52). These two sloths had reached Florida 

 at least five million years ahead of this 

 pack, a finding now confirmed from fos- 

 sils at other rare sites in Oklahoma, New 

 Mexico, and California. I like to think of 

 the Withlacoochee sloths as the "heralds," 

 in contrast to the "legions," of animals that 

 later immigrated to North America. 



The megalonychid sloth I found in the 

 Withlacoochee River was an unusually 

 small species, but a later member of the 

 family was the ox-sized Megalonyx, 

 which pushed north and eventually 

 reached Alaska. The real giant of the sloth 

 tribe was the elephant-sized Eremothe- 

 riiim, whose remains are found most 

 abundantly at Daytona Beach, Florida, but 

 which has also been discovered north to 

 New Jersey. These animals' long, curved 

 claws were at first thought to be evidence 

 that they were lionlike carnivores. But in 



1853, Joseph Leidy, the father of verte- 

 brate paleontology in North America, real- 

 ized that both species had used their claws 

 to gather edible leaves, twigs, and 

 branches. This was reinforced by his 

 recognition that the extinct ground sloths 

 were related to the living tree sloths of 

 South America. Recent studies have 

 shown that modem three-toed tree sloths 

 are more closely related to Eremotherium, 

 and that living two-toed tree sloths share 

 their ancestry with Megalonyx. Through- 

 out its long, successful history, the sloth 

 family tree has produced both small arbo- 

 real and large terrestrial branches. 



Sloths, armadillos, and anteaters, along 

 with the extinct glyptodonts — armored 

 creatures superficially resembling tor- 

 toises more than other mammals — make 

 up the most peculiar and the most primi- 

 tive group of placental mammals, the 



edentates, also known as the xenarthrans. 

 The latter name, meaning "strange joint," 

 refers to their unusual backbones. In most 

 mammals, the paired overiapping surfaces 

 that prevent dislocation between vertebrae 

 are flat or faintly curved, but in these ani- 

 mals, the surfaces are scrolled into an elab- 

 orate set of interlocking ridges and val- 

 leys. In glyptodonts, as well as in modem 

 armadillos, such infrastructure supported 

 the heavy carapace above the hindquarters 

 (in full-grown glyptodonts, the shell 

 weighed up to 200 pounds). In sloths and 

 anteaters, the trait has no obvious utility, 

 but suggests that the animals are de- 

 scended from shell-bearing ancestors. A 

 shelled ancestry is also supported by the 

 presence of a sheet of small, overlapping 

 bony scales, a kind of chain mail, in the 

 skin of many mylodont and some mega- 

 theriid sloths. 



Edentates, the ordinal name of this curi- 

 ous assemblage of animals, is a misnomer, 

 implying that they lack teeth. However, 

 only anteaters, with their long, tubular 

 snouts and sticky tongues, are truly tooth- 

 less. The other groups of edentates have 

 teeth but lack enamel, distinguishing them 

 from other orders of mammals, in which 

 enamel-crowned teeth are a hallmark. The 

 exception that proves the rule is the oldest 

 armadillo jaw, which bears ten peglike 

 teeth, typical of many later, insect-grub- 

 bing armadillos, except that each tooth re- 

 tains a thin enamel coat on its sides. (The 

 oldest-known edentates are armadillos and 

 glyptodonts found near the Rio de Janeiro 

 airport, in a sinkhole filled with sediments 

 about sixty million years old.) 



If a scale-covered carapace were not 

 unmammalian enough, modem (and pre- 

 sumably extinct) edentates have less abil- 

 ity to thermoregulate than any other order 

 of warm-blooded vertebrates. In addition, 

 armadillos have a "dumbbell bone" near 



Herbivorous edentates 



reached giant proportions in 



their native South America; 



ground sloths, such as the 



t\venty-t\vo-foot-long 



Megatherium, browsed 



placidly fivm trees by rising 



to a tripod stance with their 



tails as buttresses. Tiieir 



fellow edentates, the 

 tanklike glyptodonts, had 

 200-pound carapaces that 

 were sixty feet in diameter 



Painling by Charles R. Knight; AMNH 



51 



