phylogenetic relationships and classification of didelphid marsupials ...

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 lamb-
doid 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.).