phylogenetic relationships and classification of didelphid marsupials ...

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88 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 (e.g., Jansa and Voss, 2000; Amrine-Madsen et al., 2003; Meredith et al., 2008), unambiguous morphological synapomorphies of the family remain to be confidently identified. Because our analyses did not include any nonmarsupial outgroup, the position of the marsupial root node was not determined, and the phylogenetic interpretation of characterstate transformations on the branch separating didelphids from nondidelphid marsupials is correspondingly equivocal. If, as most recent analyses suggest, didelphids are the basalmost branch of Marsupialia (Nilsson et al., 2004; Amrine-Madsen et al., 2003; Horovitz and Sánchez-Villagra, 2003; Asher et al., 2004; Sánchez-Villagra et al., 2007; Meredith et al., 2008; Beck, 2008), then such transformations might be didelphid synapomorphies, or they could be synapomorphies of the unnamed clade that includes caenolestids and Australidelphia. Comparisons with stem metatherians that are believed to be close outgroups to Marsupialia, including {Mayulestes (see Muizon, 1998), {Pucadelphys (see Marshall et al., 1995), and {Herpetotherium (see Gabbert, 1998; Sánchez-Villagra et al., 2007) suggest that most of the craniodental traits by which didelphids differ from other marsupials are plesiomorphic, but one exception merits comment. Bone homologies on the posterodorsal braincase have received little attention in the literature, but they appear to be phylogenetically informative at many taxonomic levels within Metatheria. In particular, the presence of a large undivided interparietal bone that is wedged between the parietals anteriorly and fused to the supraoccipital posteriorly may be a didelphid synapomorphy. According to Muizon (1998: 38, fig. 6), paired interparietal ossifications are fused to the parietals in {Mayulestes, but the basis for this interpretation (which has no analog among living metatherians) is not explained, and it seems equally possible that the interparietal(s) is(are) absent in this taxon. In {Pucadelphys, paired interparietals (‘‘postparietals’’) are suturally distinct from the parietals and from the supraoccipital (Marshall et al., 1995: 50, fig. 12). The occiput of {Herpetotherium has not previously been described, but in a well-preserved specimen that we examined (127684 in the AMNH vertebrate paleontology catalog) there is no bone wedged between the parietals and the supraoccipital, which share a suture just behind the lambdoid crest. Therefore, the available evidence suggests that the didelphid condition is unique. Other alleged morphological synapomorphies of didelphids include features of the spermatozoa (Temple-Smith, 1987), postcranial skeleton (Horovitz and Sánchez-Villagra, 2003), and petrosal (Ladevèze, 2007). However, the phylogenetic interpretation of such traits is compromised by sparse ingroup sampling. Spermatozooa, for example, have not been studied from many genera (e.g., Caluromysiops, Glironia, Hyladelphys, Lutreolina, Marmosops, Thylamys), and published phylogenetic analyses of postcranial and petrosal characters have not included basal didelphids (analyzed didelphid terminal taxa in both of the osteological studies cited above belong to the subfamily Didelphinae). Hopefully, future studies will help fill in many of these taxonomic gaps and contribute to a better assessment of anatomical character support for didelphid monophyly. Only Recent didelphids are formally classified and described below, but several South American Neogene genera represented by well-preserved cranial material ({Hyperdidelphys, {Thylatheridium, {Thylophorops) are clearly members of the didelphimorph crown clade. Pending a phylogenetic analysis of their relationships with living forms, we follow current paleontological judgments of taxonomic affinity in suggesting where these fossils belong. By contrast, most other fossil didelphids are only represented by dental fragments from which few characters can be scored, and their relationships to extant taxa are correspondingly ambiguous. Subfamily Glironiinae, new CONTENTS: Glironia. DIAGNOSIS: Members of this clade differ from other didelphids by the extension of soft fur along the dorsal surface of the tail from base to tip (the dorsal surface of the tail is macroscopically naked distally in other opossums), by strongly recurved and laterally compressed manual claws (manual claws are much less strongly recurved and laterally
