phylogenetic relationships and classification of didelphid marsupials ...

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38 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 Fig. 15. Ventral midcranial view of Caluromys philander (A, AMNH 267002) and Marmosa demerarae (B, AMNH 266428) illustrating taxonomic differences in palatal morphology. In caluromyines the posterior palate slopes gently ventrally, and the palatal margin is arched (concave posteriorly) without strongly projecting lateral corners; the internal nares (or choanae, in) are very broad. In didelphines, however, the posterior palate is abruptly inflected ventrally minor palatine artery from the maxillary artery to the ventral surface of the palate. In didelphids, caenolestids, Dromiciops, peramelemorphians, and stem metatherians, this foramen is completely surrounded by bone, but the foramen is incomplete or absent in many dasyurids (e.g., Murexia and Sminthopsis) because the maxillary and palatine processes that form the posterior border of this opening in other taxa fail to ossify (Wroe, 1997). In most didelphids the posterolateral palatal foramina are small and located behind the fourth upper molar, but these openings are conspicuously larger and extend lingual to the protocone of M4 on each side of the skull in Thylamys (fig. 54) and most specimens of Lestodelphys (fig. 51). Two alternative morphologies of the posterior palate can be recognized among didelphids. In Caluromys, Caluromysiops, and Glironia, the posterior palate slopes ventrally without any abrupt inflection, the caudal palatal margin is usually broadly arched, and prominent lateral corners are not developed; behind the palate, the choanae are not strongly constricted (fig.15A). By contrast, the posterior palate of most other didelphids is abruptly inflected ventrally, and the caudal palatal margin is more or less straight with prominent lateral corners; behind the palate, the choanae are strongly constricted (fig. 15B). Most nondidelphid marsupials have posterior palates that do not conform strictly to either of these morphotypes, but they more closely resemble those of Caluromys in lacking well-developed lateral corners. NASOPHARYNGEAL AND MESOPTERYGOID REGION: The roof of the nasopharyngeal passageway (or meatus; Rowe et al., 2005) is formed by the vomer, the palatines, and the presphenoid, of which only the palatines and the presphenoid are usually exposed to ventral view. Usually concealed inside the nasopharyngeal passageway, the vomer is deeply divided by the presphenoid into paired r and the palatal margin is more or less straight with projecting lateral corners (arrows in lower panel); the internal nares are narrow. Other abbreviations: als, alisphenoid; bs, basisphenoid; max, maxillary; pal, palatine; ps, presphenoid; pt, pterygoid.
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 39 lateral processes that only occasionally extend posteriorly into the mesopterygoid fossa to contact the pterygoids (e.g., in Caluromysiops; fig. 39). Most other marsupials, however, exhibit different configurations of the nasopharyngeal roofing bones. In particular, the vomer is undivided and extends caudally to underlie the presphenoid (concealing most or all of it from ventral view) in caenolestids, dasyurids, peramelemorphians, and Dromiciops. The left and right pterygoid bones are widely separated by the presphenoid and the basisphenoid in didelphids. In other marsupials, the vomer may also extend between the left and right pterygoids, but in Dromiciops the pterygoids are in midline contact, underlying the presphenoid and concealing its suture with the basisphenoid from ventral view; the conjoined pterygoids of Dromiciops form a strong sagittal keel, which is continuous with that formed anteriorly by the vomer and posteriorly by the basisphenoid. 10 BASICRANIAL FORAMINA: A foramen that transmits the venous transverse canal perforates the alisphenoid anterolateral to the carotid canal in most didelphids (Sánchez- Villagra, 2001b; Sánchez-Villagra and Wible, 2002). Caluromys and Caluromysiops, however, consistently lack a transverse canal foramen (Archer, 1976a; Sánchez-Villagra and Wible, 2002; Voss and Jansa, 2003). According to Kirsch and Archer (1982: character 24), the ‘‘transverse canal’’ is absent in Metachirus nudicaudatus and Philander opossum, but most of the specimens we examined of both taxa had a transverse canal foramen on each side of the skull. The transverse canal foramen is normally present in most other marsupials (e.g., caenolestids, dasyurids, peramelemorphians, and Dromiciops), although its reduction or absence in a few taxa (e.g., Planigale, Tarsipes) has been noted by authors (Archer, 1976a; Aplin, 1990; Sánchez-Villagra and Wible, 2002). 10 The obviously autapomorphic midventral basicranial keel of Dromiciops is usually described as formed by the presphenoid and the basisphenoid (e.g., by Hershkovitz, 1999; Giannini et al., 2004), but the presphenoid does not participate in forming this structure in any specimen that we examined. Note that the presphenoid is incorrectly labeled as the vomer in some illustrated ventral views of the didelphid skull (e.g., Novacek, 1993: fig. 9.4B). The openings through which the mandibular division of the trigeminal nerve (V 3 ) exits the skull have been variously named by systematists. Following Gaudin et al. (1996), we use the term ‘‘foramen ovale’’ for the primary orifice through which V 3 exits the endocranial lumen of the adult skull. In most didelphids, the foramen ovale is bordered by the alisphenoid and the petrosal (fig. 16A), but the foramen is sometimes contained entirely within the alisphenoid, and both conditions can be seen on opposite sides of the same skull (Gaudin et al., 1996: fig. 6). Because it is often difficult to determine the position of this opening with respect to relevant endocranial sutures, however, we did not score the position of the foramen ovale per se. Instead, secondary enclosures of V 3 by outgrowths of the alisphenoid tympanic process provide a more accessible basis for taxonomic comparisons. Two different (apparently nonhomologous) conditions of secondary nerve enclosure occur among didelphids. In juveniles and adults of Gracilinanus, Lestodelphys, Marmosops, Metachirus, and Thylamys, the extracranial course of V 3 is enclosed by an alisphenoid process or strut that arises from the anteromedial surface of the bulla and extends anteriorly, medially, and dorsally to span the transverse canal foramen. In the smaller species with this condition (e.g., Marmosops pinheiroi; fig. 16B) the extracranial course of V 3 remains unenclosed between this process and the primary foramen ovale, but in larger species (e.g., Marmosops noctivagus) a sheet of bone produced from the posterior edge of the process extends caudally to form a more or less complete canal late in postnatal life. An alternative pattern of secondary foramen and canal formation is seen in Caluromysiops, Chironectes, Didelphis, Lutreolina, Philander, and some species of Monodelphis (e.g., M. theresa). In these taxa, V 3 is broadly enclosed by an alisphenoid lamina (fig. 16C) that extends along the posteromedial bullar surface, but there is no anteromedial process spanning the transverse canal foramen. Many juvenile specimens of some taxa scored with this condition (e.g., Didelphis) do not have a fully enclosed secondary foramen and canal, but they usually show some laminar devel-
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phylogenetic relationships and classification of didelphid marsupials ...

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