The Origin and Evolution of Mammals - Moodle

218 THE ORIGIN AND EVOLUTION OF MAMMALS
with the giant flightless predatory birds, such as the
Miocene Phorusrhacus that stood over 2 m high,
with a huge raptorial beak, and the introduction
during the Oligocene of the placental primates and
rodents.
This general picture of South American marsupial
diversity remained intact throughout the succeeding
60 million years (Marshall and Cifelli 1990).
A considerable degree of endemicity occurred, as
would be expected in a large, single continent, with
faunal differences reflecting regions with different
climates (Pascual and Ortiz 1990; Flynn and Wyss
1998). Although there were shifts in the relative
fortunes of particular groups, there were no extinctions
of higher taxa until the Plio-Pleistocene, when
a combination of large climatic changes, and the
secondary connection of North and South America,
resulted in the events referred to as the Great
American Biotic Interchange that is described
in more detail in Chapter 7. Around this time, several
South American marsupial taxa disappeared,
notably the last of the carnivorous sparassodonts
(borhyaenids), while a handful of didelphid species
dispersed northwards into Central and North
America, where they are still to be found.
Antarctica
In principle, the palaeo-biogeography of American
marsupials offers no problems, and the same is true
of Antarctica (Woodburne and Case 1996). The only
mammalian fossils so far described are from the
Middle or Late Eocene of Seymour Island, on the
Antarctic Peninsula immediately south of South
America. At the time, the oceanic current system,
and the prevailing high levels of CO 2 (Pearson and
Palmer 2000) were responsible for a warm to cool
temperate forested environment, dominated by
angiosperms. The mammals all belong to taxa that
are also found in the Palaeocene of South America.
They include marsupial polydolopoids, didelphimorphs,
and a microbiothere, and there are also
placental ungulates, xenarthrans, and bats; there is
even a possible phorusrhacid bird (Reguero et al.
2002). There was still a connection between the
South American and Antarctic land masses, and
clearly the Antarctic Eocene marsupial fauna was
continuous with that of South America, with no
more difference than would be expected in different
areas of the same large land mass. However, Drake’s
Passage between the two continents opened up in
the Late Eocene, about 36 Ma, with disastrous consequences.
A circum-Antarctic current was established
that prevented the warming influence of the
more southern oceans upon the Antarctic climate
and therefore caused an increased temperature gradient
to build up between warm tropics and the
cold, southern polar region. The event also coincided
with a reduction in CO 2 level, presaging the start of
a worldwide cooling. Before long the permanent ice
sheets characteristic of Antarctica built up, and the
last remnants of its Eocene biota disappeared.
Australia
The origin and early history of marsupials in
Australia is shrouded in ignorance. To start with,
absolutely nothing is known about any fossil
mammals of eastern Antarctica, the region of
Gondwana to which Australia was connected
during the Mesozoic. Yet eastern Antarctica was
separated from western Antarctica, and ultimately
therefore from South America, by the Transantarctic
Mountain chain, which may have posed a significant
barrier to dispersal of South American groups
of marsupials, and therefore may have been a critical
influence on the origin and nature of the Australian
fauna. The exact timing of the separation of Australia
from Antarctica by the opening up of the Southern
Ocean is not certain (Woodburne and Case 1996).
Rifting between the two had commenced by 80 Ma
and the last remaining physical connection, the
Tasman Rise foundered around 64 Ma. However, it
was not until about 52 Ma that a complete seaway
developed (Fig. 6.13). Therefore, while an easy
dispersal route may have ended at the start of
the Cenozoic, a potential sweepstake dispersal
route probably existed throughout the Palaeocene
and even into the Eocene. To add to the ignorance
about eastern Antarctica, there is no information
at all about Australian mammals prior to the
latest Palaeocene or earliest Eocene Tingamarra
Fauna.
If the marsupials found in Australasia were a
monophyletic group unrepresented elsewhere,
then their origin could be explained as the consequence
of a single entry of a ‘didelphimorph-like’
ancestor into the continent, via a sweepstake route