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The fossil record of ground sloths in North America is largely confined to the deposits of various kinds of natural cavities in limestone landscapes such as caves, sink holes and fissure fills. The bulk of American limestone solution features are found in Florida, northern California and in a belt from Missouri east through Kentucky and Tennessee to Maryland, with a scattering across the southern states from Arizona to Texas. Of these, caves are the most important because sometimes the cave environment has remained incredibly stable and provides excellent cool, dry and dark conditions for the preservation of ancient biological materials. From the abundance of the remains of certain kinds of sloths in caves, it seems that some of them must have used the caves as dens. Consequently, even soft tissues such as bits of skin, nails and tendons and dung have been preserved in caves.

The discovery of fossil sloths in the Americas dates back to the eighteenth century. One of the many interests of that great American polymathic hero Thomas Jefferson was the investigation of the life of the past. In 1799 he wrote:

It is well known that the substratum of the country beyond the Blue Ridge is limestone, abounding with large caverns ... In digging the floor of these caves ... in the county of Greenbriar, the labourers at the depth of two or three feet, came to some bones, the size and form of which bespoke an animal unknown to them.

Those West Virginian bones were later described as belonging to one of the sloth species that frequented caves, namely Megalonyx jeffersoni, which has the longest history of all the sloths in North America, surviving from 9 ma to late Pleistocene times around 14 ka. The Shashta ground sloth ([Nothrotheriops

Thomas Jefferson, 1743-1826, trained and practised as a barrister, joined the revolutionary party and drafted the Declaration of Independence (1776), sent to France with Franklin (1784), appointed secretary of state by Washington (1789), third President of America (1801) when the slave trade was prohibited, sponsored Lewis and Clark, retired 1809 and helped found the University of Virginia (1825).

shashti) encountered earlier is one of the commonest sloths to be fossilised, frequently occurring in cave deposits. Other sloths were adapted to a variety of environments, including arid deserts.

The first discovery of giant ground sloth bones occurred in eighteenth-century Paraguay. The bones from a complete skeleton were reassembled and an engraving of the rhinoceros-sized animal found its way via Madrid in Spain to Paris and 'landed' on the desk of Georges Cuvier, anatomist extraordinaire. From the engraving Cuvier was able to make a detailed analysis and comparison with the skeletal remains of the humbler and much smaller surviving tree sloth that still inhabits south Central and the northern part of South America. By using his skills as a comparative anatomist, Cuvier demonstrated similarities (correlation of parts) between the characteristic skull, jaws and clawed limbs of the fossil and extant sloths to show that they were closely related. But also in this pioneering use of the technique for the analysis of the remains of extinct creatures, he concluded that the rhino-sized fossil, which he named Megatherium (meaning 'huge beast'), must have had a radically different life style from the tree sloth.

In addition, Cuvier used the fossil ground sloth, along with the mammoth and mastodon, as the first good examples that extinction had taken place. He argued that the Earth's surviving menagerie of large, land-living animals were well enough known to assume that these grand beasts had vanished. Thomas Jefferson was not so sure. He had long been interested in fossils, especially those of the mammoth and mastodon. Although remains of these extinct elephant relatives are not preserved within the Canyon, the eighteenth-century discovery of their remains in North America was of considerable significance in the history of our knowledge of past life and its interpretation.

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