who have since radiated into 54 living genera— and you
begin to see why the effect of North American mammals
on South American ecosystems was so great.
Various amphibians, reptiles, and bird groups also
migrated in either direction, some to the north, some
to the south. The faunal interchange altered ecological
communities on both continents, producing combinations
of species that had never before been together.
The effect of the North American influx on the
South American mammal fauna was so significant
that almost half of the current mammalian families
and genera of the Neotropics originated in North
America. Thus within a period of 3 million years,
the Neotropical mammalian communities were
significantly restructured.
It is unclear how the influx of North American
mammals, as diverse as it was, affected abundances
of South American mammal species. For example,
many hoofed mammals invaded from North America.
However, their ecological counterparts in South
America, the litopterns (a group that included the
Macrauchenia, an animal that looked like a camel with
an elephant- like proboscis) and notoungulates, were
declining in numbers and diversity before the northern
camels and horses arrived. Perhaps the North American
ungulates were the primary cause of the extinction of
the various South American groups. Among predatory
mammals, the large, wolverine- like South American
borhyaenoids were essentially outcompeted by theterror birds, not by mammalian invaders from the
north. However, the phorusrhacoid terror birds may
have eventually lost out to invading mammals. There
is really only one relatively clear- cut example of what
appears to have been direct competition between a
North American species and a South American species.
This is the case of the South American saber- toothed
catlike animal Thylacosmilus and the North American
saber- toothed cat Smilodon (plate 8- 48). The animals
bore a striking anatomical similarity but were unrelated,
representing an example of convergent evolution. The
extinction of Thylacosmilus coincides closely with the
arrival of Smilodon, which crossed the land bridge into
South America from the north. Many of the extinctions
of large mammals on both continents are probably
explainable by the proliferation of humans during the
Pleistocene and not attributable to competition among
the animal groups themselves.
The Great American Faunal Interchange demonstrates
that ecological communities are subject to major
changes over time. The mixing of the two faunas created
new communities. One of the most challenging pursuits
in ecology, one related both to long- term biogeographic
history and to present ecological interactions, is our
quest to understand how various factors, including
interspecific interactions, structure communities. And
yet another major puzzle has to do with biodiversity.
Why are there so many species in the tropics? But that’s
a question for chapter 9.Plate 8- 47. The skull of a toxodon, on display at the Harvard
Museum of Comparative Zoology, Cambridge, MA. Photo by
John Kricher.Plate 8- 48. Smilodon, the saber- toothed cat, was a species that
invaded South America during the faunal interchange, perhaps
contributing to the extinction of Thylacosmilus, the catlike saber-
toothed marsupial. This skeleton is from the Carnegie Museum
of Natural History, Pittsburgh, PA. Photo by John Kricher.chapter 8 evolutionary cornucopia 133