Ground sloths have sparked curiosity and wonderment from their initial discovery in 1788. Ground sloths weren’t the first fossil animals to be discovered–humans had speculated for millennia about the origin of the strange bones they occasionally stumbled across.  However, a ground sloth was the first fossil animal to come to the attention of museum experts who in a series of fortunate circumstances recognized the bones as special; arranged to collect and preserve the nearly complete skeleton; had the skill to assemble the bones into a mount, and; could correctly identify the animal as an new and extinct species.

The first fossil skeleton of an extinct sloth was collected in 1788 near Buenos Aires, Argentina by Manuel Torres and mounted by museum preparator Juan Bautista Brú.  It resides today in the Royal Museum of Natural History, Madrid, Spain, preserved as mounted, complete with flaws.  It was given the name Megatherium (“Giant Beast”) by Georges Cuvier who recognized the animal as an extinct species of sloth.

Scientists have speculated for more than a century that the animal walked bipedally.  This was confirmed in 1986 when a fossil trackway was uncovered in Pehuén-Có, Argentina. Analysis confirmed Megatherium could walk on two legs when it wanted, and did so routinely except over rough terrain where 4-wheel drive was advantageous.

The first fossils of Megalonyx jeffersonii, Jefferson’s sloth, were discovered in 1796 by saltpeter miners excavating in a cave in Greenbrier county, Virginia (now West Virginia).  Saltpeter, or potassium nitrate, was an important chemical needed in the manufacture of gunpowder, a vital commodity in a young country that had only recently won a war of independence from Britain and was still facing serious British hostility.   The giant bones were hardly a priority, but interesting enough for several of the miners to take home as souvenirs.  An upper leg bone was saved to prop up the wobbly kettle used to process the mineral.   Eventually, word of the unusual bones reached a local resident, Colonel John Stuart, who was aware of Thomas Jefferson’s interest in scientific matters and large animals especially.  So Stuart rounded up as many of the bones as he could track down and dispatched them to Jefferson.

Unaware of the South American discovery, and lacking a scientist’s formal training in anatomy, Jefferson identified the animal as a lion-like creature at least 3X bigger than an African lion and dubbed it Megalonyx (“Giant Claw”).  Jefferson’s mistake, soon realized and corrected, continues to prompt amusement but the nimbleness of intellect that allowed Jefferson to see the bones as an coherent individual of an entirely new species unlike anything else known to science is astonishing in a time when most scientists rejected the concept of extinction. One can not doubt but that if Colonel Stuart had retrieved the skull or simply one tooth for Jefferson, or had he been able to track down the wayward leg bone that Jefferson would have avoided his error, been spared the embarrassment and would be acclaimed today as America’s first great paleontologist.

Rare Skeletons

Paleontologists can’t explain fully why fossils of Megalonyx are so rare.  Ground sloths were notably successful and widespread for millions of years, and at home in a great variety of habitats.  Surely they were a common sight on the Ice Age landscape.  One might expect their fossils to be as common as mammoth’s, which are discovered somewhere almost weekly.  But mammoths, like their close cousins the modern elephants, spent a lot of time near water, and frequently died there, where their bones could be covered up easily and preserved.  Sloths apparently spent more time away from water–perhaps they obtained enough from their diets or possessed superior water conservation adaptations.  In any case, when they died out in the open their bones were more likely to be attacked by scavengers and broken down by weathering (e.g. sun, rain, freezing, etc.).

Only five (5) known Megalonyx skeletons existed before the discovery of the Tarkio Valley sloths.  They are all woefully incomplete.  An Idaho specimen, complete from the waist up, is the best representative with only 113 bones.   If you assume sloths had about the same number of bones as humans (206), you can see even the most complete Megalonyx is barely 1/2 complete, and most of the others far less.  In comparison, T-rex skeletons are at least 3X more common and on average those are far more complete.