Not so very long ago, the Americas were home to giant mammals: monstrous-sized beavers, ground sloths weighing over 4,000 pounds, the well-known woolly mammoths and saber-tooth cats, and many more.

These megafauna, which is ancient Greek for “large animals,” thrived in the geological period called the Pleistocene epoch, which spans 1.8 million to 12,000 years ago. (For perspective, most of the dinosaurs went extinct around 65 million years ago.) The Pleistocene era ended with the end of the last ice age, along with the sudden extinction of most of the American megafauna.

Although it’s unclear what’s responsible for the mass extinctions, two main factors are suspected: dramatic climate change at the end of the ice age and arrival of humans.

A great variety of enormous mammals roamed these lands until their sudden mass extinction 10,000 to 12,000 years ago.

Ancestors of modern Native Americans, the Clovis people, arrived in North America around 12,000 years ago, crossing the Bering Straight. They may have played a part in these extinctions directly, such as through hunting, or indirectly, such as through spreading new diseases and/or disrupting established ecosystems. These factors or others caused the extinction of around 30 genera of megafauna in North America and 45 genera in South America between 10,000 to 12,000 years ago. The megafauna spanned a huge range of sizes and shapes; this article can only highlight a few, but to encounter more of these giants personally, this author recommends a trip to the La Brea Tar Pits in Los Angeles.

Although often depicted as huge wolves in popular culture, the dire wolf (Canis dirus) was actually smaller than some living northern timber wolves, reaching five feet long and weighing around 110 pounds. Making up for their relatively small size, they had massive teeth and hunted in large packs. The dire wolves spread across North and Central America, coexisting with the smaller grey wolves (Canis Lupus), and were predominant hunters in California; remains of thousands have been recovered at the La Brea Tar Pits.

Though today we think of them as a very small group of animals, rodents larger than wolves were common not so long ago. Only one existing species helps remind us what these giants were like: The largest living rodent, the capybara (Hydrochoerus hydrochaeris) looks like a four-foot-long, 140-pound guinea pig. However, unlike guinea pigs, capybaras are semi-aquatic and have webbed feet. Although they’re native to South America, you can catch them swimming at the Santa Barbara Zoo. Until 11,000 years ago, capybaras weighing around 200 to 250 pounds (Neochoerus pinckneyi) roamed the Americas.

Another colossal rodent was the giant beaver (Castoroides ohioensis). Building huge lodges in North America, these beavers reached up to nine feet long. Minus the two-foot-long tail, their bodies were comparable in size to that of a black bear.

Large cats were well-represented Pleistocene megafauna as well. The North American lion (Panthera leo atrox) was a quarter again as big as any existing large Eurasian cat. However, “lion” may be a misnomer; it may be closer to a gigantic jaguar. Spread across the Americas, these cats were common in California; around 100 have been recovered from the La Brea Tar Pits.

Multiple types of saber-tooth cats (subfamily Machairodontinae) hunted in America too. The “scimitar cat” is a smaller, lesser-popularized saber-tooth cat (genus Homotherium, literally “man’s beast”). This cat, about the size of a lion but with a hyena-like build, weighed up to 600 pounds. As their name implies, scimitar cats not only had impressive five-inch-long canines, but those teeth had razor-sharp edges. They once lived throughout Africa, Eurasia, and North America. The more popularized saber-tooth cats of the Americas (genus Smilodon) included three species, all of a compact, muscular build. The largest species (Smilodon populator), at four feet tall and seven feet long, may have weighed over 800 pounds. This mass was topped off with distinctive seven-inch-long canines.

A variety of hoofed animals also roamed ancient America, including several horse species. The extinct Western horse (Equus occidentalis) was similar in size to a modern Arabian horse, but heavier in build, while the smaller, 500-pound Equus conversidens was just one of many petite horse species. Around 11,000 years ago all horses disappeared from the American continents, but were reintroduced by Spanish explorers in the 1500s A.D.

