phylogenetic relationships and classification of didelphid marsupials ...

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34 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 Fig. 12. Lateral view of posterior braincase in Marmosa murina (A, AMNH 272816) and Marmosops impavidus (B, AMNH 272709), illustrating the presence of a fenestra (fen) that exposes the petrosal (pet) between the parietal (par) and squamosal (sq) inMarmosops. The petrosal is not laterally exposed by a fenestra in the parietal-squamosal suture of Marmosa. Other abbreviations: fc, fenestra cochleae; ssf, subsquamosal foramen. parietal suture—occur as balanced polymorphisms (neither state clearly predominating) in Lestodelphys, Marmosops incanus, and M. noctivagus. No nondidelphid marsupial that we examined has a fenestrated squamosalparietal suture. INTERPARIETAL: Although some authors (e.g., Novacek, 1993) have stated that marsupials lack an interparietal bone, a large interparietal is unequivocally present in Dromiciops (see Giannini et al., 2004: fig. 3), many diprotodontians (e.g., Tarsipes; Parker, 1890: fig. 2), and some dasyuromorphians (e.g., Myrmecobius). In these taxa, the sutures that separate the interparietal from adjacent bones (supraoccipital and parietals) are clearly visible and ontogenetically persistent. In all of the taxa we examined with such distinctly sutured interparietals, the interparietal-supraoccipital boundary coincides closely with the transverse (lambdoid) crest that marks the dorsalmost insertion of the neck extensor musculature. An interparietal is also unambiguously present in didelphids, all of which exhibit a large, unpaired, wedge-shaped or oblong element between the left and right parietals in the same position (anterior to the lambdoid crest) as the sutured interparietals of other marsupials. The didelphid interparietal, however, is fused with the supraoccipital in all of the skulls we examined (including postweaning juveniles assignable to Gardner’s [1973] age class 1), few of which show any trace of a suture. 9 Fortunately, developmental studies of didelphid pouch young are available to prove the existence of a distinct interparietal center of ossification. In Didelphis the interparietal first appears around day 8 postpartum (p) and fuses with the supraorbital by day 28p (Nesslinger, 1956), whereas in Monodelphis these events occur on days 3p and 8p, respectively (Clark and Smith, 1993). By contrast, the didelphid interparietal never appears to fuse with the parietals, the sutures between them persisting even in the largest adult specimens we examined. 9 Vestiges of the interparietal-supraoccipital suture in juvenile specimens of Monodelphis brevicaudata were described by Wible (2003). What appears to be a well-defined suture between the interparietal and supraoccipital of Didelphis albiventris in an illustration published by Abdala et al. (2001: fig. 3) was intended to represent the hypothetical boundary between bones that were indistinguishably fused in all of the specimens examined by the authors of that report (D.A. Flores, personal commun.).
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 35 Fig. 13. Posterior and lateral views of the occipital region in Lestodelphys halli (A, B, UWZM 22422) and Metachirus nudicaudatus (C, D, AMNH 267009) illustrating taxonomic differences in several cranial characters discussed in the text, including alternative patterns of contact among the interparietal (ip), squamosal (sq), parietal (par), and mastoid (5 pars mastoideus of petrosal, mas). Additionally, the incisura occipitalis (io), a distinct notch in the dorsal margin of the foramen magnum (fm), separates the right and left exoccipitals in Lestodelphys. By contrast, the incisura occipitalis is absent because medial processes of the right and left exoccipitals are in contact, excluding the supraoccipital from the dorsal margin of the foramen magnum in Metachirus. Another conspicuous taxonomic difference illustrated in these views concerns the paroccipital process (pp), which is a small, inconspicuous bony mass adnate to the mastoid in Lestodelphys. The paroccipital process of Metachirus is much larger and projects almost straight ventrally. The didelphid condition (a large interparietal fused to the supraoccipital but not to the parietals) appears to be unique among marsupials. In young dasyurids, for example, the interparietal is small and separated by open sutures from neighboring bones, or it is absent; no examined dasyurid has a large median element fused to the supraoccipital that is suturally distinct from the parietals and wedged between their posterior borders. The interparietal appears to be completely missing in peramelemorphians, none of which show any trace of a bony element wedged between the parietals anterior to the
Page 1 and 2: PHYLOGENETIC RELATIONSHIPS AND CLAS
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phylogenetic relationships and classification of didelphid marsupials ...

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