Darwin's ground sloths

On the second voyage of HMS Beagle, Charles Darwin collected thousands of plant, animal, rock and fossil specimens, including 13 species of fossil mammals. Four of these were species of ground sloth, three of which unknown to science.. 

Mylodon darwinii and Megatherium americanum were scanned in 3D and released onto the Museum's Data Portal and 3D platform Sketchfab in 2018. In the latest update to the digitisation of Darwin's fossil mammals, we have produced new 3D models of the final two species of sloth that Darwin collected, Glossotherium and Scelidotherium.

Sloths are notable for their low energy levels and slow, deliberate movements (hence their name). They are part of a larger group known as the Xenarthra, which includes their distant relatives the armadillos and anteaters.

Unlike now, most extinct sloths lived on the ground and some were many times larger than modern tree sloths.

Mylodon darwinii: Darwin's ground sloth

Mylodon darwinii is the scientific name for a species of extinct giant ground sloth. On finding the specimen, Darwin identified it as belonging to the same family as the modern sloths. Richard Owen later described it as a new species and named it in honour of its discoverer.

Mylodon lived during the Pleistocene Epoch, between 1.8 million years and 12,000 years ago. The remains of the animal have been found throughout South America, from Bolivia in the north to southern Patagonia. This shows the animal was able to adapt to cold climates.

M. darwinii weighed between 1,000 and 2,000 kilogrammes and was about three metres long. Like other sloths, it was mainly vegetarian, although a recent study by Tejada et al. suggests that M. darwinii was an opportunistic omnivore in some contexts. But unlike some of its relatives, it did not burrow or climb trees.

The exact causes of its extinction are not fully understood, although climate change and its effects on vegetation were almost certainly partly responsible.

Megatherium americanum

Megatherium americanum is the scientific name for an extinct species of giant ground sloth. The name means 'great beast from America'.

Discovered in 1787 by Manuel Torres in Argentina, the first M. americanum fossils were shipped to the Museo Nacional de Ciencias in Madrid, where the original skeleton is still on display.

The Megatherium fossils are two halves of the same skull, one held at the Hunterian Museum at the Royal College of Surgeons of England and the other at Down House (looked after by English Heritage). The fossils confirmed that, despite weighing up to four tonnes, the extinct animals were indeed related to modern-day sloths.

Glossotherium

Glossotherium is a type of extinct ground sloth endemic to South America during the Pliocene and Pleistocene (between approximately four million and 12,500 years ago). The best-known species is Glossotherium robustum, a relatively large animal around four metres long and probably weighing around 1,500 kilogrammes.

Glossotherium was first described by Richard Owen based on a partial skull collected by Charles Darwin on the Voyage of the Beagle. Owen noticed a cavity in the base of the skull, which he concluded was an attachment site for ligaments connecting the skull to the hyoid bone, a U-shaped bone that supports the tongue.

Based on the size of the cavity, Owen concluded the beast must have had a large tongue. He chose the name Glossotherium, which means 'tongue beast'. He also looked at how the lower jaw attached to the skull - and using this and his conclusion that it had a long tongue, Owen predicted that it ate insects such as termites.

While Owen was a gifted anatomist and many of his conclusions have stood the test of time, it is now accepted that Glossotherium was a herbivore. Since Darwin collected this specimen in 1833, more complete specimens of this animal have been discovered, including the front of the skull and teeth.

The wide muzzle of G. robustum suggests it was adapted for bulk-feeding on large quantities of poor-qualilty foliage such as grass. Recent detailed comparisons of the hyoid bones of ground sloths, however, suggest that G. robustum may have been able to protrude its tongue to a greater degree than many other sloths, allowing it to wrap around and grip the vegetation.

Scelidotherium

Scelidotherium leptocephalum is the scientific name for a species of extinct ground sloth, first described based on a partial skeleton discovered by Charles Darwin in 1833 on a beach in Argentina. This species is commonly found in the Pampean regions of Argentina but is also known from other Argentinian and Uruguayan sites. Known specimens are estimated to be between 800,000 and 11,000 years old.

S. leptocephalum was one of the smaller, more gracile ground sloths of the Pleistocene, weighing around 500 to 1,000 kilogrammes in weight, around the size of a large buffalo. It had a narrow, tubular skull, which suggested that it was a selective herbivore. It may have had a prehensile lip - it was able to firmly grasp objects with it – which it would have used to select particular plants or plant parts for their nutritional value.

It is highly likely that S. leptocephalum was able to burrow underground. There are several large Pleistocene caves and tunnels in the Pampean region of South America which were probably excavated by Scelidotherium leptocephalum (and probably Glossotherium robustum). These branched tunnels are up to 1.2 m high, 1.8 m wide and 20 m in length. There are scratches on the walls and roofs which match the size and shape of the claws of these ground sloths. This interpretation of them as burrowers is consistent with the shape of their limb bones.

Explore Darwin's sloths online

The M.darwinii, M.americanum, Glossotherium and Scelidotherium 3D models join the Darwin fossil mammals dataset on the Museum's Data Portal. They are also available to view on Sketchfab, a 3D modelling platform.

These fossil mammals collected by Darwin are historically and scientifically important, but also very fragile. Access to these specimens had been restricted in the past, to protect them from further damage.

By creating and openly releasing digital versions of Darwin's fossils on the Data Portal, we hope to increase access and use of the specimens online.

The previously released Toxodon, Mylodon and Megatherium models have been viewed thousands of times and downloaded and printed all over the world. They have been used in engagement activities in the Museum and elsewhere including by researchers at in the Western Science Centre in California (USA), during talks to English Heritage at Darwin's Down House and at the Chilean Congress of Palaeontology. In 2019, a Mylodon darwinii mandible cast was gifted from the Museum to the Charles Darwin Museum, Argentina.