phylogenetic relationships and classification of didelphid marsupials ...

30 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
pials. However, some peramelemorphians
(e.g., Echymipera, Perameles) appear to have
simple (unbranched) maxilloturbinals, whereas
the maxilloturbinals of examined dasyurids
(e.g., Murexia, Sminthopsis) and Dromiciops
closely resemble the elaborately dendritic
condition seen in most didelphids. A
monographic study of marsupial endonasal
features based on high-resolution X-ray
computed tomography (Macrini and Voss,
in preparation) will doubtless provide additional
points of comparison among didelphids
and other marsupial clades.
ZYGOMATIC ARCH: The bones comprising
the zygomatic arch exhibit few variable
features among didelphids. The maxillaryjugal
suture is always more or less straight or
irregularly crescentic, a frontal process of the
jugal is invariably present, and the jugalsquamosal
suture is always deeply inflected
(typically ,- or ,- shaped). Likewise, a
faceted preglenoid process of the jugal and a
well-developed postglenoid process of the
squamosal are always present, and a distinct
glenoid (entoglenoid) process of the alisphenoid
consistently forms part of the posterior
zygomatic root. Although the zygomatic arch
tends to be more gracile, to have a more
pronounced suborbital deflection, and to
have a more distinct frontal process in small
opossums (with relatively large eyes and
weakly developed masticatory muscles; e.g.,
Hyladelphys; fig. 40) than in large opossums
(with relatively smaller eyes but massive
masticatory muscles; e.g., Lutreolina; fig.
47), most didelphids exhibit intermediate
zygomatic morphologies.
Contrasting features of the zygomatic
region among other marsupial groups include:
(1) the deeply inflected maxillary-jugal
suture of peramelemorphians, which divides
the jugal into distinct anterodorsal and
anteroventral processes flanking a well-developed
nasolabial fossa (Filan, 1990); (2) the
absence of a frontal process of the jugal in
caenolestids and some peramelemorphians
(Osgood, 1921); (3) the absence of a faceted
preglenoid process of the jugal in several Old
World clades (e.g., Thylacinus and macropodoids);
(4) the absence of a distinct
postglenoid process of the squamosal in
Hypsiprymnodon, Tarsipes, and vombatids;
and (5) the absence of a glenoid process of
the alisphenoid in Myrmecobius and many
diprotodontians.
ORBITAL MOSAIC: The anteriormost part
of the didelphid orbit is formed by the
lacrimal, which is always prominently exposed
in lateral view. The lacrimal is
perforated by one or more lacrimal foramina
that are sometimes concealed within the orbit
(fig. 9A) but usually open laterally on or near
the orbital margin (fig. 9B). Most didelphids
normally have two lacrimal foramina on each
side, but Chironectes, Hyladelphys, and some
populations of Didelphis virginiana usually
have just one lacrimal foramen, and many
other didelphids that normally have two
lacrimal foramina occasionally have a single
foramen on one or both sides of the skull.
The orbital margin formed by the lacrimal is
smoothly rounded in all didelphids, none of
which exhibits lacrimal tubercles (e.g., like
those seen in macropodids; Wells and Tedford,
1995: fig. 9) or distinct crests (as in
Myrmecobius and some peramelemorphians).
Unlike the condition seen in some Old World
marsupials with maxillary-frontal contact on
the medial wall of the orbit (Flannery et al.,
1987: fig. 3), the didelphid lacrimal is always
in posteroventral contact with the palatine. 6
The medial wall of the didelphid orbit is
perforated by several openings, including the
sphenopalatine foramen (always in the palatine
bone), the ethmoid foramen (in the suture
between the orbitosphenoid and frontal), the
sphenorbital fissure (between the palatine,
orbitosphenoid, alisphenoid, presphenoid,
and sometimes the pterygoid), and the foramen
rotundum (in the alisphenoid). The
configuration of these orbital perforations in
all examined taxa is essentially similar to that
illustrated for Monodelphis by Wible (2003:
fig. 4). In particular, the foramen rotundum is
always exposed to lateral view behind the
sphenorbital fissure, from which it is invariably
separated by a bony partition. 7 Many
6 Note that elements of the didelphid orbital mosaic are
incorrectly labeled in some published illustrations (e.g.,
Hershkovitz, 1992b [fig. 19], 1997 [fig. 12]), where the orbital
process of the palatine that contacts the lacrimal is misidentified
as the ‘‘sphenoid.’’
7 According to Novacek (1986, 1993), the foramen rotundum
is confluent with the sphenorbital fissure in didelphids, but
these foramina are unambiguously separate openings in all of
the material examined by us and by Wible (2003).