Some 150 million years ago there were two great landmasses, Laurasia in the north
and Gondwana in the south. Figure 3.3 shows how Gondwanaland split apart. The
process began about 115 million years ago, with Africa and India breaking away first.
These and Madagascar separated 65 million years ago, while South America,
Antarctica, and Australia were still joined. Australia finally separated from Antarctica
much later, about 40 million years ago.
These historical movements explain some of the more peculiar distributions of
animal groups, for example why marsupials are found today only in Australia, New
Guinea, and the Americas. A fossil land mammal from an extinct marsupial family
occurred in Antarctica 40 million years ago (Woodburne and Zinsmeister 1982). This
supports the idea that Australian marsupials originated from South America via
Antarctica before 56 million years ago.
Figure 3.4 shows the distribution of the large flightless ratites (ostriches, rheas,
emus, and their extinct relatives). Similar distributions of tree-ducks, penguins, and
parrots attest to the breakup of the southern continent.
The joining of North and South America in the Pliocene provides another example
of historical events determining the nature of faunas. South America originally had
a remarkably diverse mammalian fauna resembling the radiation of the ungulate fauna
of Africa today. It included a wide range of marsupial carnivores such as big
sabertooth types (Thylacosmilus) and hyena types of the family Borhyaenidae, and
smaller mongoose types represented by the Didelphids (the group which includes
the opossum Didelphis marsupialis). These carnivores fed on the herbivorous
notoungulates, a huge placental group now entirely extinct.24 Chapter 3
(a) (b)(c)Fig. 3.3Gondwanaland
at different time periods
before the present. The
thin line around each
continent is the limit of
water less than 1000 m
deep. The dot indicates
the South Pole. (a) At
150 million years BPthe
southern continents
were joined. (b) At 65
million years BPAfrica
had separated from
the other continents,
which were still joined.
(c) The present-day
distribution of
continents. (After
Norton and Sclater
1979.)