For over a century and a half dinosaurs have been the unofficial symbols and ambassadors of paleontology, but this was not always so. It was fossil mammals, not dinosaurs, which enthralled the public during the turn of the 19th century, and arguably the most famous was the enormous ground sloth Megatherium. It was more than just a natural curiosity. The bones of the “great beast” represented a world which flourished and disappeared in the not-so-distant past, but, as illustrated by Christine Argot in a review of its history, illustrations of what Megatherium looked like have been in flux since the time of its discovery.
No doubt humans have been finding the remains of giant ground sloths for quite some time, but the story of Megatherium as we know it began in 1788. It was in that year that Manuel Torres discovered the nearly complete skeleton of an immense, strange animal on the banks of the river Luján about 65 kilometers west of Buenos Aires in northern Argentina. The following year it was shipped to the Cabinet of Natural History in Madrid, Spain where it was assembled and illustrated by Juan Bautista Bru, and it was on the basis of these illustrations that the French anatomist Georges Cuvier was able to determine what this spectacular animal was. He gave it the name Megatherium americanum, and appraised it as an extinct, peculiar creature most similar of the small arboreal sloths which still clung the branches of South American rainforests. As Cuvier did not accept the idea of evolution there could be no genetic connection between the ancient monster and the living sloths, but the similarity between the two could not be denied.
Yet Cuvier’s work on Megatherium was largely descriptive – it would be up to other naturalists to reconstruct its lifestyle and habits. One of the first to do so was another French paleontologist, François Jules Pictet de la Rive (often simply referred to as Pictet), who considered several hypotheses about Megatherium in the 1850′s. The first was that the sloth living underground, burrowing through the soil like an enormous mole. It certainly had the claws for digging, but this idea was too absurd, especially considering how populations of these creatures might undermine the landscape. (Though, in his 1836 contribution to the Bridgewater Treatises [which is not mentioned by Argot], the English geologist William Buckland envisioned Megatherium as an animal which dug into the ground for roots.) An alternative was that, like living sloths, Megatherium lived in the trees, but again its large size seemed to preclude this possibility; what sort of tree could hold such a large animal?
The only viable hypothesis was that Megatherium was a terrestrial animal which used its large claws to tear off branches or even uproot trees, perhaps sitting back on its tail (a part of the skeleton which had been unknown to Cuvier) to munch on leaves and treelimbs. Indeed, in his own analysis of the mammal made around the same time as Pictet’s, the English anatomist Richard Owen proposed that Megatherium could stand up on its hind limbs in some instances and was not wholly constrained to a quadrupedal stance.
The image of a shuffling Megatherium which could rear up to feed was fleshed out by the artist Benjamin Waterhouse Hawkins. His restoration depicts two rotund individuals with slightly elongated snouts, one Megatherium on all fours and another reaching up to the branches of a tree, giving life to the hypotheses which Pictet and Owen had developed. H.N. Hutchinson also presented a colorful portrayal of Megatherium in his popular work Extinct Monsters in which he described the attack of a short-trunked, long-tongued Megatherium on a tree:
…it is not difficult to form a mental picture of the great beast laying siege to a tree, and to conceive the massive frame of the Megatherium convulsed with the mighty wrestling, every vibrating fibre reacting upon its bony attachment with the force of a hundred giants; extraordinary must be the strength and proportions of the tree if, when rocked to and fro, to right and left, in such an embrace, it can long withstand the efforts of its assailant. It yields, the roots fly up, the earth is scattered wide upon the surrounding foliage, and the tree comes down with a thundering crash, cracking and snapping the great boughs like glass. Then the coveted food is within rea,ch, and the giant reaps the reward of his Herculean labours.
The skeletal outline of such an event was impressively put on display at the Museum of natural history of Paris in the late 1860′s under the direction of paleontologist Albert Gaudry. He thought that the image of a Megatherium rearing up against a tree-trunk would make the greatest impression upon visitors, and this pose has since been adopted for other Megatherium and giant sloth mounts in museums elsewhere.
As Argot notes, however, there is still much to learn about the appearance and habits of Megatherium despite the length of time it has been known to scientists. While recent studies of its skull have confirmed that it had a long tongue but probably did not have a trunk, the way in which the ponderous animal moved is still somewhat difficult to determine. That it could walk on all fours or rear up on two legs is obvious, but trackways found in South America hint that it may have walked bipedally, too, and one attempt to test its ability to walk on two legs found nothing preventing it from doing so. Over two centuries since the discovery of the skeleton figured by Bru, we are still figuring out how to restore the magnificent Megatherium.
Christine Argot (2008). Changing Views in Paleontology:
The Story of a Giant (Megatherium, Xenarthra) Mammalian Evolutionary Morphology A Tribute to Frederick S. Szalay DOI: 10.1007/978-1-4020-6997-0_3