- North America used to be crawling with giant mammals, from dire wolves to big cats.
- Horses and camels evolved on the continent while others, like bison, crossed over from Asia.
- Most of these mammals went extinct around 10,000 years ago and scientists are still debating why.
Towering 13 feet tall and weighing a hefty 7 tons, the African elephant is the biggest land mammal on Earth. But that wasn't always the case.
Tens of millions of years ago, shortly after the dinosaurs went extinct, mammals grew to epic proportions, dwarfing the animals we see today.
"There used to be cow-sized wombats in Australia, and there were armadillos the size of cars in South America," Emily Lindsey, an assistant curator at the La Brea Tar Pits and Museum, told Insider.
Some evolved on these continents while others crossed the Bering Land Bridge, which once connected Asia and Alaska.
North and South America were home to woolly mammoths, mastodons, giant sloths, and much more.
Columbian and woolly mammoths
Height: 12 to 13 feet tall (Columbian mammoths); 9 to 11 feet tall (woolly mammoths)
Weight: 10 tons (Columbian mammoths); 4 to 6 tons (woolly mammoths)
When did they go extinct? 13,000 to 10,000 years ago (Columbian mammoths); 4,000 years ago (woolly mammoths)
Two types of mammoths roamed North America. Both are members of the proboscidean family, which also includes modern elephants, and stem from a species that evolved in Africa 5 million years ago.
The Columbian mammoth arrived in North America about 1.5 million years ago, traveling over the Bering Strait, which once connected Asia and Alaska. They fanned out at least as far south as Mexico, possibly all the way to Costa Rica.
In North America, "you only get the woolly mammoths up in the north, starting around the Great Lakes," Lindsey said. They arrived later, roughly 100,000 years ago, and didn't venture as far south.
One of the woolly mammoth's most distinctive features is its thick fur. The Columbian mammoth was far less shaggy.
American mastodons
Height: 8 to 10 feet tall
Weight: 4 to 5 tons
When did they go extinct? Around 11,000 years ago
A tad smaller than their woolly mammoth relatives, American mastodons were also more widely spread. Their fossils have been found all over the continent, from Alaska to central Mexico.
Some mastodon fossils show them living in North America as far back as 16 million years ago. The forest dwellers likely fed on trees and shrubs.
In addition to their smaller stature, mastodons had shorter, straighter tusks than mammoths. But paleontologists mainly distinguish them by their teeth. Mammoth molars are fairly flat. American mastodons have bumpier molars.
Camels
Height: 7 feet tall
Weight: 1,800 pounds
When did they go extinct? 10,000 years ago
"We think of camels as this very exotic creature that we associated with very different places, with sand dunes, but camels actually are native to North America," Lindsey said.
Its relatives evolved 44 million years ago during the Eocene period. Over time, Camelops made its way to the Canadian Yukon and Mexico, making its home in woodlands, grasslands, and wetlands.
The Camelops was about a foot taller than modern camels, but it's difficult to tell from their fossils if they had a hump.
These animals crossed the Bering land bridge into Asia 7 million years ago and gave rise to modern camels. More recently, they arrived in South America via the Isthmus of Panama, another land bridge that formed between North and South America 3 million years ago.
Some of Camelops' ancestors went extinct, Lindsey said. Others continued to evolve. "Those are the llamas and guanacos that you see in South America today," she said.
Horses
Height: 4.5 feet tall (Equus scotti)
Weight: 1,100 pounds (Equus scotti)
When did they go extinct? About 10,000 years ago
Over 50 million years ago, horses started evolving in North America. There were many species, all different from today's versions.
"When they start out, they're very small," Lindsey said, comparable to small dogs. Instead of hooves, they had five toes on their feet.
Then, 15 million years ago, the landscape and climate started changing. It was drier. Grasslands emerged.
Horses became larger, with longer legs and fewer toes. Some migrated to Eurasia, becoming zebras and wild Asian horses.
"Then they get wiped out in the Americas and brought back about 12,000 years later when the Spanish come," Lindsey said.
Short-faced bears
Height: 11 feet tall when standing on their hind legs
Weight: 1,500 to 2,200 pounds
When did they go extinct? Between 12,7000 and 11,000 years ago
Hundreds of thousands of years ago, the Arctodus simus cast a looming shadow over boreal forests, plains, tundra, grasslands, and subtropical woodlands. Known as the short-faced bear, it ranged across the US and Canada, into Florida.
"The short-faced bears were huge," Lindsey said. They were far larger than 8-foot-tall grizzly bears and "would've been terrifying."
Studies have shown that its muzzle was perhaps not as short as its name suggests. Some researchers also think the bear may have been an omnivore instead of solely a meat eater.
Dire wolves
Height: 3 feet tall at the shoulder
Weight: 130 to 150 pounds
When did they go extinct? An estimated 10,000 years ago
Dire wolves were big. "But they're not as big as they're shown in 'Game of Thrones,' I'm sorry to say," Lindsey said. She calls them the linebacker version of wolves, about 20% larger than gray wolves.
Once thought to be the cousins of gray wolves, dire wolves evolved separately over 5 million years ago in North America. The finding came courtesy of ancient DNA.
Lindsey said the news was surprising because they're so similar behaviorally and morphologically. For example, they likely lived in packs like modern dogs, wolves, and coyotes.
