The Origin and Evolution of Mammals - Moodle

286 THE ORIGIN AND EVOLUTION OF MAMMALS
There have been a number of attempts to
explain the details of the Great American Biotic
Interchange, traditionally by invoking competition:
supposedly inferior South American mammals succumbing
to the competitive onslaught of superior
North American forms (Simpson 1980). However,
there are serious anomalies to such a simple
explanation. For one thing, the extinctions of the
South American taxa do not always coincide
with the entry of their supposed North American
competitors. The borhyaenids, which occupied the
large carnivore niches, had disappeared prior to the
Pliocene, with the single exception of the marsupial
sabre-tooths Thylacosmilus, leaving the phorusrhacid
birds as the most abundant top carnivores.
The meridiungulates, too, had already undergone a
severe decline in diversity before the interchange.
Of the two orders remaining by this time, litopterns
were represented by Macrauchenia and a few
related genera, and Notoungulata by the one family
Toxodontidae. In this case, both groups survived
until well after the interchange and are recorded in
the Late Pleistocene. Furthermore, a close relative
of Toxodon was one of the forms that entered North
America during the exchange. A second complication
to a simple competitive explanation is that the
interchange was associated with considerable environmental
changes.
All more recent authors, such as Webb (1985a,
1991), Marshall (1988), and Vrba (1992, 1993) have
appreciated that the actual pattern of immigrations
and extinctions must have resulted from a complex
interplay of environmental changes, ecological
opportunism, and biotic interactions, and have
pointed to the effect that the alternation between
interglacial and glacial episodes might have
had. Webb (1991) proposed a two-phase model
(Fig. 7.27). The first phase occurred when the land
bridge formed, 2.5 Ma. It was during an interglacial
period of warm, humid conditions, and continuous
dense rain-forest covered much of South America,
the Isthmus, and southernmost North America,
much as is the case today. Mammals that were
adapted to rain-forest conditions could readily disperse
from the much larger area of South American
rain-forest into the far smaller area of rain-forest
that occupied the low latitudes of North America,
where most of the survivors are still to be found.
The second phase of the model commenced with
the onset of an arctic glaciation 2.4 Ma. The tropical
regions became cooler and drier, and the forests
were largely transformed into savannah and light,
dry woodland. Conditions now favoured expansion
southwards of mammals from the very large
area of subtropical and temperate North America
that were already adapted to this kind of habitat.
Once into South America by this route, they could
extend their range to higher latitudes as the next
warmer and wetter, interglacial phase returned,
Arid
Rain forest
Savanna
Temperate forest
Arid
Rain forest
Savanna
Figure 7.27 Shift in the extent and distribution of biomes between
interglacial and glacial phases across Central and South America.
(a) Interglacial phase. (b) Glacial phase (Webb 1991).
(a)
(b)