The Origin and Evolution of Mammals - Moodle

192 THE ORIGIN AND EVOLUTION OF MAMMALS
largest specimens of Virginia opossum. They are a
structurally conservative group with many primitive
characters, and few morphological features are clear
synapomorphies defining the group. The primitive
marsupial dentition is retained, and the incisors are
unmodified. The most distinctive character of the
upper molar teeth is their dilambdodont condition,
in which the crest connecting the metacone to the
paracone tends to be W-shaped (Fig. 6.1(b)). Whether
this is a primitive, or a convergently derived character
shared with other marsupial groups is discussed
later. The didelphimorph ankle joint has certain
minor distinctive features described by Szalay (1994).
The majority of didelphids are arboreal, with an
opposable first digit and often a prehensile tail. The
diet is omnivorous and very adaptable, to which
feature Szalay (1994) attributed the lack of dental
specialisation. Among the more specialised members,
the yapok Chironectes, and Lutreolina are the
only marsupials that are even semi-aquatic. They
have webbed feet and a sealable pouch. The
caluromyine woolly opossums are specialised fruit
and nectar eaters, hanging by their prehensile tails
in the branches of the high forest canopy.
Paucituberculata
The Caenolestidae or shrew-opossums are the sole
living family of this South American order. There are
only three genera, comprising five species, and they
live primarily in the Andes. All are small, with body
weights ranging from 15–40 g, and they possess an
elongated snout for the mainly insectivorous diet.
The fur is particularly thick to provide the insulation
necessary for their high-altitude habitat.
The caenolestid dentition (Figs 6.1(c) and 6.7(c))
is modified by reduction of the number of lower
incisors and greatly increasing the length and
procumbency of the first pair, a condition described
as diprotodont. They use them to pierce their prey.
The upper molars are also very distinctive, having
evolved a roughly square shape in crown view.
Microbiotheria
Dromiciops gliroides (Fig. 6.10(a)), the monito del
monte, is the sole living species of this order. It is a
small, mouse-like form, weighing up to 30 g and
confined to the cool, humid forests of Chile and
Argentina. Its diet is primarily insects.
The single most characteristic feature of this
group is the complex structure of the tympanic
bulla, housing the middle ear structures. It is a
composite structure involving the alisphenoid and
the petrosal bones, which completely encloses the
ectotympanic. There are also characteristic features
of the molar teeth, notably a reduction of the stylar
shelf of the uppers, and elongation of the talonid of
the lowers (Fig. 6.1(e)).
Dasyuromorphia
These are the Australian carnivores, and consist of
19 genera containing 73 species. Amongst their
diagnostic characters there is a reduction of the
incisor teeth to four uppers and three lowers, and
possession of lower molars in which the talonid
has been reduced in size relative to the trigonid
(Fig. 6.1(d)). The braincase and the ankle joint also
have distinctive osteological features.
The great majority of dasyuromorphs are members
of the family Dasyuridae, the quolls, Tasmanian
devil, antechinuses, dunnarts, and marsupial mice.
They occupy a wide range of habitats, from rain
forest through woodland and savannah to very arid
regions. All are basically carnivores or insectivores,
although the Tasmanian devil has large teeth and
a short powerful jaw which it uses for dealing
with carrion as well. Two other families, both
monospecific, complete the major living subgroups
of Dasyuromorphia. The termite-eating numbat,
Myrmecobius fasciatus has a somewhat elongated
snout and long tongue for collecting the termites
that it digs up with its strong front claws. The
Tasmanian tiger or wolf, Thylacinus cynocephalus,
can be counted as the largest recent dasyuromorph
species, although there have been no confirmed
reports of a living specimen since 1936 when one
died in Hobart Zoo. There have nevertheless been
many unconfirmed claims of sightings of living
thylacines or their dung (Paddle 2000).
Notoryctemorphia
Two species of the genus Notoryctes constitute the
living marsupial moles. They occur in the sandy
central deserts of Australia, consuming insects,
notably burrowing beetle larvae, and small lizards.
The body is profoundly adapted for a fossorial
existence. The forelimbs are very short and powerful,