The Origin and Evolution of Mammals - Moodle

220 THE ORIGIN AND EVOLUTION OF MAMMALS
a microbiothere could have reversed back along the
sweepstake route into Antarctica and South America
by Tiupampan times.
The second possible explanation for the Australian
marsupial fauna is that the initial radiation of the
taxon Australidelphia did not occur in Australia,
but in other parts of Gondwana, namely Antarctica
and/or South America. Under this scenario, if the
microbiotheres are the sister group of the rest of
the australidelphians (Fig. 6.36), as weakly supported
by Amrine-Madsen et al. (2003), then the
divergence between them must have occurred during
the earliest Palaeocene, and only a single dispersal
into Australia by the ancestor of the
Australian radiation need have occurred. This event
could have taken place any time in the Palaeocene,
prior to the age of the first Australian fossil marsupials.
However, if the microbiotheres are taxonomically
nested within the Australidelphia, there would have
to have been a dispersal into Australia of the respective
ancestors of each of the lineages more basal
than the microbiotheres. For example Horovitz and
Sánchez-Villagra’s (2003) cladogram (Fig. 6.3(a)),
requires three such dispersals: a dasyuromorphian,
a notoryctemorphian–peramelemorphian, and a
diprotodontian. Woodburne and Case (1996) have
proposed a solution along these lines. They argue
that the dilambdodont condition evolved independently
in didelphids and basal australidelphians. They
then point to the dentition of Andinodelphys from the
Tiupampan Palaeocene of South America, which
they claim has plesiomorphic australidelphian features,
including the pre-dilambdodont condition
of a linear centrocrista, and prominent stylar cusps
on the upper molars. They infer therefore that
Andinodelphys represents the basal stock of all the
Australidelphia, and that the origin of the group
occurred in Gondwana, outside Australia. Their view
also implies that some divergence had already
occurred by Tiupampan times, because microbiotheres
are distinct by then. The authors thus propose
that representatives of the Australian groups dispersed
via eastern Antarctica and into Australia by a
sweepstake route, prior to the complete separation of
Australia and Antarctica. Acceptance of this hypothesis
hinges primarily on their interpretation of
Andinodelphys. Godthelp et al. (1999) do not accept
its australidelphian affinities but regard it as
a didelphid although, as already pointed out, they
cannot actually distinguish basal didelphids from
basal dasyuromorphs on teeth alone.
The third possible explanation for the biogeography
of the Gondwanan marsupials is that
Australia, Antarctica, and South America shared
a single, taxonomically broad marsupial fauna in
the Early Palaeocene. Following the isolation of
Australia, differential extinction accounted for its
subsequent fanna. For this hypothesis to be true,
there would have to have been ameridelphians such
as polydolopoids and true didelphimorphs in
Australia at one time, as well as the australidelphians.
Possible support for this pan-Gondwana
hypothesis comes from interpretation of the
Tingamarran fauna as including representatives of
the didelphimorphs, polydolopoids, and possibly
also microbiotheres and protodidelphids. Not one
of these identifications is indisputable, but all are
possible. Under this scenario, it would be expected
that the Gondwanan placentals would also be
represented in the Palaeocene of Australia, particularly
those known to have existed in the Eocene of
Antarctica, namely xenarthrans, and the ungulate
orders of litopterns and astropotheres (Reguero
et al. 2002).
It is impossible at present to choose between these
alternatives, but at least a solution is realistically
likely to be found. The first alternative would be
supported by the discovery of very early Palaeocene
faunas in Australia of australidelphians including
microbiotheres; the second by Palaeocene faunas
in Antarctica yielding other australidelphian groups
as well as microbiotheres; the third by the
undoubted presence in Australia of polydolopoids,
didelphimorphs, other ameridelphian groups, and
also Gondwanan placentals. In short, Early
Palaeocene mammalian faunas of Australia, and
Antarctica, if and when discovered, may be
expected to reveal the critical evidence.
Whatever the detailed history of their arrival in
Australia, the subsequent radiation of marsupials
in that continent was profoundly affected by the
absence of placental ungulates, the mammals that
occupied the medium-sized and large herbivore
roles everywhere else in the world, including
South America and also the Eocene of Antarctica.
Among lots of smaller and medium-sized species,