Researchers know quite a bit about how dire wolves hunted, too. "Scientists have looked at injuries in dire wolves, as well as saber-tooth cats, that were found in the tar pits," Lindsey said.
They found injuries to the fossils' paws and skulls. "That's consistent with the type of injuries we see in, say, wolves today that are chasing down animals on the landscape," Lindsey said. A wolf's prey might trample its toes or kick it in the head.
Saber-tooth cats
Height: 3 feet at the shoulder (smilodon and scimitar)
Weight: 300 to 500 pounds (scimitar cat); up to 750 pounds (smilodon)
When did they go extinct? Roughly 10,000 years ago (smilodon); between 10,000 and 28,000 years ago (scimitar)
You may have grown up calling them saber-toothed tigers, but the smilodon isn't a close relative of tigers. Their common name comes from their impressive canine teeth, which could be up to 7 inches long.
Another genus of saber-tooth cats, Homotherium, is known as scimitar cats. Their teeth weren't as long as smilodon and were serrated like a steak knife. Like the smilodon, they lived in North America during the Pleistocene. The two cats were similarly widespread, from the Yukon to Florida.
But these two predators had different prey, according to a study from a few years ago. While the smilodon went after tapirs (pig-like animals with trunks) and deer, scimitars preferred baby mammoths.
"Saber-tooth cats tend to get a lot of injuries in their lower back, which is consistent with this idea that they were ambush predators," Lindsey said. The smilodons would sit and wait before leaping onto a large animal and wrestling it to the ground.
"It's very easy to torque your back that way," Lindsey said. "And so they get a lot of lower back arthritis type of injuries."
Lindsey was also part of a study that diagnosed a smilodon with hip dysplasia based on a CT scan of a fossil. "It's an animal that probably couldn't have survived to adulthood as it did without other animals taking care of it," she said. "So that indicates that these are probably social creatures. There's all kinds of amazing, fun things you can learn from bones."
Giant sloths
Height: 19.6 feet when upright
Weight: Up to 3 tons
When did they go extinct? 10,000 years ago or into the Late Holocene
For most extinct Pleistocene megafauna — animals weighing more than 100 pounds — there are still living analogs. Instead of mammoths, there are elephants. Camelops has its camel and llama relatives.
"But you don't have anything in the world that is even kind of like a 3,000-pound sloth," Lindsey said. "That's just something that is completely gone."
Unlike their modern counterparts, these behemoths spent most of their time on the ground instead of in trees.
North America had several species of giant sloths. Among the largest were the Eremotherium, which have mostly been found in the southeast along Florida and the Gulf Coast, as well as in Central and South America.
"They like sort of the wetter, more tropical regions," Lindsey said. "They got to be about the size of elephants." While very large, the Eremotherium wasn't as big as the Megatherium — another species of giant sloth — which lived throughout South America.
The giant sloths had a diverse diet, including being able to reach higher into the trees thanks to their stature and ability to stand on their hind legs. Exactly when they went extinct is disputed, but their adaptability may have helped them survive a bit longer than many other mammals they lived alongside.
American lion
Height: 4 feet at the shoulder
Weight: Up to 770 pounds
When did they go extinct? Between 10,000 and 12,000 years ago
Saber-tooths weren't the only big cats that made their home in North America. Panthera atrox was one of the largest members of the cat family ever.
"The American lions were huge," Lindsey said. "They're much, much bigger than modern lions."
While fairly rare in the fossil record, there's evidence the big cats did make their way throughout North America. Scientists have found their bones and teeth from Canada to Mexico.
Researchers have debated about whether these gigantic cats are more closely related to modern lions or jaguars. Its skull shares similarities with lions', but its jaw resembles those of jaguars and tigers.
Due to its enormous size, the American lion was able to make meals out of bison and wild horses.
Ancient bison
Height: 7.5 feet tall
Weight: 3,500 pounds
When did they go extinct? Roughly 10,000 years ago
Unlike the Camelops and horses that evolved in North America, other big beasts migrated in. That includes ancient bison.
"They are the most recent arrivals into North America of all the Pleistocene megafauna," Lindsey said.
The first bison species appeared around 3 million years ago in South Asia and China. Eventually, some crossed the Bering Land Bridge. They start showing up in the North American fossil record between 300,000 and 130,000 years ago.
The ancient bison, Bison antiquus, was 25% larger than those living today. Their territories included grasslands, woodlands, and wetlands from Canada to Mexico.
Another species of bison on the continent was the Bison priscus or steppe bison. A recent study suggested modern bison — Bison biso — evolved from this species.
Why did they go extinct?
Was it the changing climate, human interactions, or a combination that caused so many large mammals to go extinct around 10,000 years ago?
"That is something that scientists have been debating for about 70 years," Lindsey said. The climate was changing at the end of the Ice Age on a global scale. Humans were also spreading throughout the world.
"These two things are so closely tied in time, the climate changes and the spread of human populations, it's been really hard to say to what degree each of those factors was involved in the extinctions," Lindsey said.
Humans may have more of an impact than just hunting, too. A 2023 paper tied fires started by humans to some of the extinctions in what is now California.
If that did have an impact on the animals in that region, the causes might vary for others. "Different ecosystems are going to have different parameters of what pushes them to the brink," Lindsey said. "But there is something about the combination of humans and an unstable climate or a rapidly changing environment that pushes ecosystems again and again towards tipping points."