phylogenetic relationships and classification of didelphid marsupials ...

2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 41
opment; subadults and young adults show
progressively more complete enclosure; and
old adults (large specimens with heavily worn
teeth) usually have completely enclosed
foramina and canals (Abdala et al., 2001).
All other didelphid taxa (Caluromys, Chacodelphys,
Cryptonanuns, Glironia, Hyladelphys,
Marmosa, Tlacuatzin, and most species of
Monodelphis) lack secondary enclosures of
the mandibular nerve except as rare (usually
unilateral) variants.
Although many nondidelphid marsupials
(e.g., caenolestids, Dromiciops, most dasyuromorphians)
have only a primary foramen
ovale, secondary foramina ovales are also
widely distributed. Unfortunately, these traits
remain to be widely surveyed, and inconsistent
terminology (discussed by Gaudin et al.,
1996) makes it difficult to interpret some
published descriptions of marsupial basicrania.
Although other developmental mechanisms
have obviously been responsible for
secondary enclosures of the mandibular
nerve in some taxa (e.g., Phascolarctos),
putative homologues of both didelphid patterns
of secondary foramen formation can be
recognized among certain Old World marsupials.
At least some specimens of Thylacinus,
for example, have a secondary foramen
formed by an anteromedial bullar strut that
spans the transverse canal foramen, whereas
the secondary foramen ovale of Recent
peramelemorphians (e.g., Echymipera, Perameles)
is formed by a broad medial bullar
lamina.
EAR REGION: All didelphids have an
osteologically well-defined middle ear cavity
or hypotympanic sinus (sensu van der
Klaauw, 1931: 19). A cup-shaped tympanic
process (or ‘‘wing’’) of the alisphenoid
invariably forms the anterior part of the
r
floor of the middle ear, but this structure
exhibits significant taxonomic variation in
size. The alisphenoid tympanic process is
large and extends far enough posteriorly to
closely approximate or contact the rostral
tympanic process of the petrosal in Caluromys
and Caluromysiops (see Reig et al.,
1987: fig. 44B, D). In other opossums, a
distinct gap separates the alisphenoid tympanic
process from the rostral tympanic
process of the petrosal, such that at least
part of the floor of the middle ear is
membranous (e.g., in Marmosa and Monodelphis;
Reig et al., 1987: fig. 41B, D). In
Lestodelphys and Thylamys a rather indistinct
medial process of the ectotympanic
makes a small contribution to the floor of
the middle ear, partially filling in the gap
between the tympanic processes of the
alisphenoid and petrosal (Reig et al., 1987:
fig. 42B, D). The roof of the hypotympanic
sinus in all Recent didelphids is almost
exclusively formed by the alisphenoid, with
the petrosal making only a small, often
negligible, contribution. The didelphid squamosal
does not participate in forming any
part of the floor or roof of the middle ear
cavity.
Caenolestids, dasyurids, peramelemorphians,
and Dromiciops resemble didelphids in
that the alisphenoid forms most of the
anterior part of the floor of the middle ear.
However, whereas caenolestids and some
peramelemorphians (e.g., Echymipera) have
a small alisphenoid tympanic process that
does not contact the rostral tympanic process
of the petrosal, the alisphenoid tympanic
process is large and broadly contacts the
petrosal in dasyurids, Dromiciops, and other
peramelemorphians (e.g., Perameles). The
hypotympanic sinus roof of dasyurids and
(fo), which is bordered by the alisphenoid (als) and the petrosal (pet); the extracranial course of the nerve is
unenclosed in this taxon. In Marmosops, however, the extracranial course of V 3 is partially enclosed by a
bony strut (st) that extends from the anteromedial surface of the alisphenoid tympanic process (atp) across
the transverse canal foramen (tcf); the nerve then emerges from a so-called secondary foramen ovale.
Another kind of secondary enclosure is seen in Monodelphis, where the nerve emerges from a secondary
foramen ovale formed by a medial lamina (lam) of the alisphenoid tympanic process. Other abbreviations:
bs, basisphenoid; bo, basioccipital; cc, carotid canal; ect, ectotympanic. Scale bars 5 5 mm.