The Origin and Evolution of Mammals - Moodle

the Diprotodontia also included genera of
diprotodontids, kangaroos, and wombats that
adopted various roles as large herbivores. To date
absolutely nothing is known of the radiation of
these and the other australidelphians before the
Late Oligocene, about 25 Ma. At this time,
Australia was largely occupied by rain forests,
tropical and temperate, in which lived a rich
marsupial fauna containing members of most of
the modern families, and about as many again no
longer extant. Therefore, whatever the situation
today, it is clear that the australidelphian marsupials
were primarily adapted for non-arid, forested
conditions, an environment that prevailed for
another 10 million years. However, by the Late
Miocene, drier conditions had spread bringing
with them a great extension of grasslands in place
of forests. This was caused in part by the general
LIVING AND FOSSIL MARSUPIALS 221
worldwide cooling of the later Miocene, and in
part by the northward drift of Australia, eventually
colliding with Asia and causing the rise of the New
Guinean mountains which cast a rain shadow
(Archer et al. 1991). One consequence was a great
increase in the diversity of grazing diprotodonts.
These were mostly the macropodine kangaroos,
with their pattern of tooth replacement modified
so that new teeth continued to replace old, worn
teeth, and wombats with high-crowned, openrooted
molars that continuously grew.
From this point, despite further reduction
of forests and desertification of the centre of the
continent, the fauna remained unchanged in
essence until the arrival of humans 50,000–60,000
years ago, and the simultaneous commencement of
the extinction of the larger marsupial species, as
discussed at the end of the next chapter.