The Origin and Evolution of Mammals - Moodle

earing two huge claws on digits three and four for
digging. The hindlimbs have three large claws for
pushing soil and sand backwards as the animal
moves through the substrate. The snout is pointed
and bears a protective plate, the eyes are vestigial,
and external ears and vibrissae are absent. The molar
teeth (Fig. 6.1(g)) are highly modified, with fusion of
the metacone and paracone of the uppers and loss
of the talonid of the lowers. These teeth are thus a
version of the zalambdodont tooth, which evolved
convergently in a number of other marsupial and
placental groups.
Peramelemorphia
There are 18 species in the seven living genera of
bandicoots and bilbies that belong to this order. They
are relatively small insectivores and omnivores, up
to the size of a rabbit, that are distributed throughout
Australia and New Guinea, in a wide variety of habitats.
Peramelemorphs are one of the two syndactylous
groups of marsupials, in which the second and
third digits of the hindfoot are enclosed in a single
sheath. The function of the syndactyl foot in bandicoots
is apparently grooming of the fur, rather than
locomotor specialisation (Hall 1987).
The most remarkable character of the peramelemorphs
is the presence of a chorio-allantoic placenta.
During the course of development of the embryo,
a normal marsupial type of yolk sac placenta is
supplemented by a placental type of chorio-allantoic
placenta. The effect of this is to accelerate the
development of the embryo so that the neonate
emerges from the birth canal to enter the pouch at
a significantly younger age. However, it is no more
developmentally advanced at parturition than are
the neonates of other marsupials.
The peramelemorph dentition is characterised by
laterally compressed incisors. The molars tend to
be square-shaped, and the talonid of the lowers
is as large or larger than the trigonid (Fig. 6.1(f)).
Diprotodontia
There are 40 living genera containing about 128 living
species in this primarily herbivorous group, ranging
from the minute, nectivorous 7 g pygmy possum and
10 g honey possum to the large kangaroos weighing
close to 100 kg. The various diprotodont families of
phalangers, possums, gliders, koalas, wombats, and
LIVING AND FOSSIL MARSUPIALS 193
kangaroos of Australasia combine two distinctive,
though individually not unique characters. The
dentition is diprotodont, with great enlargement of
the first pair of lower incisors to form a base against
which the upper incisors can work for browsing and
grazing. The second is the syndactylous hindfoot, in
which the second and third digits are combined in
a single tissue sheath to form a functionally single
digit. There are also certain characteristic features
of the basic postcanine dentition of diprotodonts,
notably crenulated molars, and details of the molar
cusp pattern (Fig. 6.1(h) ands (i)), which is based
on a bilophodont arrangement. However, there is a
wide variety of detailed tooth form in this ecologically
disparate taxon.
Interrelationships
For many years, two conflicting derived characters
dominated the question of the evolutionary interrelationships
of the marsupials. Diprotodonty
(Fig. 6.2(a)), the enlargement of one of the pairs of
lower incisors, occurs in both the South American
caenolestids and the Australian diprotodonts,
suggesting a relationship between these two. But
syndactyly (Fig. 6.2(b)), the coupling of the second
and third digits of the hindfoot, occurs in the
peramelemorphs and diprotodonts, suggesting
a different, mutually incompatible sister-group relationship.
The biogeographic coincidence of the two
Australian orders might be seen to have supported
the Syndactyla hypothesis, except that discussions
at the time were also coloured by the common
opinion that the Australian thylacine and the South
American fossil borhyaenids were related, for they
possess very similar adaptations for a dog-like,
carnivorous mode of life; if this relationship was
true, then biogeographic evidence could not count
for much. How the remaining marsupial groups
fitted into either scheme, beyond the assumption
that dasyuromorphs and didelphimorphs were
more or less ancestral, was obscure.
Kirsch’s (1977) application of the serological technique
to marsupials produced one of the earliest
results of molecular systematics. He demonstrated
that all the Australian taxa constituted one monophyletic
group, and the American taxa a second,
and therefore that the diprotodont dental condition