peared from Australia. Meanwhile, giant ground sloths (Fig. 197), mastodons, and
woolly mammoths (Fig. 198) disappeared from North America. A possible
exception was the dwarf woolly mammoth, which might have survived in the
Arctic until about 4,000 years ago. The loss of these animals also caused their
main predators, the American lion, saber-tooth tiger, and dire wolf, to go extinct.
About 8,000 to 9,000 years ago, conditions were much hotter and dryer
than today. The global environment reacted to the changing climate with
declining forests and expanding grasslands. The climate change disrupted the
food chains of many large animals. When deprived of their food resources,
they simply vanished. Also by this time, humans were becoming proficient
hunters and roamed northward on the heels of the retreating glaciers. On their
journey, they encountered an abundance of wildlife, many species of which
they might have hunted to extinction.
In North America, 35 classes of mammals and 10 classes of birds went
extinct.The extinctions occurred between 13,000 and 10,000 years ago, with
the greatest die out peaking around 11,000 years ago. Most mammals
adversely affected were large herbivores weighing more than 100 pounds, with
many weighing up to a ton or more. Unlike earlier episodes of mass extinc-
tion, this event did not significantly affect small mammals, amphibians, reptiles,
and marine invertebrates. Strangely, after having survived several previousFigure 197Giant
ground sloths became
extinct at the end of the
last ice age.
Historical Geology