What Was the Giant Ground Sloth? Megatherium and the Animal That Pulled Down Trees

Megatherium americanum was a ground sloth of the pampas of Argentina, Uruguay and Bolivia, related to the tree sloths and the anteaters, and built like nothing living: a body the size of an elephant's on hind legs and a heavy tail, with forelimbs that could reach up while the animal stood, and a skull with peg-like teeth and a long muscular tongue. Its footprints, found in Argentina, show that it walked on the sides of its feet, with the claws curled inward, and sometimes on two legs. The skin, known from a smaller relative found mummified in a Chilean cave, was studded with small bones, ossicles, like chain mail under the hair, which may have been defence against the sabre-toothed cats that shared its range.

The teeth and jaws of Megatherium were built for browsing, and the animal seems to have reared up on its hind legs and tail as a tripod to pull down branches with its forelimbs and strip them with the tongue, reaching leaves that no other animal on the pampas could touch. A 2003 analysis of its jaw suggested it could also have taken meat, scavenging or even killing with its claws, and the idea has not been widely accepted; the isotopes in its bones point to plants. The ground sloths were a large and varied family:

• •Megatherium: the largest, about four tonnes, on the grasslands of southern South America
• •Eremotherium: as large, in the forests of Brazil and Central America, reaching as far north as the southern United States
• •Megalonyx: a Jefferson sloth, named for Thomas Jefferson, who described its claws in 1797 and hoped it was still alive in the west; the size of an ox, found across North America to Alaska
• •Nothrotheriops: the Shasta ground sloth of the American southwest, whose dung, preserved in dry caves, records a diet of desert plants
• •The Caribbean sloths: small island species, the last ground sloths of all, surviving on Cuba and Hispaniola until about 4,000 years ago

The Madrid skeleton, found on the banks of the Lujan river near Buenos Aires in 1787, reached Cuvier as drawings, and he recognised in it an animal related to the sloths but unlike anything alive, and used it with the mammoth and the mastodon to argue in 1796 that species had gone extinct, an idea that most naturalists rejected because it implied that creation was incomplete. Darwin found ground sloth bones in Argentina on the Beagle voyage in 1832, the first fossils he collected, and their resemblance to the living sloths of the same continent, giants replaced by dwarfs, was one of the observations that set him thinking about descent. The animal that hung upside down and the one that stood six metres tall were kin, and the question of why one had gone and the other stayed was a question about how nature changed.

The ground sloths of the mainland disappeared between about 12,000 and 10,000 years ago, in the extinction that took the mammoths, mastodons, horses, camels and sabre-toothed cats of the Americas, and the two suspects are the same as everywhere: the end of the ice age and the arrival of people. The human case is stronger for the sloths than for most. Butchered sloth bones with cut marks have been found in Argentina and Uruguay, sloth footprints in New Mexico are overlain by human ones that seem to follow and harry the animal, and the island sloths of the Caribbean died out within a thousand years of humans reaching each island, thousands of years after the mainland ones, which is the pattern of hunting rather than climate. A slow, large, unafraid animal that reproduced rarely was exactly the prey that a new predator with spears would take first.

For most of the twentieth century the family tree of sloths was drawn from the shapes of skulls and teeth, which can easily mislead when unrelated animals evolve similar features for similar lives. In recent decades two new kinds of evidence have been added. Ancient DNA can be recovered from sloth bones, hair and dung preserved in cold or dry caves, and where DNA has broken down, especially in warm climates, collagen proteins in bone survive much longer and can be sequenced instead. Studies using the two methods, published independently, reached largely the same answer, and it overturned part of the old tree.

The two living kinds of sloth turn out not to be close relatives of each other. The two-toed and three-toed sloths each sit within a different branch of the ground sloth family, which means that hanging upside down in trees, the slow metabolism and the hooked claws evolved at least twice, separately, from ancestors that lived on the ground. Biologists call this convergent evolution, when unrelated lines arrive at the same solution because they face the same problem, and the sloths are now one of its clearest examples. The animals in the canopy are not simply shrunken ground sloths but two independent experiments in the same way of life.

Some fruit you may eat could have been shaped by animals like Megatherium. In 1982 the ecologists Daniel Janzen and Paul Martin pointed out that a number of Central and South American trees produce large fruits with big seeds that no living native animal swallows and spreads well. The avocado is the best-known example: fruit that simply drops beneath the parent tree mostly rots or sprouts in its shade. Janzen and Martin proposed that such fruits were adapted to the extinct megafauna, including ground sloths and gomphotheres, elephant relatives that once lived in the Americas.

On this idea, a giant herbivore would eat the fruit whole, walk for kilometres and pass the seed in a heap of dung, a ready-made seedbed far from the parent. When the big animals vanished, the trees lost their partners but lived on, since long-lived trees can persist for millennia with poor dispersal, and people later took over by cultivating some of them. The writer Connie Barlow called such plants ghosts of evolution. The hypothesis is hard to test directly, and some fruits on the list may have other explanations, but it suggests that an extinction can leave gaps in an ecosystem that are still visible today.

Some of the most detailed evidence about ground sloths comes from their droppings. In dry caves in the American Southwest, such as Rampart Cave in Arizona, thick deposits of sloth dung survived for thousands of years. By examining the plant remains inside, researchers could identify exactly what the animals ate, including desert plants such as globemallow and Mormon tea.

Radiocarbon dating of the dung helped pin down when ground sloths disappeared from the region, around the end of the last ice age. Sadly, the Rampart Cave deposit was badly damaged in 1976 when a fire, probably started by a visitor, smouldered in the dung for months. Similar caves in South America have preserved sloth dung, hair and even skin.

The giant ground sloths were a family of large relatives of today's tree sloths, the biggest, Megatherium, an elephant-sized browser of the South American grasslands that reared up on its hind legs and tail to pull down branches, with peg teeth, forearm-length claws and bone-studded skin. Its skeleton persuaded Cuvier that extinction was real and its bones set Darwin thinking, and it died out about 10,000 years ago with the other American giants, with humans the likelier cause, the island species lasting until 4,000 years ago.