Megatherium: The Giant Ground Sloth Found in Argentina in 1787

Megatherium is a giant ground sloth whose first known skeleton was dug up in 1787 beside the Luján River in Argentina and shipped to Madrid. Mounted for display in the Spanish royal natural history collection, it was studied from engravings by the French anatomist Georges Cuvier, who named it in 1796. The animal, as large as an elephant yet clearly related to living sloths, became one of the first fossils to persuade scientists that species could become extinct.

A giant from the pampas

In 1787 the Dominican friar Manuel de Torres found large bones eroding from the banks of the Luján River, west of Buenos Aires. The region then belonged to the Spanish Viceroyalty of the Río de la Plata. The bones were excavated with the help of local officials and packed into crates.

They were sent to Spain and reached Madrid in 1788, where they entered the Real Gabinete de Historia Natural, the Royal Cabinet of Natural History. A popular anecdote holds that King Charles III asked for a living example of the animal to be sent, but the story is not well documented.

Mounting the skeleton in Madrid

The task of assembling the bones fell to Juan Bautista Bru de Ramón, the cabinet’s painter and preparator. With no living animal to compare, he worked out the arrangement himself, and the result was one of the earliest mounted skeletons of a fossil vertebrate anywhere in the world.

Bru also made detailed drawings, which were engraved by Manuel Navarro. They were later published in 1796 by José Garriga in a Spanish description of the skeleton. Some of Bru’s errors, such as parts of the shoulder and foot, were corrected in later remountings, but the skeleton is still on display at the National Museum of Natural Sciences in Madrid.

Cuvier names the “great beast”

Proof sheets of Bru’s engravings reached Paris through a French diplomat. In 1796 the young Georges Cuvier, working at the National Museum of Natural History, published a short paper based on them. He identified the animal as an edentate, the group that includes sloths, anteaters and armadillos, and named it Megatherium, “great beast”.

That same year Cuvier argued that the mammoths and mastodons of Europe and North America were distinct from living elephants. Megatherium fitted the same pattern: a huge animal, obviously unknown alive, that could hardly have escaped notice if it still existed. Together with the Mosasaurus of Maastricht, these fossils underpinned Cuvier’s argument that extinction was real and that Earth had a long history of lost worlds.

What Megatherium was like

Megatherium americanum lived in South America during the Pliocene and Pleistocene epochs and died out roughly 12,000 years ago. Large individuals reached about 6 metres from head to tail, and weight estimates generally range from about 3 to 4 tonnes, with some higher.

It had massive hind limbs, a stout tail and long, curved claws. Footprints and skeletal evidence suggest it walked on all fours but could rear up on its hind legs to reach leaves, perhaps using its tail as a prop. Its teeth point to a diet mainly of tough plants, although some researchers have suggested it may occasionally have scavenged.

Darwin and later finds

In September 1832, during the voyage of HMS Beagle, Charles Darwin found fossil bones at Punta Alta near Bahía Blanca, Argentina, including Megatherium remains alongside other extinct mammals. The fossils were later studied in London by Richard Owen.

Darwin was struck by the fact that South America’s extinct giants were related to its living sloths and armadillos. This pattern of related animals succeeding one another in the same region became one of the observations that led him towards the theory of evolution.

A world of giant sloths

Megatherium was not alone. Ground sloths were a diverse group in the Americas, ranging from animals the size of a large dog to giants such as Megatherium and Eremotherium, which spread as far north as the southern United States. Unlike today’s small tree sloths, which hang from branches in tropical forests, they lived on the ground and moved on all fours.

Their ancestors evolved in South America, which was an island continent for tens of millions of years. When the Isthmus of Panama formed, about 3 million years ago, ground sloths crossed into North America, while many northern animals moved south. Fossils of ground sloths, including dried dung and skin from caves, show that some survived until only a little over 10,000 years ago.

By the numbers

Timeline

• 1787: Manuel de Torres finds the skeleton by the Luján River.
• 1788: The bones arrive in Madrid at the Royal Cabinet of Natural History.
• c. 1788–1795: Juan Bautista Bru mounts the skeleton and draws it for engraving.
• 1796: Cuvier names Megatherium; José Garriga publishes the Madrid description.
• 1832: Darwin collects Megatherium fossils at Punta Alta, Argentina.

Why it matters

The Luján Megatherium was one of the first fossil skeletons ever assembled for display and one of the first giant extinct mammals described scientifically. It helped Cuvier establish extinction as a fact, and its relatives in South American rocks later fed into Darwin’s thinking. Its disappearance also belongs to the wider Quaternary megafauna extinction at the end of the Ice Age.