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Fossils-Giant Sloths
by Michael Guest
Historical Geology-GLG 102
Spring 2005                
 

GIANT SLOTHS OF NORTH AMERICA
 

Two families of Giant Ground Sloth’s once lived in North America; the Megalonychidae (giant claw) at about 2.5 meters, and the Megatheriidae with the latter being subdivided into the larger Megatherium and Eremotherium, both about 6 meters long, and the smaller Nonrotheres at about 1.2 meters.  The earliest known North American megalonychid, Pliometanastes protistus lived in Florida about 8 million years ago.   Ground sloths were large relatives of the modern two-toed sloths (Choloepus spp.) and three-toed sloths (Bradypus spp.) but unlike these tree dwelling animals spent all of their time on the ground.

The four species of herbivorous ground sloths which inhabited the United States at the end of the last Ice Age were Megalonyx jeffersonii, Glossotherium harlani, Eremotherium laurillardi, and Nothrotheriops shastensis.













 

Jefferson’s ground sloth.

Jefferson's ground sloth (Megalonyx jeffersonii).  Thomas Jefferson the third president of the United States was a paleontologist.  He gave the name Megalonyx to giant ground sloths during his 1797 presentation to the American Philosophical Society. He advised Lewis and Clark to be on the look out for evidence of the animal on their expedition in search for the Northwest Passage.  The sloth species was formally named in his honor in 1822 by Anselme Demarest, a French anatomist.  This is the largest known species of Megalonyx, and when fully grown was about the size of an ox (2.5 to 3.0 m long). The more southerly specimens were somewhat smaller than those found in the north.   The skull was short, broad and deep with a blunt muzzle. The jaw contained five upper teeth on each side and four lower teeth on each side. The peg-like teeth had a space separating the front fangs from the wider cheek teeth.  Unlike other ground sloths it had plantigrade hind feet, whereby its weight was borne on the sole similar to humans.  Also the three central claws of the hind foot touched the ground, which would have enabled it to move more easily than other ground sloths. Its strong, broad tail would have acted as a stabilizer when it stretched upwards after food.
 

Jefferson's ground sloth first appeared in North America, from as far south as Patagonia, during the second last (Illinoian) glaciations (probably more than 150,000 years ago); its remains have been reported from about 80 localities in America including the eastern two-thirds of the United States and from central Mexico to California to Washington and inland Idaho. It is the only ground sloth found as far north as the Yukon Territory and Alaska. In more southerly regions of North America, Jefferson's ground sloth lived in woodlands, and is believed to have browsed on leaves, twigs.  It became extinct about 9,400 years ago.

Harlan's ground sloth (Glossotherium harlani). Was quite similar to Jefferson's and about the size of an oxen, it is one of the two ground sloths found trapped in the La Brea Tar Pits, California. An interesting feature of the Harlan's ground sloth were the small bones deep under the skin around the neck, shoulders and back which may have served as armor against attacking predators.

Laurillard's ground sloth (Eremotherium laurillardi).  Megatherium (great beast) was named by Richard Owen, anatomist, in 1856; although the first Megatherium fossil was found in Brazil in 1789.  This was the earliest of the giant ground sloths known as megatheres to have migrated from South America north across the Panamanian Land Bridge.  It was the largest of the American giant sloths, weighing between three and five tons, and could rear up high into trees after leaves and twigs.  While all other giant sloths had four fingers with only two or three claws, this one had five fingers, four of them with large claws, the biggest being nearly a foot long; it became extinct about 12,500 years ago.  Skeletons may be seen in two Florida museums, North Carolina State Museum of Natural Sciences, Raleigh, NC, the National Museum of Natural History in Washington DC, and the British Museum of Natural History, London.

                                       Excavated in 1975 from the Pleistocene fossil site

       called the Daytona Bone Bed.

Shasta ground sloth (Nothrotheriops shastensis).  Was a slighter bear-sized ground sloth which reached a length of about 1.2 meters.  It became extinct around 9,000 years ago.  Considerable remains of this species including a skull, backbone, hair, and large claws were uncovered in 1930-31 at the Gypsum Cave located in Clark County, Nevada. Other remains were found in the La Brea Tar Pits, California.

DNA studies of caramelized feces, “coprolites” about 20,000 years old, found at the Gypsum Cave site in Nevada by Max Planck researcher Svante Pääbo have shown that the giant ground sloths lived on plants from at least eight families such as  capers, mustards, lilies, grasses and grapes. At the time of their existence the nearest water source where grapes might have grown was six miles or so away. A fair distance for an animal believed to have been capable of moving just one to two miles an hour, even though its footprints were three feet long.

The cause of demise of the giant ground sloths is still open for debate.  There are three likely causes – extinction by hunting, by human borne disease, or a severe change in climate, or any combination of these.  Extinction by hunting was extensively researched by paleontologist John Alroy of the University of California at Santa Barbara, who with the aid of his computer program “proved” they could have been killed off in as little as one thousand years.  Not everyone is satisfied with this explanation, not the least objection being the widespread nature of the sloth’s distribution, and the lack of any evidence of human evidence, (stone points for instance, such as have been found at Mammoth kill sites) with their remains.  Ross D. E. MacPhee of the American Museum of Natural History in New York City, agrees with the idea that humans may well have caused the animals extinction, but indirectly through the introduction of hyper lethal disease. This, possibly via their dogs or vermin, from whose diseases the animals could not have been immune.  Again no evidence has been found thus far to support this theory.  Paleontologist Russell W. Graham of the Denver Museum of Nature and Science blames the loss on the weather.  He  states the Pleistocene epoch witnessed considerable climatic instability causing some habitats to disappear and shrinking their geographical ranges, a death sentence for animals which need large ranges.  The sloths and other large animals maintained viable populations through most of the Pleistocene, but the Younger Dryas event (a major abrupt century-scale warm-cold oscillation that took place in the circum-North Atlantic region during the last glacial period) pushed them over the edge, Graham says.

 

Matt Smith (left), an artisan with the Natural History Museum of Livingston, Mont., and S. David Webb, a curator with the Florida Museum of Natural History at the University of Florida, examine the claws and arm bones of a Giant Ground Sloth.

 

Web sites researched for this paper

http://www.ucmp.berkeley.edu/museum/pbios/abstracts_29to40.html
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http://www.capefearmuseum.com/groundsloth.htm

http://www.crystalinks.com/sloth.html

http://www.moas.org/permanent.html

http://www.mnsu.edu/emuseum/archaeology/sites/northamerica/gypsum_cave.html

http://state.nd.us/ndfossils/Education/animals/sloth%20030612.html

http://www.customerservicecareers.com/wiki4/g/Ground_sloth.asp

http://en.wikipedia.org/wiki/Ground_Sloth

http://www.mpg.de/english/illustrationsDocumentation/documentation/pressReleases/1998/news20_98.htm

http://www.karencarr.com/News/sloth/mammoth_kill.html

http://www.agu.org/revgeophys/overpe00/node7.html

http://www.napa.ufl.edu/2000news/slothph.htm  (photo)

http://www.museum.state.il.us/exhibits/larson/sloth.html (photo)

http://dest.travelocity.com/DestGuides/multimedia_photogallery/0,3161,AOLCOM%7C4074%7cus%7c41980,00.html
(not working currently)