The Origin and Evolution of Mammals - Moodle

Extinct Notoryctemorphia
At present the sole fossil relatives of the marsupial
mole are teeth and isolated skeletal elements from
the Riversleigh deposits. The molar teeth are not as
extremely modified as those of Notoryctes
(Fig. 6.1(g)), and when a description is published it
may indicate how the notoryctid-like version of the
zalambdodont condition evolved from a basic marsupial
tooth (Archer et al. 1991).
Extinct Diprotodontia
As the principal herbivorous Order of Australian
marsupials, diprotodonts are, as expected, the most
abundant and diverse component of the fossil, as
well as of the living, fauna. In fact, the family level
diversity of the group at their first appearance in
the Late Oligocene and through the Early Miocene
exceeds that of the present day. Of the total of 16
diprotodont families recognised by McKenna and
Bell (1997), only two are not known during this
period, the monospecific Tarsipedidae (honey
possum), and dispecific Acrobatidae (feather-tailed
glider and feather-tailed possum). On the other
hand, eight of the mid-Cenozoic families have
subsequently disappeared. These extinct families
include some very peculiar and highly specialised
diprotodonts.
Wynyardiidae. Members of this family only occur
in the Late Oligocene/Early Miocene, although they
were fairly abundant during this time. Two complete
skeletons of Muramura (Fig. 6.12(a)) from the
Etadunna Formation of the Lake Eyre Basin have
been found (Pledge 1987), so it is a particularly
well-known group. Muramura was dog-sized and its
unspecialised syndactyl hindfeet indicate a simple,
terrestrial quadrupedal locomotion. The molars had
a primitive degree of development of transverse
lophs, and the stylar cusps on the uppers are still
distinguishable. As far as the details of the wynyardiid
braincase are concerned, Aplin (1987)
believed it to be the most plesiomorphic of all
diprotodonts. Therefore, on dental, cranial, and
postcranial structure, the wynardiids appear to be
close to the base of the diprotodont radiation.
Diprotodontidae. The diprotodontids (Fig. 6.12(i)
and (j)) include the largest marsupials ever to
LIVING AND FOSSIL MARSUPIALS 213
evolve, ranging in size from that of a sheep to giants
larger than hippos. The Late Pleistocene Diprotodon
optatum was almost 3 m long, stood 2.6 m high at the
shoulder, and weighed as much as 2.78 tonnes
(Wroe et al. 2003). The diprotodontid dentition was
modified for browsing on relatively soft vegetation,
with enlarged, bilophodont molar teeth. Unlike the
other major herbivore family, the macropodids,
diprotodontids remained fully quadripedal. The
group finally became extinct in the Late Pleistocene,
as part of the general megafaunal mass extinction of
that time.
Palorchestidae. This family consisted of forms
quite similar to diprotodontids and they were at
one time actually included in the latter family. Also
like the diprotodontids, they survived until the
Late Pleistocene, when the best-known genus
Palorchestes was the size of a horse. However,
palorchestids do differ from diprotodontids in
details of the dentition, and also in the structure of
the ear region, in which they more closely resemble
the wombats. Very large, powerful claws tended to
develop on the forelimbs suggesting a diet of tubers
and roots, and in the later forms such as Palorchestes
the snout was elongated in a manner that suggests
that it bore a mobile proboscis or short trunk.
Thylacoleonidae. Famously the most extraordinary
diprotodonts of all were the marsupial lions, which
are also one of the comparatively rare examples of
the evolution of a carnivore from a specialised
herbivorous ancestor. The Late Oligocene–Early
Miocene thylacoleonid Priscileo was a relatively
small, cat-sized animal (Rauscher 1987), and
Wakaleo (Murray et al. 1987) had a 15-cm skull
(Fig. 6.12(f)). By the Plio-Pleistocene, Thylacoleo
(Fig. 6.12(g)) had evolved into the size and general
form of a leopard. The dentition is modified for
extreme carnivory (Nedin 1991), or perhaps a
hyaena-like scavenging that included dealing with
large bones. The first upper and sole lower incisors
are enlarged and caniniform. The upper third
premolar (Fig. 6.12(h)) is an enormous shearing
blade that worked against the almost equally large
lower third premolar plus the first molar. The rest
of the molars are reduced in both number and
relative size.