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The Great American Interchange 



More than twenty million years ago, 

 huge pieces of the earth's crust, moving to 

 the slow rhythms of continental drift, en- 

 croached upon the western margins of the 

 American continents, pushing up the 

 mountain ranges that still form the "back- 

 bone of the Americas" from Alaska to 

 Tierra del Fuego. These global forces were 

 also responsible for forging, two to three 

 million years ago, a land bridge in Panama 

 between North and South America. To 

 creatures that could not swim or fly, the 

 bridge opened continent-sized new realms 

 and unleashed hordes of competitors and 

 predators. 



In a movement known as the Great 

 American Interchange, land animals ex- 

 panded their ranges north and south in one 

 of the greatest-known minghngs of distinct 

 continental faunas in the earth's history. A 

 dozen land mammal families from South 

 America ranged northward through the 

 tropics into temperate North America. 

 Nearly half were edentates — mainly sloths, 

 but also armadillos, glyptodonts, and 

 anteaters. Other kinds of animals that made 

 the trek north included porcupines, the 

 giant aquatic capybaras, opossums, and the 

 now-extinct, rhino-sized plant eaters 

 known as toxodonts. 



North America's emigrants were even 



more varied. South America had previ- 

 ously hosted no carnivores. The indigenous 

 hoofed animals and rodents had been 

 nearly free of predation. During the Inter- 

 change, raccoons, weasels, dogs, bears and 

 cats, including sabertooths, entered the 

 continent. The hoofed contingent included 

 mastodonts, tapirs, horses, peccaries, lla- 

 mas, and deer. Rabbits and various rodent 

 families also seized new opportunities in 

 the vast lands south of the equator. Most of 

 the newcomers spread and diversified, 

 many traversing the tropics and following 

 the high Andean route before reaching 

 south temperate lands. 



The most successful of the northerners 

 by any measure were the cricetid rodents, 

 or New World mice. Within two million 

 years, they produced some fifty new gen- 

 era, bursting into arboreal and terrestrial 

 settings, sylvan and pastoral habitats, low- 

 lands and uplands, and even producing one 

 offshoot that specializes in fishing in An- 

 dean streams. 



Fully half of the land mammal genera 

 that now live in South America came by 

 way of the Panama land bridge during the 

 Interchange. In contrast, only three genera 

 from South America still survive in temper- 

 ate North America — the porcupine, opos- 

 sum, and armadillo. — S. D. W. 



Thefonnation of the Panama land bridge opened the way for two-way traffic 



between the American continents. In this scene of Florida some two and a half 



million years ago, a sloth known as Glossotherium; an armadillo; a large, 



flightless ground bird; and aquatic capybaras — all immigrants — share a 



cypress swamp with native North American beavers. 



Painting by Eiy Kisll 



the tip of their nose. Useful in burrowing 

 for food and shelter, this extra bone, called 

 a prevomer, is retained from the ancient 

 mammallike reptiles. These traits, as well 

 as molecular comparisons, indicate that 

 the edentates were the first branch from 

 the base of the placental mammal tree. 



Sloths arose from armadillo stock, but 

 starting more than thirty million years ago, 

 they made an extraordinary switch in 

 adaptive strategy, becoming plant-eating 

 giants. They played as important a role as 

 the native South American ungulates, ri- 

 valing these other large herbivores, such 

 as the now-extinct toxodonts, in abun- 

 dance and diversity. Their success as her- 



bivores is quite remarkable when one con- 

 siders their descent from short-legged, 

 armor-encased burrowers, with shallow, 

 feebly muscled jaws and peglike teeth 

 with no enamel. How could sloths even 

 begin to compete with fleet ungulates 

 whose deep jaws and elaborately enam- 

 eled teeth were already well adapted to 

 processing coarse vegetation? 



Perhaps part of the explanation for the 

 improbable success of herbivorous eden- 

 tates was that South America had no effi- 

 cient carnivores to take advantage of the 

 sloths' lack of speed. Evolution does not 

 produce perfection in all departments; 

 rather, like politics, it is the art of the pos- 



sible. South America, with its great tropi- 

 cal girth, offered vast opportunities for 

 beasts that could feed readily and live well 

 on low-grade fodder. With no need for 

 speed, sloths had the advantage of low me- 

 taboMsm, and they easily converted their 

 powerful digging claws and feet into leaf- 

 and branch-stripping devices. Their hind 

 feet became twisted, so that the claws 

 faced inward, while the outer side faced 

 the ground. This allowed the smaller sloths 

 to climb trees and the bigger ones to clear 

 their claws from the ground. With the aid 

 of a powerful tail, they rose up on a solid 

 tripod base to feed from trees. A set of fos- 

 silized sloth tracks found in the prison 



52 Natural History 4/94 



