phylogenetic relationships and classification of didelphid marsupials ...

2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 33
Fig. 11. Dorsal views of the interorbital region in Marmosa murina (A, AMNH 267368), Marmosops
noctivagus (B, AMNH 262402), and Gracilinanus agilis (C, MVZ 197439) illustrating taxonomic
differences in the development of processes and beads. Distinct postorbital processes (pop) are normally
present in fully adult specimens of most Marmosa species, but they are usually absent in Marmosops and
Gracilinanus. Supraorbital beads (b), consisting of upturned dorsally grooved ridges, are present in some
but not all species of Marmosops. Other abbreviations: ioc, interorbital constriction; poc, postorbital
constriction. Scale bars 5 5 mm.
Dromiciops, and in most small-bodied Old
World marsupials, but well-developed sagittal
crests are present in adult specimens of
some dasyuromorphians (Dasyurus, Sarcophilus,
Thylacinus), some peramelemorphians
(Macrotis), and some diprotodontians (e.g.,
phalangerids, {Wakaleo, {Ekaltadeta).
The taxonomic distribution of parietalalisphenoid
versus frontal-squamosal contact
on the lateral braincase of metatherian
mammals has often been discussed by authors
(e.g., Archer, 1976a; Marshall et al.,
1995; Muizon, 1998; Wroe et al., 1998), not
all of whom have correctly reported the
distribution of these alternative conditions
among didelphids (Voss and Jansa, 2003). In
fact, parietal-alisphenoid contact is exhibited
by all didelphids with the unique exception of
Metachirus (fig. 44). Among other marsupials,
parietal-alisphenoid contact is also found
in caenolestids (Osgood, 1921: pl. 20, fig. 2),
most dasyuromorphians (e.g., Dasycercus;
Jones, 1949: fig. 8), some fossil peramelemorphians
(e.g., Yarala; Muirhead, 2000),
and many diprotodontians, whereas squamosal-frontal
contact is present in some dasyuromorphians
(e.g., Sminthopsis; Archer, 1981:
fig. 4), all Recent peramelemorphians, and a
few diprotodontians (Wroe et al., 1998).
Dromiciops, the only microbiotherian for
which the lateral braincase morphology is
known, also exhibits squamosal-frontal contact.
8
In most didelphids (Chironectes, Didelphis,
Glironia, Lutreolina, Marmosa, Metachirus,
Monodelphis, Philander, andTlacuatzin) the
petrosal is only exposed on the occiput
(behind the lambdoid crest) and on the
ventral surface of the skull (between the
exoccipital, basioccipital, and alisphenoid;
see Wible, 1990: fig. 1). In taxa conforming
to this morphology, there are no gaps among
the bones that normally form the lateral
surface of the braincase (squamosal, parietal,
and interparietal; fig. 12A). By contrast, the
petrosal capsule that encloses the paraflocculus
and the semicircular canals (5 pars
mastoideus or pars canalicularis) is consistently
exposed through a fenestra in the
suture between the parietal and squamosal
in Chacodelphys, Cryptonanus, Gracilinanus,
Thylamys, and some species of Marmosops
(fig. 12B). Both conditions—presence and
absence of a fenestra in the squamosal-
8 Hershkovitz (1999: fig. 25) erroneously depicted the lateral
braincase of Dromiciops with alisphenoid-parietal contact and
with a large ‘‘orbitosphenoid’’ ossification wedged between the
frontal, alisphenoid, and parietal bones. The correct morphology
of lateral braincase elements in this taxon was illustrated by
Giannini et al. (2004: fig. 2G).