The large, moose-like deer (Cervalces scotti), at an impressive 1,000 pounds, was larger than modern moose. The woodland musk-ox (Bootherium bombifrons), the only ox that evolved and stayed in North America, was even more massive. Large camels (genus Camelops) also once called America home; some were seven feet tall at the shoulder, slightly taller than modern camels, and weighed just over 2,000 pounds.

Claiming the title for the largest mammalian land carnivore of the time was the giant short-faced bear (Arctodus simus). The largest males were probably just over five feet at the shoulders, and eleven and a half feet tall when standing on their hind legs. Weighing an amazing 2,500 pounds, they’re the largest bear ever to exist.

With some members even larger than these amazing bears, over 80 genera of Pleistocene ground sloths existed. Although three-toed and two-toed tree sloths still exist, these are much smaller than their ancient relatives. Giant ground sloths had huge stout claws and great crushing ability. Smaller giant ground sloths, weighing a mere 1,400 pounds, were browsers (family Megatheriidae) and ate leafy material, reaching for high branches on their hind limbs. Grazing ground sloths (family Mylodontidae) preferred open stretches of grassy land. These massive sloths were over six feet tall, and weighed around 4,400 pounds. Another family (Megalonychidae) is famous for being first described by Thomas Jefferson in 1794, as he discovered giant remains in western Virginia caves (the species was consequently named Megalonyx jeffersoni). Some members weighed nearly 3,000 pounds.

Related to the ground sloths, giant armadillos the size of cars once inhabited these lands. Unlike modern insect-eating armadillos, these giants ate plants. Two families of giant armadillos existed, each having tortoise-like armor; bone segments formed a huge shell around their back. Members of one family (Pampatheriidae) were relatively small, weighing 440 pounds, while the largest members of the other family (Glyptodontidae) weighed around 4,400 pounds.

Even larger still, the American mastodon (Mammut americanum) resembled a furry elephant, and is often mistaken for the woolly mammoth. Mastodons browsed cold North American woodlands and weighed an awesome 12,000 pounds. They have a distinct large hole in their skulls, which is their nasal cavity. It’s theorized that discovery of these remains by ancient Greeks and others may have inspired myths about the giant Cyclops.

Largest of all, some legendary Woolly Mammoth (genus Mammuthus) bulls may have weighed 20,000 pounds. For comparison, the current largest land mammal is the African elephant, with bulls usually around 13,000 pounds. The mammoths spread across North America and Eurasia, eating tundra grass. They had meter-thick fur with a thick undercoat the mastodon may not have had, as well as longer, more curved tusks. Ancient cultures often believed their remains to be the bones of giants. Only in the last few centuries have scientists recognized them as a species distinct from Asian elephants, which they are quite similar to. Many specimens have been found nearly intact due to their having left their bodies in frozen areas; enough DNA has been recovered to sequence their genome.

Although we may never know exactly what caused the massive extinction of so many prehistoric behemoths, their existence and unlikely rapid demise reminds us of the importance of present conservation efforts. Today we can only catch a glimpse of what these giants were like, watching some of their lucky little cousins that still scurry across the lands.

For more on Pleistocene megafauna of America, see Chester Stock’s Rancho La Brea: A Record of Pleistocene Life in California, W. David Lambert and Crawford S. Holling’s “Causes of Ecosystem Transformation at the End of the Pleistocene: Evidence from Mammal Body-Mass Distributions,” Gary Haynes’ “American Megafaunal Extinctions at the End of the Pleistocene” Wikipedia’s “Megafauna,” Wikipedia’s “Pleistocene Megafauna,” Wikipedia’s “New World Pleistocene Extinctions.”

Biology Bytes author Teisha Rowland is a science writer, blogger at All Things Stem Cell, and graduate student in molecular, cellular, and developmental biology at UCSB, where she studies stem cells. Send any ideas for future columns to her at


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