phylogenetic relationships and classification of didelphid marsupials ...

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28 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 Fig. 7. Left lateral cranial and mandibular views of Marmosa murina showing principal osteological features mentioned in the text. Abbreviations: als, alisphenoid; ap, angular process; atp, alisphenoid tympanic process; conp, condylar process; corp, coronoid process; ect, ectotympanic; exo, exoccipital; fpj, frontal process of jugal; fr, foramen rotundum; fro, frontal; hpp, hamular process of pterygoid; iof, infraorbital foramen; ip, interparietal; jug, jugal; lac, lacrimal; lc, lambdoid crest; lf, lacrimal foramina; maf, masseteric fossa; max, maxillary; mef, mental foramina; nas, nasal; pal, palatine; par, parietal; pc, pars cochlearis (of petrosal); pcf, paracanine fossa; pm, pars mastoideus (of petrosal); pop, postorbital process; pre, premaxillary; rmf, retromolar fossa; sq, squamosal; ssf, subsquamosal foramen; sup, supraoccipital; zps, zygomatic process of squamosal. rostrum beyond I1 and contains a distinct suture between the right and left bones (fig. 8A). Taxa that exhibit a well-developed premaxillary rostral process include Caluromys, Gracilinanus, Marmosa, and most examined species of Marmosops. Aspects of rostral process development that impact phylogenetic character scoring, including some discrepancies and ambiguities in the published literature on didelphid premaxillary morphology, were previously discussed by Voss and Jansa (2003: 22–23). Didelphid nasal bones always extend anteriorly beyond the vertically oriented facial processes of the premaxillae to form a well-defined median apex, and they always extend posteriorly between the lacrimals. Despite such consistency, taxonomic differences in nasal length can be defined with respect to other anatomical landmarks. The tips of the nasal bones are produced anteriorly beyond I1 in most opossums, and the nasal orifice is consequently not visible from a dorsal perspective (figs. 37–44, 49–54). In Chironectes, Didelphis, Lutreolina, andPhilander, however, the tips of the nasals do not extend so far anteriorly, and the nasal orifice is dorsally exposed (figs. 45–48). The nasals are wider posteriorly than anteriorly in most didelphids, but they are not so wide posteriorly as to contact the lacrimals (except as rare, often unilateral, variants). The transition from the narrow anterior part of each nasal to the broader posterior part may be gradual or abrupt and usually occurs at or near the maxillaryfrontal suture. By contrast with this widespread condition, a few taxa (Chacodelphys, Thylamys, Marmosops incanus) have uniformly narrow nasals with subparallel lateral margins (e.g., fig. 54). Due to the usual absence of nasolacrimal contact, the maxillae and frontals are normally in contact on both sides of the rostrum.
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 29 Fig. 8. Ventral view of rostrum in Marmosa rubra (A, MVZ 153280) and Monodelphis brevicaudata (B, AMNH 257203) illustrating taxonomic differences in premaxillary morphology. A broad, shelflike rostral process (rp) extends the suture between right and left bones well anterior to the incisors in M. rubra, but the premaxillae form only narrow alveolar rims anterior to I1 in M. brevicaudata, where the left and right bones are separated by a small tissue-filled gap (not a distinct suture). Most of the lateral surface of the didelphid rostrum is formed by the maxilla, which is prominently perforated above P2 or P3 by the infraorbital foramen. The foramen is notably larger in Chironectes than in other didelphids, presumably to accommodate the hypertrophied infraorbital nerve that conducts trigeminal sensory fibers from the well-developed mystacial vibrissae of this semiaquatic taxon (Sánchez-Villagra and Asher, 2002). Aside from small nutrient foramina that may be variably present or absent in some taxa, there are no other lateral rostral openings in didelphids; in particular, no fenestra is ever present at or near the conjunction of the maxillary, nasal, and frontal bones. Other marsupials exhibit a wide range of rostral morphologies. In all examined dasyuromorphians, for example, the nasals are very short and do not extend anteriorly beyond the facial processes of the premaxillae; rather than forming an acute apex, they are notched medially, and the incisive foramina (in the floor of the nasal orifice) are exposed to dorsal view (e.g., in Dasyurus; Flores et al., 2006: fig. 4B). In Recent peramelemorphians, the nasals do not extend posteriorly between the lacrimals (Freedman, 1967: fig. 2), whereas Recent caenolestids have a prominent fenestra on each side of the rostrum at or near the conjunction of the nasal, frontal, and maxillary bones (Thomas, 1895; Osgood, 1921: pl. 20, fig. 2). In a few Old World marsupials (e.g., Lasiorhinus, Tarsipes) the nasals contact the lacrimals (separating the maxillae from the frontals), whereas the premaxillae contact the frontals (separating the nasals from both the lacrimals and the maxillae) in dactylopsiline petaurids (Tate, 1948a). By contrast, many other marsupials (e.g., Dromiciops) essentially resemble didelphids in rostral morphology. NASAL CAVITY: The complex morphology of the bony turbinals inside the didelphid nasal cavity is difficult to visualize without Xray computed tomography, which has only been used to describe the endonasal skeleton of Monodelphis domestica (see Rowe et al., 2005). However, some turbinal structures are accessible to direct inspection. The maxilloturbinals, for example, are thin, dorsally scrolling sheets of bone that occupy the lower part of the didelphid nasal fossa, where they arise from the inner surface of the maxilla on each side. In most didelphids, the greater curvature of each maxilloturbinal scroll throws off secondary lamellae, which branch to form tertiary lamellae, some of which branch again to form an elaborate dendritic mass that fills much of the ventrolateral part of the nasal lumen. By contrast, the maxilloturbinals are simple, slender, unbranched (or sparsely ornamented) scrolls in all examined species of Monodelphis. The morphology of the nasal cavity remains to be surveyed among other marsu-
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