North America - Giant Beasts!

When humans first entered North America is hotly debated. However, many scientists believe that humans arrived comparatively recently, long after the end of the last major glaciation (about 20,000 calendar years ago). The earliest radiocarbon dates documenting human presence in either North or South America that are widely accepted are only on the order of 12,000-13,000 years ago (about 14,000 calendar years ago).

The most obvious difference concerns the mammalian megafauna. Today there are only about a dozen species of North American mammals that are megafaunal (greater than 100 lbs in body size). As recently as 11,000 calendar years ago, there were more than three times that number.
Whatever the exact date may turn out to be, we know from the archeological and paleontological records that, when humans first arrived in the New World, they entered a faunal landscape that greatly differed from the one that we live in today.

Their ranks included the Woolly Mammoth, a close relative of living elephants, and Giant Ground Sloths whose closest living relatives are the comparatively tiny Tree Sloths of South America. In addition to these species, there were American horses, American camels, American lions, and a host of less familiar creatures. Within a period possibly as short as 400 years, the last representatives of these species (and many others, to a total of perhaps 135 on both continents) had become extinct.

What happened to all these mammals?

"Megafaunal" by whose standards?

Scientists use different definitions of what counts as a large animal.

When talking about the end-Pleistocene extinctions, for example, some scientists refer to any animal over 5 kg as "large." Using this definition, about 90% of the genera dying out in North America 11,000 calendar years ago could be described as large.

Other scientists restrict the term "megafaunal" to refer to any animal over 100 lbs (~44 kg). Under this definition, only about three-quarters of the species that went extinct fit the "megafaunal" bill.

Megafaunal by anyone's standards!
Man standing next to the bones of a Woolly Mammoth leg in the AMNH collection.