Paleo Profiles: Megatherium

A Megatherium, from the London Natural History Museum

Visitors to the Natural History Museum in London in the second half of the nineteenth century gasped in awe upon seeing a giant skeleton. Visitors today may do so at the animatronic Tyrannosaurus, the long-necked sauropod Camarasaurus, or perhaps Dippy, the Diplodocus who until recent years dominated the Entrance Hall. However, in the 1800s the guests were not gasping at a dinosaur as they would today - instead they were in awe of a sloth. The Megatherium was one of the largest mammals to ever walk on land; only the largest of mammoths, elephants, and the great Paraceratherium exceeded it in size. 

Discovery and Fossils

Megatherium has been known to science for a long time - long before science officially described dinosaurs. In 1788 on the banks of the Lujan River in Argentina Manuel Torres came across several large bones which were then shipped to Madrid where they were stored in the Museo National de Ciencias Naturales. This coincided with the European Enlightenments which have been seen as one of the intellectual movements which gave rise to present-day science. As a result, anatomists and early biologists were eager to examine newly found bones, and live specimens. Eventually, an anatomist named Georges Cuvier - a French anatomist and biologist who would be considered to be a 'founding father' of palaeontology. Looking at the bones he described them as resembling that of a sloth - originally he even stated that, like sloths, it lived in the trees - except that these bones were from an animal the size of an elephant. His paper in 1796 named this extinct animal Megatherium americanum, 'Giant best from America'. Cuvier continued studying Megatherium eventually changing his mind on the sloth's biology; he argued it really lived a subterranean life using its big arms and claws to dig tunnels. He was half correct; Megatherium could dig but it did not lead a subterranean life.

Georges Cuvier

We have many examples of Megatherium fossils from across South America. It was an incredibly diverse animal having eight different species which occupied all of the continent from Patagonia in the south to Colombia in the north. Close cousins of the Megatherium also dominated the southern US and Central America as well. Normally, palaeontologists have to deal with a few remains and, therefore, have to reconstruct animals from better preserved relatives or logical guesses. The number of complete skeletons we have of Megatherium means that we do not have that issue in this case.  On his voyage with the Beagle Charles Darwin even brought some skeletons back with him in the 1830s. Other genus of ground sloths have even given us fossilised hair and droppings which paints a better picture of what the Megatherium was like in life. 

Biology

The size of the Megatherium from Prehistoric-Wildlife.com

The ground sloths share a basic body shape with their relatives in the trees, except that their bodies have adapted for life on the land instead. The hind legs are short with the lower bones being as thick as the femur; they had short but thick tails; robust bones; a broad pelvis; and a low body - this allowed the Megatherium to eat from the trees while remaining firmly on the ground. A stable lower-body allowed the sloth to raise itself onto two feet to browse from the trees. Its upper body was surprisingly flexible to reach branches; prehensile lips (with a possible prehensile tongue) to choose the best foliage; and strong, muscular jaws to effectively cut through the vegetation. As this animal was feeding it would rear up, using its tail as a tripod to steady itself, and then use its arms and lips to physically feed. The Megatherium's hands were also decked out in five curved claws which could be up to 70-cm (around 2.4-foot) in length. These could be used to tear leaves from the trees or bend branches closer to its mouth - or alternatively be used as a weapon. Megatherium would have walked on all-fours, but from footprints we know that it could spend considerable amounts of time in a bipedal stance. Even when on all-fours it had a unique way of walking; to protect its claws from unnecessary wear it would walk on the sides of its feet. Pictured below is a relative of the sloths - the giant anteater. Like the Megatherium they are heavily reliant on their claws so when they walk in a quadrapedal stance they do so on their knuckles.

A giant anteater walking on its knuckles, from Arkive by Joe Kolowski, Accessed 05/01/2019

Megatherium was in the Order Pilosa which included other giant ground sloths, and today is comprised of modern sloths, anteater, and tamanduas. Expanding it out slightly they also belong to the Superorder Xenarthra which also includes armadillos - the giant armadillo even walks like a ground sloth. The Megatherium differs thanks to its size - the largest were 6 metres long (20 foot) and weighed up to 4 tonnes. Modern elephants are of an equivalent size. A lot of this came from the animal's bones and muscles - it was big to make itself stable. As a result, adult Megatherium had little to fear in regards to predators. However, due to its size, weight being at the lower half of the body, and walking style this made Megatherium a slow animal.

