yard of the Nevada State Penitentiary in 

 Carson City shows just how sloths ambu- 

 lated on huge, splayed-out feet. 



By making just a few modifications in 

 their unimpressive teeth, sloths were able 

 to chew vast quantities of leaves. (Glyp- 

 todonts, too, with some orthodontia, be- 

 came efficient large herbivores.) Despite 

 their lack of enamel, sloths developed tall- 

 crowned, elaborately folded teeth with 

 tracts of a hard substance called vitroden- 

 tine to supplement the soft dentine. Al- 

 though sloth teeth wore down faster than 

 enameled teeth, they compensated by 

 growing continuously throughout the ani- 

 mal's life. The jaws deepened, and the 



cheek bones expanded to support a com- 

 plex set of chewing muscles. In short, 

 sloths cobbled together the necessary bio- 

 logical machinery to overcome the inade- 

 quacies of their armadilloid heritage. 



All of the ground sloths, and the 

 glyptodonts as well, slid into extinction 

 just over 10,000 years ago in North and 

 South America. The climate had changed, 

 and bands of human hunters had swept 

 across the Bering Strait and throughout the 

 New World. At about the same time, most 

 of the hemisphere's other large herbivores 

 disappeared. When the first Megalonyx 

 was unearthed at Big Bone Lick on the 

 Kentucky frontier some 1 80 years ago, its 



dense, iron-stained bones were brought to 

 the White House, where President Jeffer- 

 son, an avid amateur paleontologist, stud- 

 ied them. When Jefferson commissioned 

 Lewis and Clark to explore the western 

 territories, he also asked them to make a 

 careful search for living Megalonyx. If not 

 for the deadly combination of climate 

 change and overhunting of the creatures 

 by Paleo-Indians a few thousand years 

 earlier, Lewis and Clark might have been 

 successful. Instead, sloths are now discov- 

 ered and studied mainly by paleontolo- 

 gists. Today only two kinds of tree sloths 

 exist, diminutive tropical sur\'ivors of their 

 big, far-ranging, extinct brothers. 



53 



