phylogenetic relationships and classification of didelphid marsupials ...

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-