The final two specimens of Darwin's fossil mammals to have been scanned belonged to an extinct giant ground sloth.

In the latest update to the digitisation of Darwin's fossil mammals project, researchers have uploaded the specimens of Megatherium collected by Darwin on the Voyage of the Beagle in the 1830s. The Megatherium fossils are two halves of the same skull, one held at the Royal College of Surgeons of England Museum 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. But what actually was Megatherium? 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.

Present day sloths are relatively small mammals. They are less than a metre in length, average around 5 kg in weight and spend most of their time suspended from branches in trees. They are currently restricted to South and Central America. 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. In contrast to the present day, most extinct sloths lived on the ground and some were many times larger than modern tree sloths.

Megatherium americanum was up to 10 times the size of living sloths reaching weights of up to four tonnes (similar to a present day bull elephant). On its hind legs, M. americanum would have stood a full 3.5 metres (12 feet) tall. Megatherium americanum is known from Argentina, Uruguay and Bolivia. Fossils of the animals have been found in deposits dating from the Middle Pleistocene (around 400,000 years ago) to the beginning of the Holocene (around 8,000 years ago). We know that they overlapped with humans in time as Megatherium fossils have been found with cut marks on them, suggesting that these giant sloths were on the menu thousands of years ago.

Despite its magnificent claws, M. americanum was a vegetarian. This has been confirmed through chemical analysis of its teeth which record what it ate during life.

It was able to stand and walk on its hind legs, making it the largest bipedal mammal of all time. Footprints attributed to M. americanum dating to around 14,000 years old have been found in Argentina, just 40 km from a site where the young Charles Darwin collected the remains of this animal between 1832 and 1833. These specimens collected by Darwin are historically and scientifically important, but also very fragile meaning that physical access to these specimens has had to be restricted to protect the specimens from further damage.