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 89 compressed in other opossums), and by postorbital processes that are formed by the frontals and parietals (postorbital processes are absent or are formed only by the frontals in other opossums). REMARKS: The name Glironiidae as used by Hershkovitz (1992a, 1992b, 1999) was a nomen nudum because it was not accompanied by a statement of diagnostic characters (ICZN, 1999: Article 13). To our knowledge, no family-group name based on Glironia is technically available from any other publication. Glironia Thomas, 1912 Figure 37 CONTENTS: venusta Thomas, 1912 (including aequatorialis Anthony, 1926; and criniger Anthony, 1926). MORPHOLOGICAL DESCRIPTION: Combined length of adult head and body probably ca. 170–210 mm; adult weight probably ca. 100– 200 g (measurements from the only two adult specimens known to have been measured by the American method and accompanied by weight data are in table 4). Rhinarium with two ventrolateral grooves on each side of median sulcus; dark circumocular mask present; pale supraocular spot absent; dark midrostral stripe absent; throat gland unknown (no suitably preserved adult male specimens have been examined for this feature). Dorsal body pelage unpatterned, brownish; dorsal underfur gray; dorsal guard hairs short and inconspicuous; ventral fur grayish or gray-based whitish, with or without self-whitish pectoral markings. Manus paraxonic (dIII 5 dIV); manual claws strongly recurved, laterally compressed, and much longer than fleshy apical pads of digits; dermatoglyph-bearing manual plantar pads present; central palmar epithelium smooth; carpal tubercles absent. Pes unwebbed; dIV longer than other pedal digits; plantar surface of heel naked. Pouch absent; mammae 2–0–2 5 4, all abdominal/inguinal; cloaca present. Tail about as long as combined length of head and body or a little longer, slender and muscular (not incrassate), and densely furred from base to tip except along ventral midline; naked ventral caudal surface modified for prehension with raised tubercles near base and apical pad bearing dermatoglyphs. Premaxillary rostral process absent. Nasals long, extending anteriorly beyond I1 (concealing nasal orifice from dorsal view), and conspicuously widened posteriorly near maxillary-frontal suture. Maxillary turbinals elaborately branched. Lacrimal foramina concealed within anterior orbital margin or exposed laterally, usually two on each side. Orbits very large; supraorbital crests well developed; flattened triangular postorbital processes present, formed by both frontal and parietal bones. Left and right frontals and parietals separated by persistent median sutures. Parietal contacts alisphenoid on lateral braincase (no frontal-squamosal contact). Sagittal crest absent. Petrosal not laterally exposed through fenestra in parietal-squamosal suture (fenestra absent). Parietal-mastoid contact present (interparietal does not contact squamosal). Maxillopalatine fenestrae usually present, but small; palatine fenestrae absent; maxillary fenestrae absent; posterolateral palatal foramina small, not extending anteriorly between M4 protocones; posterior palatal morphology Caluromys-like (without prominent lateral corners, the choanae not constricted behind). Maxillary and alisphenoid not in contact on floor of orbit (separated by palatine). Transverse canal foramen present. Alisphenoid tympanic process smoothly globular, without anteromedial process or posteromedial lamina enclosing extracranial course of mandibular nerve (secondary foramen ovale absent), and not in contact with rostral tympanic process of petrosal. Anterior limb of ectotympanic suspended directly from basicranium. Stapes triangular with large obturator foramen. Fenestra cochleae exposed, not concealed by rostral and caudal tympanic processes of petrosal. Paroccipital process small, adnate to petrosal. Dorsal margin of foramen magnum bordered by exoccipitals and supraoccipital, incisura occipitalis present. Two mental foramina present on lateral surface of each hemimandible; angular process acute and strongly inflected. Unworn crowns of I2–I5 asymmetrical (‘‘incisiform’’), with longer anterior than posterior cutting edges; I5 separated from I4 by small diastema in some specimens (e.g.,
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PHYLOGENETIC RELATIONSHIPS AND CLAS
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CONTENTS Abstract .................
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ABSTRACT This report summarizes a d
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