The Origin and Evolution of Mammals - Moodle

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Figure 6.13 Southern Gondwana in the Early Eocene, 53 Ma.
from Antarctica, followed by radiation into the
five indigenous orders. No such simple picture as
this is possible, however, because of the relationships
of the microbiotheres. These are taxonomically
australidelphians, yet occur in South America,
from the Palaeocene to the Recent, and in the
Eocene of western Antarctica. They may possibly
occur in the Tingamarra fauna, but this has yet to be
substantiated.
There are a number of possible solutions to what
is termed the ‘Dromiciops problem’. One is that
the whole australidelphian radiation, including the
origin of the microbiotheres, did indeed occur in
Australia after the immigration of a single ancestor,
but that microbiotheres dispersed from Australia
into Antarctica and South America. This sequence
of events would have to have been completed by
the Early Palaeocene for the microbiothere Khasia
LIVING AND FOSSIL MARSUPIALS 219
to appear in the Tiupampan fossils of that age,
which means hardly any time at all after the initial
entry of marsupials from North America into the
southern continents in the first place. Meanwhile,
the microbiotheres must have gone extinct in
Australia, if not by the Eocene, then certainly before
the great Oligocene–Miocene faunas of Riversleigh.
This view is consistent with Godthelp et al.’s (1999)
description of the Tingamarran genus Djarthia,
whose teeth they cannot distinguish from primitive
members of either Australidelphia or Ameridelphia.
Its upper molars are dilambdodont, and they
propose that this is the condition of the common
ancestor of both these groups; Djarthia is therefore
seen as a conservative descendant of the original
single immigrant. However, their interpretation
still requires the initial dispersal event to have
occurred by the very start of the Palaeocene, so that