The Origin and Evolution of Mammals - Moodle

was high. It lasted from 5 to 3 Ma and was followed
by the extended cool period marked by the cycle of
Plio-Pleistocene ice ages and intervening interglacials
that continues at the present time.
Amongst many minor extinctions, radiations, and
dispersal events recorded in the Plio-Pleistocene,
there was one major episode, referred to as the Great
American Biotic Interchange, which concerns the
exchange of faunas following the connection of
South America and North America by the Isthmus of
Panama (Simpson 1980; Stehli and Webb 1985). Prior
to this event, the isolated South American mammalian
fauna was completely different from anywhere
else. All the carnivores, large and small, were
marsupials. The large grazing and browsing herbivores
were the archaic placental meridiungulates, of
which only the litopterns and notoungulates still
existed at the time of the interchange. The Xenarthra
occupied the specialist roles of ant-eating on the part
of armadillos and anteaters, and leaf-browsing by
the glyptodonts, ground sloths, and tree sloths.
Apart from the inevitable bats, two other groups of
placentals had arrived in South America from elsewhere,
radiated, and became a fully integrated part
of the indigenous fauna. As described earlier, hystricognath
rodents are first recorded in the Early
Oligocene about 31 Ma, and platyrrhine primates
25 Ma in the Late Oligocene. Both are believed to
have come from West Africa by means of rafting
across the then much narrower Atlantic Ocean.
Even before the formation of the Isthmus, a
handful of mammals had dispersed between North
and South America. Two kinds of ground sloths
appear in the fossil record of Florida about 8 Ma,
and at about the same time a procyonid raccoon
occurred in Argentina. They are presumed to have
made the journey by island-hopping across the
Caribbean Islands. The next record of an exchange
is the occurrence of cricetid rodents in South
America around 3 Ma, which was only shortly
before the formation of the land bridge. The bridge
was finally completed 2.5 Ma as a consequence
of tectonic changes, and the first actual wave of
immigrants crossed in either direction. Going
northwards were more xenarthrans, consisting of
additional species of ground sloths, armadillos, and
the glyptodont Glyptotherium. They were accompanied
by capybaran rodents and also one of the
LIVING AND FOSSIL PLACENTALS 285
flightless, predatory phorusrhacid birds. A second
wave shortly afterwards, about 1.9 Ma, saw a second
contingent of xenarthrans including a giant
anteater Myrmecophaga, a rhinoceros-like notoungulate
Mixotoxodon and the first didelphid marsupial
enter southern USA and Mexico. There are also
four genera of New World monkeys living in the
forests of Central America today, although no fossil
record of exactly when they arrived there.
The taxonomic traffic over this period in the
opposite, southerly direction was considerably
more extensive. At the time of completion of the
landbridge, 2.5 Ma, there is evidence for a mustelid
skunk, a tayassuid peccary, and the horse genus
Hippidion in South America and these were shortly
afterwards joined by a procession of dogs, cats,
bears, more horses, gomphothere elephants, tapirs,
camels, deer, shrews, and several rodent groups.
The subsequent history of the respective immigrants
was just as asymmetrical as the initial invasions.
In South America, the northern immigrants
had a higher speciation rate and a lower extinction
rate than the native taxa, leading eventually to the
present day situation where 44% of the families and
54% of the genera of mammals are of North
American origin, either directly or by evolution
within South America after the interchange
(Marshall 1988). In fact, only two of the immigrant
families have become extinct, the horses and the
gomphothere elephants. In contrast, many of the
indigenous South American groups have disappeared,
including the large carnivorous borhyaenid
marsupials, and both the meridiungulate groups
Litopterna and Notoungulata. The invasion by
southern mammals into North America had vastly
less ultimate consequence. The ground sloths,
glyptodonts, and toxodonts disappeared. Of the rest
of the immigrants, only three genera of anteaters,
two of tree sloths, and two of armadillos still occur,
in the company of the four monkeys mentioned, a
few rodents, and about half a dozen genera of
didelphid marsupials. Even then, they are largely
restricted to the tropical forests of Central America,
only a few extending even as far as southern Mexico
and Florida. The porcupine Erethizon dorsatum, and
the Virginia opossum Didelphis virginiana alone have
spread deep into North America, and the latter was
largely due to human intervention.