Beavers the size of bears. Sloths weighing more than 3,000 pounds. Seven-foot camels roaming the shores of marshy seas. This was the Ice Age world that humans encountered when they first crossed from Asia into North America roughly 14,000 years ago.

“The North America we know today is not a normal, natural landscape,” says Stuart Fiedel, an archeologist who investigates the prehistory of the Americas. “For most of Earth’s history, certainly through the Pleistocene, there were big mammals all over the place. After humans arrive, though, the really big mammals rapidly disappear, except in Africa and parts of Asia.”

When the glaciers receded in the late Pleistocene (also called the Ice Age), North America was home to dozens of thriving species of extra-large mammals known as megafauna. But around 10,000 years ago, nearly all of those giant creatures were wiped out.

What caused the mass extinction of Ice Age megafauna? That’s an open debate. Most archeologists blame over-hunting by paleo-humans, who would have encountered large prey animals unafraid of the hairless, two-legged newcomers.

Others say that overkill of megafauna by humans was only one cause. The mass extinctions may have also been triggered by a sudden climatic shift that rapidly cooled the planet 12,800 years ago (the Younger Dryas), or animals could have been stricken with diseases carried by paleo-humans and their dogs.

Until that mystery is solved, learn more about some of the extinct giants of Ice Age North America.

1. Giant Short-Faced Bear

Giant, indeed. The giant short-faced bear (Arctodus simus) was the largest carnivorous mammal to ever roam North America. Standing on its hind legs, an adult giant short-faced bear boasted a vertical reach of more than 14 feet. The most striking difference between modern North American bears and the giant short-faced bear were its long, lean and muscular legs.

“The short-faced bear was huge and had much longer legs than modern bears relative to the body,” says Fiedel, “which has given rise to the idea that it was a ‘cursorial’ predator, meaning that it ran after prey.”

By examining skeletal remains, paleontologists calculate that the short-faced bear could reach speeds topping 40 miles per hour, making it the fastest bear to ever live. Combined with large nasal cavities, it’s likely that the short-faced bear used its powerful sense of smell to detect nearby carcasses, and its speed and size to chase off the competition.

Giant short-faced bears are not related to any living species of bear in North America. Its closest living relative—although distant—is the diminutive spectacled bear (or Andean bear), the only bear species native to South America.

2. Saber-Toothed Tiger and Scimitar Cat

The saber-toothed tiger (Smilodon fatalis) is one of the most iconic animals of Ice Age North America. Saber-tooth skeletons pulled from sites like the La Brea tar pits in Los Angeles were armed with upper canines averaging seven inches long and a jaw that could open an astonishing 130 degrees.

Saber-toothed cats aren’t related to modern tigers—or any living felines at all—and their bodies stray significantly from today’s tigers or lions. They were a foot shorter than an adult lion, but almost twice as heavy (over 600 pounds in some cases). Saber-toothed cats had relatively short legs and a bobbed tail, meaning they were built for ambush attacks, not long sprints.

But saber-toothed tigers weren’t the only big-boned feline hunters of the late pleistocene. The scimitar cat (Homotherium serum), with its four-inch canines, roamed all of North America and was built more like a modern African lion with long front legs for pulling down big prey. An adult scimitar cat could easily take down a young mammoth and drag it back to its cave.

3. Dire Wolf

The dire wolf gets its sinister-sounding name from its Latin name, Canis dirus, meaning “terrible wolf.” Despite its wolf-like appearance—they were roughly the same height and length as modern wolves—DNA analysis suggests that the dire wolf wasn’t closely related genetically to the gray wolf at all, but was a distant relative of modern jackals.

“Ancient DNA studies have really shaken up pleistocene paleontology,” says Fiedel. “The dire wolf was less of a wolf than a super-sized, ferocious coyote.”

Dire wolves roamed every inch of North America from the frozen Canadian north down through Mexico and thrived in every imaginable ecosystem from boreal forests to grassland plains to tropical wetlands. Like gray wolves, dire wolves hunted in packs of 30 or more and fed on large prey like mammoths, giant sloths and Ice Age horses.           

4. Mammoth and Mastodon

The woolly mammoth (Mammuthus primigenius) is one of the most famous extinct Ice Age megafauna. Standing 12 feet tall at the shoulders and weighing six to eight tons, the woolly mammoth grazed the northern steppes of Ice Age North America using its colossal, 15-foot curved tusks to dig under the snow for food and defend itself against predators.

But fossil evidence shows there were at least four distinct species of massive, elephant-like animals that called North America home in the late pleistocene.