When and Where

An example of the Great Interchange, Wikipedia - the blue originated in the North, the olive the South

One of the reasons why we have so many Megatherium fossils is due to it living quite recently. Megatherium first appeared in the early Pliocene, sometime between 5 and 3 million years ago, or when humanity's first ancestors started becoming bipedal. Megatherium's ancestors evolved during a time of great change. Moving continents lead to the creation of the Isthmus of Panama bridging the gap between North and South America. This severed the link between the Atlantic and warmer waters leading the Atlantic, and also the Arctic and Antarctic, to become much cooler. Two continents now being connected allowed animals to migrate: deer, camelids, and big cats were just some who moved south as sloths, giant terror birds, and hummingbirds were just some who moved north. Hence, we have sloths closely related to Megatherium living in Central and North America. The Eromotherium was so similar to Megatherium that it was once, and sometimes still is, classed as Megatherium. As giant ground sloths existed in woodlands and grasslands this have them a wide area to call home - we have found Megatherium for that reason in nine countries, the most recent being found in the Peruvian Andes. Existing throughout the Ice Age it managed to last until the last few thousand years. We can even carbon date the youngest Megatherium fossils, and the giant sloths went extinct around 8,500 BCE.

Diet

Like existing sloths Megatherium was adapted to eating plants. Their jaws were adapted to slice through thick foliage allowing them to browse from the trees as well as through various grasses and agaves found in America's grasslands. Most of this is not speculation either. Fossilised dung from Megatherium and other ground sloths have shown us their diets - inside the fossilised droppings we find a wide variety of plants, in one up to over 70 different types. However, Richard Fariña and Ernesto Blanco have put forward a controversial theory: Megatherium could be an omnivore. They argued that short triceps meant that Megatherium could move its arms quickly making its claws an effective weapon. They did stress that Megatherium was not a hunter - instead, using its immense size, it would scare predators from their kills to supplement their diets. The BBC documentary Walking with Beasts portrayed it as doing exactly that. This theory has been criticised in recent years. Palaeontologists have pointed out that we likely would have found claw marks from sloths or evidence in dung if meat was such a large part of their diet to be classed as an omnivore. Zoologists have found deer eating baby birds in the wild and hippos even cannibalising one another so it is not too far-fetched for Megatherium to occasionally scavenge. Undoubtedly, the Megatherium would almost entirely be herbivorous.

Habitat

A Megatherium in its habitat in Walking with Beasts

Megatherium lived in the grasslands and woodlands across South America - imagine llaneros herding their cattle today and coming across a giant ground sloth. Their bodies being adapted to an occasional bipedal lifestyle would indicate that woodlands, or the outskirts of woodlands, would be the ideal habitat for a Megatherium. They would live alongside a wide variety of fauna including the lama like Macrauchenia, a giant armadillo (the size of a small car) with a clubbed tail called Doedicurus, the terror birds (two metre tall predator birds), and the Smilodon, better known as the sabre-toothed cat. A fully grown Megatherium would have no fear, other than other sloths, but younger ones would have potential predators. Smaller ground sloths, although still fairly large, in North America have been found to have dug holes. This could be a potential way to clean themselves - dirt baths are particularly effective way to clear fur from ticks - but also a potential place to hide from a predator. They likely had one predator: humans.

Extinction

Megatherium is one of the first animals lost in the Sixth Mass Extinction - the mass extinction caused by humanity. When humanity expanded into Europe, Australasia, and the Americas most of the megafauna went extinct - including the giant sloths. We even have fossil evidence to show that humans did hunt smaller cousins to the Megatherium - cuts and breaks on sloth bones show evidence that humans had used their bodies for resources. It is quite likely that Megatherium could have met the same fate. One theory has suggested that humans accidentally caused a mass extinction as fauna went extinct thanks to disease introduced by domesticated animals. However, humans were not the only reason why the sloth went extinct. From around 12,000 years ago the climate started rapidly changing - which has been suggested why humanity adopted farming - which drastically affected habitats worldwide. Climate change had restricted Megatherium's habitat which started driving the sloth into extinction - human hunters ensured that it did go extinct. Megatherium was joined in extinction by the rest of the ground sloths as well as much of the megafauna of the Americas.

Thank you for reading. The sources I have used are as follows:

-'Megatherium', Prehistoric-Wildlife.com, (Accessed 04.01/2019)

-Jeremy Green and Daniela Kalthoff, 'Xenarthran Dental Microstructure and Dental Microwear Analyses, with New Data for Megatherium americanum (Megatheriidae)', Journal of Mammology, 96:4, (2015), 645-658

-Gideon Mantell, 'The Megatherium', Scientific American, 37:7, (1852), 291

-'Sabre Tooth',Walking with Beasts, (2001), BBC, 13 December

-M. Susana Bargo, 'The ground sloth Megatherium americanum: Skull shape, bite forces, and diet',  Acta Palaeontologica Polonica, 46:2, (2001), 173-192

-Francois Pujos, 'Megatherium celendinense sp. nov. from the Pleistocene of the Peruvian Andes and the phylogenetic relationships of Megatheriines', Palaeontology, 49:2, (2006), 285-306

-Pip Brewer, 'What was Megatherium?', Natural History Museum, (22/11/2018), Accessed 04/01/2019
-Tim Haines and Paul Chambers, The Complete Guide to Prehistoric Life, (London: BBC Books, 2005)

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