Evolution__3rd_Edition

..
. . . and South American mammals
Some groups had migrated before
...
. . . but the main event came after
3 million years ago
CHAPTER 17 / Evolutionary Biogeography 513
Figure 17.14
A reconstruction of Doedicurus, a Pleistocene glyptodont. The
glyptodonts were a strange group of armored South American
mammals related to armadillos. Reprinted, by permission of the
publisher, from Simpson (1980). © 1983 Scientific American
Books.
shows no sign of possessing these forms. It instead evolved its own distinctive mammalian
fauna.
The South American mammals of the Paleocene and Eocene fall into three groups:
marsupials, xenarthrans (armadillos, sloths, anteaters), and ungulates. Armadillos, tree
sloths, and opossums still survive in South American forests, but they formerly lived
along with many other curious, and now extinct, forms. There were marsupial sabertooth
carnivores (Figure 15.4, p. 429), ground sloths (the group from which the giant
ground sloth Megatherium of the Pleistocene evolved), and the most heavily armored
mammals that ever lived a the glyptodonts (Figure 17.14), which were first described
from Darwin’s collections made during the Beagle voyage.
New arrivals came in from the outside, on rare occasions, from the early Oligocene
on. They probably immigrated by waif dispersal, hopping from island to island before
there was a continuous land bridge between the continents. Rodents are a major group
which first appeared in the Oligocene. It is so uncertain where they came from that
experts still dispute whether the South American rodents are more closely related to
African or North American species (though the latter is the more widely favored
source). The South American rodents, like the other mammalian groups in that land,
also in turn evolved peculiar South American forms, including one called Telicomys
gigantissimus (in the Pleistocene) that is the biggest rodent ever to have lived and was
almost as large as a rhinoceros.
In the late Miocene, about 8–9 million years ago, further small additions to the
fauna arrived. These were the procyonids (racoons and allies) who came from North
America, and the cricetid rodents. These too almost certainly entered by waif dispersal.
It is possible that North and South America had tectonically wandered closer together
at that time, but the connection can have been neither close nor lasting, because it was
another 6 million years before the South America mammal fauna encountered the full
range of the outside world’s mammalian types.
Then, about 3 million years ago, the Bolivar trough finally disappeared and the modern
Panamanian land bridge formed. The vegetation on both sides of the bridge was
probably savannah, not the tropical rainforest of modern times. Mammals adapted to
the similar vegetation of the two sides could move freely both ways, and it was now that
the mustelids (skunks), canids (dogs), felids (cats), equids (horses), ursids (bears), and
camels invaded South America from the north, while the dasypodids (armadillos),
didelphids (opossums), callithricids (marmosets), and edentate anteaters moved
rather less dramatically in the opposite direction a in both cases accompanied by many
other less well known forms. This extraordinary clash and exchange of faunas is known