phylogenetic relationships and classification of didelphid marsupials ...

42 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
Dromiciops is almost exclusively formed by
the alisphenoid (as in didelphids), but a
broad petrosal shelf forms part of the sinus
roof in caenolestids and some peramelemorphians.
Diprotodontians exhibit a very wide
range of bullar morphologies (reviewed by
Aplin, 1990), many of which include substantial
squamosal participation.
The epitympanic recess of didelphids is a
shallow concavity in the ventral surface of
the petrosal pars canalicularis that encloses
the mallear-incudal articulation (Wible, 2003:
fig. 7D). 11 Although this morphology is
widespread among other marsupials, a few
(e.g., Macrotis, Perameles) have an enormously
inflated epitympanic recess that
vaults high above the ossicles, leaving the
mallear-incudal joint effectively unenclosed
by bone.
The promontorium of the petrosal pars
cochlearis is not marked by any vascular
grooves in didelphids. Instead, the most
conspicuous feature of this otherwise
smooth, bulbous structure is the rostral
tympanic process (sensu MacPhee, 1981;
Wible, 1990). The rostral tympanic process
is small, more or less conical, and perhaps
serves only to anchor the posterior (ventral)
limb of the ectotympanic annulus in some
opossums (e.g., Glironia), but it is anteroposteriorly
expanded, dorsally concave, and
substantially contributes to the ventral enclosure
of the middle ear in others (e.g.,
Marmosa; Reig et al., 1987: figs. 38–42). In
Caluromys and Caluromysiops, the rostral
tympanic process extends along the entire
length of the promontorium from the fenestra
cochleae to the anterior pole (Sánchez-
Villagra and Wible, 2002). The rostral
tympanic process of caenolestids and most
peramelemorphians is small and more or less
conical, but dasyurids, some peramelemorphians
(e.g., Macrotis), and Dromiciops have
an anteroposteriorly extensive and dorsally
11 Because the illustrations in Wible (1990) are so useful in
other respects, it is important to note that the epitympanic
recess is mislabeled in two views of the tympanic surface of the
petrosal of Didelphis virginiana. As noted by Muizon (1998:
90), the concavity so labeled in Wible’s figure 4A and 4C is
really the posterior part of the hypotympanic sinus roof; the true
epitympanic recess is a smaller depression posterolateral to the
hypotympanic sinus roof and separated from it by a low ridge,
the petrosal crest.
concave process that forms a substantial part
of the bullar floor in these taxa.
In most didelphids, the fenestra cochleae is
exposed in lateral or ventrolateral view as
illustrated for Didelphis by Wible (1990: fig.
1). In others (e.g., Caluromys, Caluromysiops,
Lestodelphys, Marmosa rubra, Monodelphis
emiliae, andThylamys), the fenestra cochleae
is concealed within a chamber or sinus
formed by a laminar outgrowth of the pars
canalicularis that approximates or contacts
the rostral tympanic process of the pars
cochlearis. This lamina, which appears to be
homologous with the caudal tympanic process
of MacPhee (1981), is invariably fused
posteroventrally with the paroccipital process
of the exoccipital, which also throws forward
a small ‘‘tympanic’’ outgrowth. The resulting
auditory chamber—corresponding to the
‘‘periotic hypotympanic sinus’’ of Archer
(1976a) and the ‘‘post-promontorial tympanic
sinus’’ of Wible (1990)—is always small
and uninflated in opossums.
The fenestra cochleae is also exposed in
caenolestids and some peramelemorphians
(e.g., Echymipera), in which tympanic processes
of the petrosal are either small or
absent. In other peramelemorphians (e.g.,
Perameles) and in dasyurids, however, the
fenestra cochleae is concealed within a
postpromontorial sinus formed by the seamless
fusion of the rostral and caudal tympanic
processes; in these forms (as in Caluromys
and other didelphids with similar petrosal
features), the sinus is small and uninflated.
The fenestra cochleae of Dromiciops is
enclosed in an inflated (bubblelike) postpromontorial
sinus that, although strikingly
unlike the smaller sinuses of other plesiomorphic
marsupials, is also assumed to have
resulted from fusion of caudal and rostral
tympanic processes (Sánchez-Villagra and
Wible, 2002: fig. 11).
No other bone (besides the alisphenoid,
ectotympanic, and petrosal) participates in
auditory sinus formation among Recent
didelphids, which have only one or two
hypotympanic cavities as described above.
Caenolestids and Dromiciops are similar to
didelphids in this respect, but Recent dasyuromorphians
and some peramelemorphians
(e.g., Macrotis and Perameles) have a welldeveloped
epitympanic sinus formed by the