phylogenetic relationships and classification of didelphid marsupials ...

32 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
Fig. 10. Oblique dorsolateral view of the left orbital floor in Thylamys venustus (A, AMNH 263562)
and Monodelphis peruviana (B, AMNH 272695) illustrating taxonomic differences in sutural patterns. In
Thylamys (and most other didelphids), the maxillary (max) and alisphenoid (als) bones are separated by
the palatine (pal), but the alisphenoid extends anteriorly across the palatine to contact the maxillary in
Monodelphis. Other osteological abbreviations: fro, frontal; hp, hamular process of pterygoid; jug, jugal;
lac, lacrimal; par, parietal; sq, squamosal.
the supraorbital margins are consistently
smooth and essentially featureless in most
specimens of Cryptonanus, Lestodelphys,
Thylamys, and other species of Marmosops
and Gracilinanus (e.g., G. agilis; fig. 11C).
Distinct postorbital processes are consistently
absent in many nondidelphid marsupial
groups (e.g., caenolestids, peramelemorphians,
macropodoids), but they are present
in some dasyuromorphians (e.g., Thylacinus,
Sarcophilus) and in a few diprotodontians
(e.g., Petaurus breviceps, thylacoleonids).
Other aspects of interorbital morphology are
too variable among nondidelphid marsupial
clades to describe succinctly here, but none
appear to provide an unambiguous basis for
phylogenetic inference or taxonomic diagnosis.
DORSOLATERAL BRAINCASE: The right and
left frontals and parietals are separated by
ontogenetically persistent median sutures in
most didelphids (fig. 6), but the midfrontal
suture is incomplete or absent in most
juveniles and in all examined subadult and
adult specimens of Chironectes (fig. 45),
Didelphis (fig. 46), Lutreolina (fig. 47), and
Philander (fig. 48). All examined juveniles
and one young adult specimen (FMNH
84426) of Caluromysiops have complete mid-
frontal and midparietal sutures, but the right
and left frontals and parietals are co-ossified
in all of the remaining (fully adult) specimens
of Caluromysiops that we examined. Most
nondidelphid marsupials have ontogenetically
persistent median sutures separating the
braincase roofing bones, but the left and
right frontals and parietals are co-ossified in
all examined subadult and adult caenolestids
(Osgood, 1924: figs. 1–3; Patterson and
Gallardo, 1987: fig. 1). The midparietal
suture is also fused in most examined
postjuvenile peramelemorphians.
The scars that mark the dorsalmost origin
of the temporalis muscle on each side of the
braincase are widely separated, and no
sagittal crest is developed in most didelphids.
In large adult specimens of Caluromys,
Lestodelphys, and Monodelphis, however, a
small sagittal crest is sometimes developed
over the interparietal or along the midparietal
suture. By contrast, much larger sagittal
crests that extend anteriorly onto the frontals—an
unambiguously different condition—are
consistently developed in adult
specimens of Caluromysiops, Chironectes,
Didelphis, Lutreolina, andPhilander. Sagittal
crests are altogether absent in caenolestids,