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water laden with sediment surged along the Mississippi River toward the Gulf
of Mexico, widening the channel to several times its present size. Many other
rivers overreached their banks to carve out new floodplains.
The warming paved the way for the Climatic Optimum.This period of
unusually warm, wet conditions began 6,000 years ago and lasted for 2,000
years.As the Climatic Optimum unfolded, many regions of the world warmed
on average about 5 degrees Celsius. The melting ice caps released massive
floodwaters into the sea, raising sea levels 300 feet higher then when the
Holocene began.
The inland seas filled with sediments. Subsequent uplifting drove out the
waters, leaving behind salt lakes. Great Salt Lake in Utah is today only a remnant
of a vast inland sea. During a long wet period between 12,000 and 6,000 years
ago, it expanded to several times its current size and flooded the nearby salt flats.
MEGAHERBIVORE EXTINCTION
About 3 million years ago, the uplifting of the Panama Isthmus separating
North and South America created an effective barrier to isolate Atlantic and
Pacific marine species, which began to go their separate evolutionary ways.
Simultaneously, extinctions impoverished the once rich fauna of the western
Atlantic. Meanwhile, South America, which had been isolated from the rest of
the world for 80 million years, witnessed a vigorous exchange of terrestrial
species with North America across the Panama land bridge, causing many
species unable to compete with the new arrivals to go extinct.
The connecting landform between North and South America halted the
flow of cold water currents from the Atlantic into the Pacific. In addition, the
closing of the Arctic Ocean from warm Pacific currents helped initiate the
Pleistocene glaciation. Unlike ice ages of the past, the Pleistocene was unusual
for its low extinction rates possibly because species were already well adapted
to colder conditions.Those species living today are highly robust, having with-
stood the extreme environmental swings over the last 3 million years, when
glaciers spanned much of the Northern Hemisphere.
When the last ice age was drawing to a close between 12,000 and 10,000
years ago, an unusual extinction killed off large terrestrial plant-eating mam-
mals called megaherbivores.These animals grew to enormous size and roamed
the ice-free regions in many parts of the Northern Hemisphere.The giantism
might have resulted from similar circumstances that led to the great size of
some dinosaurs, including an abundant food supply and lack of predation.
Woolly rhinos, mammoths, and Irish elk disappeared in Eurasia. The great
buffalo, g iant hartebeests,and giant horses disappeared in Africa. More than 80
percent of the large mammals and a significant number of bird species disap-
267QUATERNARY GLACIATION