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Unearthed: Finding Fossil Sloths

This blog post is part of a series from Smithsonian Libraries highlighting Unearthed, a new collection of paleobiology literature in Biodiversity Heritage Library curated by Smithsonian Libraries in celebration of the opening of the National Museum of Natural History’s David H. Koch Hall of Fossils – Deep Time exhibit. Follow the series throughout 2019 as Smithsonian scientists and librarians discuss the collection, identify their favorite books within it, and explain the importance of paleobiology literature in research today. This post was written by Ryan Haupt, Research Fellow in the Department of Paleobiology at the National Museum of Natural History.  

I study sloths, and as popular as they have become on the internet, the thing most people do not realize is that we are living in a world majorly deprived of most types of sloth. Today there are only two types of sloths: two-fingered (genus Choloepus) and three-fingered (genus Bradypus). (You may have heard them referred to a two-toed and three-toed, but they both have three digits on their hind limb, so the difference is on the forelimb, in other words, their hand, hence: fingers.) There are only a few species of each kind of sloth and they all live in the trees of tropical rainforests in Central and South America. But back in prehistoric times there were many types of sloths: more than 80 genera across five different families. They lived as far north as the Yukon Territory and Alaska, as far south as Patagonia, and stretched from coast to coast in the United States and even colonized islands throughout the Caribbean. The smallest of these fossil sloths were still bigger than any sloth alive today, and the biggest extinct sloths were the size of modern elephants (but could probably walk around on two legs like an awkward bear). All this means there is a lot to learn about the sloths that are no longer with us.

Ryan Haupt and giant sloth fossil. Photo taken by Emily Thorpe in the collections of the Denver Museum of Nature and Science.

The earliest scientific description of a fossil sloth was written by then Vice President Thomas Jefferson, based on some bones found in cave in what was then Virginia, now West Virginia, by some men mining saltpeter (potassium nitrate), which crystalizes out of deposits of bat guano, and was an important economic resource as a major chemical component of gunpowder. The bones were given to the land owner, who knew that Thomas Jefferson had an interest in Natural History, so had them sent to Monticello. Jefferson presented his findings at the American Philosophical Society in Philadelphia in 1797, which was then published in writing in 1799. Jefferson interpreted the animal as something similar to a large lion, though in reality probably closer in size to a large bear, and named the animal Megalonyx, which means “great claw,” because, like living sloths, ground sloths also had impressive claws.

Jefferson’s sloth description in Transactions of the American Philosophical Society. v.4(1799). Available in the Biodiversity Heritage Library “Unearthed” collection.

In addition to thinking this animal was some great hunter, Jefferson did not like the idea that this animal was extinct (remember, this is still half a century before the publication of Darwin’s The Origin of Species). After working through some of the logic of his argument Jefferson concludes, “If this animal then has once existed, it is probable on this general view of the movements of nature that he still exists…” According to a possibly apocryphal tale, when Jefferson was preparing Lewis and Clark for their journey to the interior of the continent, he showed them the massive claws of Megalonyx and warned them to stay vigilant should the beast still be stalking the forests of the then still “untamed” parts of Western Virginia, Ohio, and beyond.

Even 100 years after Thomas Jefferson’s first description there were questions as to whether or not relatives of Megalonyx still existed in the far reaches of South America, owing in part to some amazingly-preserved mummified sloths that look “recent” compared to many fossils, with bits of skin and fur still clinging to desiccated bone. As far as we can tell, we sadly live in a world bereft of ground sloths, but in some ways, we are still trying to answer some of the earliest questions Jefferson and others had about their no longer observable lifestyles.

For example, there is a fossil site in South America that has several different types of ground sloths and seemingly no predators to eat them, leading to a question of how so many large herbivores avoided scouring the plant life bare. One paleontologist has proposed that the elephant-sized Megatherium, one of the largest ground sloths ever to evolve, may have scavenged or even hunted the smaller animals in their range, harkening back to Jefferson’s lion-esque Megalonyx. While this seems implausible, it is an idea and one that we can try to test, which is where my research comes in.

Plate depicting Megatherium americanum fossilized skeleton from Extinct monsters : a popular account of some of the larger forms of ancient animal life (1893) by H.N. Hutchinson. Available in the Biodiversity Heritage Library “Unearthed” collection.

One focus of my research is what ground sloths ate. Paleontologists use a lot of different techniques to determine diet in animals we cannot directly observe anymore, such as the type of teeth the animal had and the other types of fossils found in the same area (including fossil poop if we can find any!). We also use less obvious methods like the microscopic scratches seen on the chewing surfaces of teeth, or even the actual atoms in the fossils themselves (because those atoms ultimately come from the atoms of food, so you really are what you eat). Sloth teeth are softer than normal mammal teeth, and grow continuously throughout their entire lives, which makes them better suited to the persistent chewing of leafy browse or grasses than the tearing of flesh and crunching of bone. Two species of ground sloth have been found in caves associated with fossil feces (sometimes called coprolites) and when taken apart or analyzed chemically the feces only shows evidence of poorly digested plants. And when we look at the ratios of different isotopes trapped in bone, teeth, and even the poop, we see evidence of a mix of plants in the diet, but little evidence of animal protein being eaten. One avenue of my research is testing some of these fossil findings against modern sloths, to make sure that our interpretations of these data are correct. Two-fingered sloths are often given a little bit of dog food in captivity, whereas three-fingered sloths are not, so I have actually been able to chemically test the isotopes of a slightly omnivorous sloth against a pure herbivore to see how that compares to the fossils!

So, from these lines of evidence we can conclude that ground sloths were herbivores much like modern sloths, and while there is much more research to be done, it is incredibly helpful to be able to look back to the earliest days of paleontology through the Smithsonian Libraries’ collections and see how the thinking about sloths has changed drastically even as we still work to answer some of the earliest questions about these magnificent, lazy, internet-famous beasts.

Megatherium cuvieri from Catalogue of casts of fossils, from the principal museums of Europe and America (1866). Available in the Biodiversity Heritage Library “Unearthed” collection.

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