phylogenetic relationships and classification of didelphid marsupials ...

2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 39
lateral processes that only occasionally extend
posteriorly into the mesopterygoid fossa
to contact the pterygoids (e.g., in Caluromysiops;
fig. 39). Most other marsupials, however,
exhibit different configurations of the
nasopharyngeal roofing bones. In particular,
the vomer is undivided and extends caudally
to underlie the presphenoid (concealing most
or all of it from ventral view) in caenolestids,
dasyurids, peramelemorphians, and Dromiciops.
The left and right pterygoid bones are
widely separated by the presphenoid and the
basisphenoid in didelphids. In other marsupials,
the vomer may also extend between the
left and right pterygoids, but in Dromiciops
the pterygoids are in midline contact, underlying
the presphenoid and concealing its
suture with the basisphenoid from ventral
view; the conjoined pterygoids of Dromiciops
form a strong sagittal keel, which is continuous
with that formed anteriorly by the
vomer and posteriorly by the basisphenoid. 10
BASICRANIAL FORAMINA: A foramen that
transmits the venous transverse canal perforates
the alisphenoid anterolateral to the
carotid canal in most didelphids (Sánchez-
Villagra, 2001b; Sánchez-Villagra and Wible,
2002). Caluromys and Caluromysiops, however,
consistently lack a transverse canal
foramen (Archer, 1976a; Sánchez-Villagra
and Wible, 2002; Voss and Jansa, 2003).
According to Kirsch and Archer (1982:
character 24), the ‘‘transverse canal’’ is
absent in Metachirus nudicaudatus and Philander
opossum, but most of the specimens we
examined of both taxa had a transverse canal
foramen on each side of the skull. The
transverse canal foramen is normally present
in most other marsupials (e.g., caenolestids,
dasyurids, peramelemorphians, and Dromiciops),
although its reduction or absence in a
few taxa (e.g., Planigale, Tarsipes) has been
noted by authors (Archer, 1976a; Aplin,
1990; Sánchez-Villagra and Wible, 2002).
10 The obviously autapomorphic midventral basicranial keel
of Dromiciops is usually described as formed by the presphenoid
and the basisphenoid (e.g., by Hershkovitz, 1999; Giannini et
al., 2004), but the presphenoid does not participate in forming
this structure in any specimen that we examined. Note that the
presphenoid is incorrectly labeled as the vomer in some
illustrated ventral views of the didelphid skull (e.g., Novacek,
1993: fig. 9.4B).
The openings through which the mandibular
division of the trigeminal nerve (V 3 ) exits
the skull have been variously named by
systematists. Following Gaudin et al.
(1996), we use the term ‘‘foramen ovale’’ for
the primary orifice through which V 3 exits
the endocranial lumen of the adult skull. In
most didelphids, the foramen ovale is bordered
by the alisphenoid and the petrosal
(fig. 16A), but the foramen is sometimes
contained entirely within the alisphenoid,
and both conditions can be seen on opposite
sides of the same skull (Gaudin et al., 1996:
fig. 6). Because it is often difficult to
determine the position of this opening with
respect to relevant endocranial sutures, however,
we did not score the position of the
foramen ovale per se. Instead, secondary
enclosures of V 3 by outgrowths of the
alisphenoid tympanic process provide a more
accessible basis for taxonomic comparisons.
Two different (apparently nonhomologous)
conditions of secondary nerve enclosure
occur among didelphids.
In juveniles and adults of Gracilinanus,
Lestodelphys, Marmosops, Metachirus, and
Thylamys, the extracranial course of V 3 is
enclosed by an alisphenoid process or strut
that arises from the anteromedial surface of
the bulla and extends anteriorly, medially,
and dorsally to span the transverse canal
foramen. In the smaller species with this
condition (e.g., Marmosops pinheiroi; fig.
16B) the extracranial course of V 3 remains
unenclosed between this process and the
primary foramen ovale, but in larger species
(e.g., Marmosops noctivagus) a sheet of bone
produced from the posterior edge of the
process extends caudally to form a more or
less complete canal late in postnatal life.
An alternative pattern of secondary foramen
and canal formation is seen in Caluromysiops,
Chironectes, Didelphis, Lutreolina,
Philander, and some species of Monodelphis
(e.g., M. theresa). In these taxa, V 3 is broadly
enclosed by an alisphenoid lamina (fig. 16C)
that extends along the posteromedial bullar
surface, but there is no anteromedial process
spanning the transverse canal foramen. Many
juvenile specimens of some taxa scored with
this condition (e.g., Didelphis) do not have a
fully enclosed secondary foramen and canal,
but they usually show some laminar devel-