Mr Darwin’s lost sloth

A delightfully grouchy looking ground sloth, Mylodon darwinii. (Art by Tabitha Paterson)

Ground sloths are weird. The two-toed and three-toed varieties of memetic fame that we are left with only hint at the absurdity of different genera such as Eremotherium, Megalonyx, and Nothrotheriops: bear-sized to elephant-sized behemoths, covered in shaggy fur, and sporting enormous curved claws.

The great diversity of Pleistocene sloths shuffled around (yes, they walked on the outside of their pedes, as if club-footed), a wide variety of habitats from frigid Alaska to tropical Florida to bleak Patagonia, and even the Caribbean islands. The species Mylodon darwinii was probably about the size of a giant panda and lived along the western coast of South America, even down into Patagonia. You may have spotted something familiar about the latin name of the species. This sloth was named after a certain Mr Charles Darwin.

Darwin’s Beagle voyage is well known for sparking his ideas on how evolution worked. His keen observations of the finches (and mockingbirds and giant tortoises) on the different Galapagos islands struck a chord. The voyage is not so well known for his discovery of several new species of Pleistocene mammals.

Hopping down the South American coast, and occasionally riding inland on horse, Darwin collected a lot of fossils. Back then (in the 1830s) there were very few big mammals known and with some specimens being only teeth or bones, Darwin had little to compare them to. Boxes of bones were sent back to England and the famous anatomist Richard Owen identified a number of enormous mammals, from the giant armadillo-like Glyptodon clavipes to the wonderfully weird  Macrauchenia patachonica.

These extinct giants hinted to Darwin at some kind of relationship between living and extinct forms. He writes at length about it in his Voyage of the Beagle, written 20 years before On the Origin of Species. Here he describes how living animals are always found in the same region as their extinct relatives. Discovering so many incredible Twilight Beasts was pivotal in developing the young Darwin’s thinking about how and why evolution happened.

Amongst the 15 or so new mammals identified by Richard Owen in the crates of fossils was the fairly large sloth, Mylodon darwinii. This large beast was described on the basis of a jawbone and named in honour of its discoverer.

Mylodon is one of the better known extinct sloths, thanks to copious material found preserved in the cold, dry cave systems of Chile and Argentina. In fact, one site, imaginatively named Cueva del Milodon (Mylodon Cave), has produced an abundance not just of skeletal material, but dung, nail, hair, and skin. When the site was first discovered in the 1890s, the remarkable preservation of a complete Mylodon skin (clearly removed from an animal by ancient human hands) encouraged many people to think that ground sloths still survived in the remoter parts of South America. This optimism has lasted for over a century, with the occasional expedition still being mounted to search for giant sloths in the jungles of the Matto Grosso. It’s a wild goose chase for sure.

The Cueva del Milodon material has been studied intensely, and radiocarbon dated to the late Pleistocene (>10,000 years old). The cold, dry conditions of the cave have preserved the soft tissue due to a natural freeze-drying process. The bones from this site are so well-preserved that ancient DNA has been recovered from a number of the species found there.

Written by Ross Barnett (@DeepFriedDNA) and Jan Freedman (@janfreedman)

Art work by Tabitha Paterson (@TabithaPaterson)

Further reading:

Akersten, W. A. and McDonald, H. G. (1991), ‘Nothrotheriops from the Pleistocene of Oklahoma and paleogeography of the genus’, The Southwestern Naturalist, 36 (2), 178-85. [Full article]

Cartelle, C. and De Iuliis, G. (1995), ‘Eremotherium laurillardi: The Panamerican late Pleistocene Megatheriid sloth’, Journal of Vertebrate Paleontology, 15 (4), 830-41. [Full article]

Fernicola, J. C., Vizcaino, F, and de Iuliis, G. (2009), ‘The Fossil Mammals collected by Charles Darwin in South America during his travels on board the HMS Beagle’, Revista de la Asociatión Geológica Argentina. 64 (1), 147-59. [Full article]

Höss, M. , et al. (1996), ‘Molecular Phylogeny of the extinct ground sloth Mylodon darwinii‘, Proceedings of the National Academy of Sciences of the U.S.A., 93, 181-85. [Full article]

Lönnberg, E. (1900), ‘On a remarkable Piece of Skin from Cueva Eberhardt, Last Hope Inlet, Patagonia’, Proceedings of the Zoological Society of London, 379-84.

McDonald, H. G., Harington, C. R., and G., De Iuliis. (2000), ‘The ground sloth Megalonyx from Pleistocene deposits of the Old Crow Basin, Yukon, Canada’, Arctic, 53 (3), 213-20. [Full article]

Owen, R. (1840), ‘Part I. Fossil Mammalia’, in C. R. Darwin (ed.), The zoology of the voyage of H.M.S. Beagle, under the command of captain Fitzroy, R. N., during the years 1832 to 1836 (London: Smith, Elder and Co.). [Full text]

Steadman, D. W. (2005), ‘Asynchronous extinction of late Quaternary sloths on continents and islands’, Proceedings of the National Academy of Sciences of the U.S.A., 102 (33), 11763-68. [Full article]

Vizcaino, S. F., Farina, R. A., and Fernicola, J. C., (2009), Young Darwin and the ecology and extinction of Pleistocene South American fossil mammals’, Revista de la Asociatión Geológica Argentina. 64 (1), 160-69. [Full article]