phylogenetic relationships and classification of didelphid marsupials ...

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36 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 Fig. 14. Palatal morphology of Thylamys venustus (AMNH 261254) illustrating nomenclature for fenestrae, foramina, and other features described in the text. Dental loci (I1–M4) provide convenient landmarks for defining the size and position of palatal structures. Abbreviations: if, incisive foramen; m, maxillary fenestra; mp, maxillopalatine fenestra; p, palatine fenestra; plpf, posterolateral palatal foramen. lambdoid crest, even as juveniles. The condition in caenolestids is uninterpretable because no sutures persist on the posterodorsal braincase of any examined specimen. The didelphid interparietal participates in two alternative patterns of contact among bones of the posterior braincase and the occiput. In most opossums the interparietal does not contact the squamosal because the parietal is in contact with the mastoid (fig. 13A, B). However, in Chironectes, Didelphis, Lutreolina, Philander, and in one examined specimen of Metachirus (fig. 13C, D) the interparietal contacts the squamosal and prevents the parietal from contacting the mastoid. Two species of Monodelphis (M. brevicaudata and M. emiliae) exhibit both conditions with approximately equal frequency among the specimens we examined. Most examined nondidelphid marsupials exhibit parietal-mastoid contact, but acrobatids and Tarsipes exhibit broad interparietalsquamosal contact (Parker, 1890: fig. 2), and phalangerids uniquely exhibit squamosalexoccipital contact (Flannery et al., 1987: fig. 4C, D). PALATE: The bony palate is variously perforated by foramina and fenestrae that exhibit considerable variation in occurrence, size, and position among marsupials. Unfortunately, inconsistent terminology has long inhibited clear communication about taxonomic differences. The nomenclature for palatal perforations adopted herein is illustrated in figure 14 and compared with other terminology in table 6. The incisive foramina are prominent slots in the anterior palate that transmit the nasopalatine ducts (Sánchez-Villagra, 2001a) together with nerves and blood vessels that remain to be identified in marsupials (Wible, 2003). In didelphids, these openings extend from the upper incisor arcade posteriorly to or between the canines. Although didelphid incisive foramina are always bordered posteriorly and posterolaterally by the maxillary bones, their dividing septum is formed primarily by the premaxillae. By contrast, the enormously elongated incisive foramina of caenolestids extend posteriorly between the premolar rows (opposite P1 or P2), and the posterior part of the dividing septum is formed by the maxillae (Osgood, 1921: pl. 20). The long incisive foramina of peramelemorphians do not extend behind C1, but the posterior part of their dividing septum is likewise formed by the maxillae. Most other marsupials (e.g., dasyuromorphians and Dromiciops) have incisive foramina that essentially resemble those of didelphids, with the conspicuous exception of some diprotodontians (e.g., Macropus; Wells and Tedford, 1995: fig. 9E) in which these openings are completely contained by the premaxillae. Among Recent didelphids, only Caluromys and Caluromysiops consistently lack wellformed maxillopalatine fenestrae (figs. 38, 39). Although small perforations in the maxillary-palatine sutures occur in most examined specimens of both genera, these are obviously vascular openings (perhaps homologous with the major palatine foramina of placental mammals) that do not resemble the nonvascular fenestrae of other didelphids. Glironia is sometimes said to lack palatal fenestrae (e.g., by Archer, 1982; Reig
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 37 TABLE 6 Terminology for Marsupial Palatal Foramina and Fenestrae This report Other names incisive foramen anterior palatal foramen (Tate, 1933), anterior palatine vacuity (Osgood, 1921), palatine fissure (Rowe et al., 2005), premaxillary vacuity (Archer, 1976a, 1981) maxillary fenestra mesolateral fenestra (Creighton, 1984) maxillopalatine fenestra major palatine foramen (Wible, 2003), maxillary vacuity (Archer, 1976a, 1981), maxillopalatine or mesolateral vacuity (Hershkovitz, 1992b, 1997), palatine fenestra (Clemens et al., 1989), palatine foramen (Novacek, 1993), posterior palatine vacuity (Osgood, 1921) palatine fenestra palatine vacuity (Archer, 1976a, 1981), posteromedial fenestra (Creighton, 1984), posteromedial or palatine vacuity (Hershkovitz, 1992b, 1997) posterolateral palatal foramen lateral posterior palatal foramen (Tate, 1933), minor palatine foramen (Wible, 2003), palatine canal (Clemens et al., 1989), posterolateral fenestra (Creighton, 1984), postero-lateral foramen (Osgood, 1921), postero-lateral palatine foramen (Archer, 1976a), postpalatine foramen (Marshall et al., 1995; Muizon, 1998) et al., 1987), but distinct maxillopalatine openings are present in most adult specimens that we examined (e.g., INPA 2570; fig. 37). When present, the left and right maxillopalatine fenestrae of didelphids are invariably separated by a broad median septum, and they never extend posteriorly behind the molar rows. Maxillopalatine fenestrae are very widely distributed among other marsupials, but they are absent in a few taxa (e.g., Myrmecobius, Dactylopsila, Phascolarctos). Among nondidelphid marsupials that possess these openings, the right and left fenestrae are often confluent because no median septum is developed (as in Rhyncholestes and Perameles gunnii), or they may extend posteriorly well behind the molar rows (as in acrobatids and some burramyids). Many didelphids have, in addition to maxillopalatine fenestrae, separate openings in the posterior palate that are entirely contained within the palatine bones. Palatine fenestrae are consistently present in adult specimens of Cryptonanus, Didelphis, Gracilinanus, Lutreolina, Philander, Thylamys, some species of Marmosa, and some species of Marmosops (e.g., M. creightoni; fig. 52). Presence is also the modal condition for other taxa (e.g., Lestodelphys) in which individuals without palatine perforations are uncommon variants (Martin, 2005). By contrast, palatine fenestrae are consistently absent in Caluromys, Caluromysiops, Chironectes, Glironia, Hyladelphys, Metachirus, Monodelphis, some species of Marmosops (e.g., M. pinheiroi; fig. 53), and most species of Marmosa. The single intact skull of Chacodelphys has palatine fenestrae, but these are incompletely separated from the maxillopalatine openings; the normal morphology for this taxon remains to be determined. Palatine vacuities are less widely distributed than maxillopalatine openings among other marsupials, but they occur in some dasyurids (e.g., Sminthopsis; Archer, 1981: fig. 5A), some peramelemorphians (e.g., Isoodon; Lyne and Mort, 1981: fig. 9), and a few diprotodontians (e.g., Phascolarctos, Vombatus). Most didelphids with fenestrated palates have only maxillopalatine or maxillopalatine and palatine openings. A few species, however, have additional fenestrae that are located in the maxillary bone between the maxillopalatine fenestra and the toothrow (at the level of M1 or M2) on each side of the palate. Maxillary fenestrae are normally present in Chacodelphys, Gracilinanus, many species of Thylamys (e.g., T. pusillus), and in Tlacuatzin canescens. Most other didelphids lack maxillary vacuities except as rare (usually unilateral) variants. These palatal openings are not known to occur in nondidelphid marsupials. The posterolateral palatal foramen perforates the maxillary-palatine suture behind M4 or posterolingual to M4 on each side of the skull in most metatherians. According to Archer (1976a), this foramen transmits the
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