The American mastodon (Mammut americanum) is the most ancient of the North American “elephants.” Its ancestors crossed the Bering Strait from Asia roughly 15 million years ago and evolved into the American mastodon 3.5 million years ago. The mastodon was shorter and stockier than the later mammoths, and the shape of its teeth indicate that mastodons didn’t graze on grass like mammoths, but ripped off leaves and entire tree branches for food. 

Mammoths came to North America much later in two waves. Around 1.5 million years ago a mammoth called Mammuthus meridionalis crossed the Bering land bridge and branched into two distinct mammoth species: the relatively hairless Columbian mammoth (Mammuthus columbi) and the Jeffersonian mammoth (Mammuthus jeffersonii). Then, as recently as 500,000 years ago, the woolly mammoth arrived.

Mammoths spread everywhere in Ice Age North America, ranging from Canada down to Honduras. Nearly all mammoths and mastodons were wiped out in the great megafauna extinction 10,000 years ago, but archeologists have dug up remains showing that lone bands of mammoths still roamed arctic islands as recently as 4,500 years ago.

“There were mammoths living down to the time when the Egyptians were building the pyramids,” says Fiedel.

5. Giant Ground Sloths

Today, sloths are molasses-moving tree dwellers roughly the size of a dog. But in Ice Age North America, sloths were an entirely different beast. The giant ground sloths of the late Pleistocene were bear-sized herbivores that stood 12 feet on their hind legs and weighed up to 3,000 pounds.

One giant sloth species, the Jefferson ground sloth (Megalonyx jeffersonii), was named for Thomas Jefferson, who initially believed that sloth fossils were a type of colossal cat that he dubbed the Megalonyx (“giant claw”).

Like modern sloths, these huge ground sloths were probably slow, awkward walkers with some species shuffling on the curled-in ankles of their hind legs. Ground sloths also had elongated claws like modern sloths, which they would have used to strip leaves and dig for roots.

DNA analysis indicates that all extinct North American ground sloths, including the massive Harlan's ground sloth, originated in South America and migrated north along with ancient opossums and armadillos.

“North and South America were separated until around 2 million years ago when the Isthmus of Panama emerged,” says Fiedel. “Most mammals migrated north to south, but sloths came the other way. In some periods, there were giant ground sloths as far north as Alaska.”

6. Camelops

The very first camels on the planet evolved in North America around 44 million years ago. Those ancient camels migrated westward over the Bering land bridge around 7 million years ago, later becoming the one-humped (dromedary) and two-humped (Bactrian) camels of North Africa and Asia. Other ancient species of North American camels migrated south and became the llamas and alpacas of South America.

One of the most widespread camel species in Ice Age North America was the camelops (Camelops hesternus) or “yesterday’s camel.” This two-toed, furry camel stood seven feet tall at its shoulders, weighed 1,800 pounds and had no hump at all. The camelops could travel long distances on its long and lean legs, but it’s not clear if they could survive long periods without water like modern camels.

7. Giant Beaver

The giant beaver of Ice Age North America was the largest rodent to ever live. It measured up to seven feet long, weighed more than 250 pounds and its two front teeth (incisors) were as long as bananas.

Although similar in appearance to modern beavers—with oversized front teeth and a flat tail—the giant beaver is more closely related to the capybara of South America. The incisors and molars of the giant beaver were rounded and blunt, lacking the power and precision to fell large trees. And their brains were relatively smooth, a sign that they weren’t intelligent enough to construct dams.

“Why then do they have the same kind of giant teeth?” asks Fiedel. “Maybe they’re eating bark or tree pith but not doing anything with the wood that they chop down. Let’s just say, nobody has found a giant beaver dam.”

8. American Zebra

It was long believed that horses were first introduced to North America by Spanish settlers in the 16th century, but archeological evidence has rewritten that history. It’s now clear that indigenous horses roamed North America for 55 million years before going extinct along with other Ice Age megafauna roughly 10,000 years ago.

One of the oldest and most widespread ancient horse species in North America was the American zebra (Equus simplicidens), also known as the Hagerman horse. The American zebra is the oldest known member of the genus Equus, which includes all modern horses, both wild and domesticated.

In both appearance and genetics, the American zebra is most closely related to the modern zebra. The American zebra stood around five feet tall at the shoulders, had a stocky build, and faint stripes along its neck and flank.

Around 2 to 3 million years ago, herds of American horses traveled west over the land bridge into Asia, eventually spreading to Africa. Those ancient horses were the distant ancestors of the domesticated horses that the Spanish re-introduced to North America 500 years ago.