomeles. 13 The first upper incisor is styliform 13 Following Thomas (1887), we assume that upper teeth are lost from (or added to) the posterior (distal) end of the incisor row. Although there appears to be no developmental data to support this notion, we observed that I5 is small and occupies a separate alveolus from I2–I4 (whose alveoli are usually at least partially confluent) in some taxa with five upper incisors (e.g., Glironia, Hyladelphys), and that the loss of such a tooth would simply enlarge an already existing diastema between the incisors and C1 rather than opening a fresh gap in the dental arcade.
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