tica drifted over the South Pole, it acquired a permanent ice sheet that buried
most of its terrain features (Fig. 188).
The Mid-Atlantic Ridge system, which generates new ocean crust in
the Atlantic Basin, began to occupy its present location midway between the
Americas and Eurasia/Africa about 16 million years ago. Iceland is a broad
volcanic plateau of the Mid-Atlantic Ridge that rose above sea level. About 3
million years ago, the Panama Isthmus separating North and South America
uplifted as oceanic plates collided. Prior to the continental collision, South
America had been an island continent for the past 80 million years, during
which time its mammals evolved undisturbed by outside competitors.
The barrier created by the land bridge isolated Atlantic and Pacific
species. Extinctions impoverished the once rich fauna of the western
Atlantic. The new landform halted the flow of cold water currents from the
Atlantic into the Pacific. Along with the closing of the Arctic Ocean from
Figure 188The Antarctic
Peninsula ice plateau,
showing mountains literally
buried in ice.
(Photo by P. D.Rowley,
courtesy USGS)TERTIARY MAMMALS