44). Ground sloths, now all extinct, were robust, large
terrestrial sloths with long claws (plate 8- 45). They were
probably largely herbivores, feeding on foliage, though
some have suggested they may have been partially meat
consumers. As you might guess, ground sloths were
closely related to the much smaller tree sloths of the
Neotropics, a point not lost on Charles Darwin when
he first examined fossil ground sloth bones during the
voyage of the Beagle. Capybaras belong to the same rodent
subgroup (Caviomorpha) as chinchillas and guinea
pigs. One of the two extant capybaras, Hydrochoeris
hydrochaeris, abundant in parts of South America, is the
world’s largest rodent (chapter 12). Phorusrhacoid birds,
now all extinct, have earned the nickname “terror birds.”
Some stood almost as tall as a human being. They were
flightless but could move quickly, running on long legs.
Their huge raptor- like beaks were easily capable of killing
small to moderate- size mammals. At the time they lived,
they were among the top carnivores of South America.
Other northward invaders included Didelphis
virginiana, the familiar Virginia Opossum (plate 8- 46),
North America’s only marsupial mammal. Opossums
invaded approximately 1.9 million years ago and
continue to expand their range northward today. Finally,
at least one species of toxodon, an odd, husky mammal
that belonged to a group known as the notoungulates,
came north from South America. Toxodons were bulky
mammals whose appearance suggests a cross between
a cow and a hippopotamus (plate 8- 47). The collective
impact of the South American invaders was modest at
best. Only armadillos and opossums remain, both of
which are thriving. The ground sloths were probably
killed by humans as the human population spread
southward from Siberian Beringia. The other groups
just drop out of the fossil record.
Many North American animals made the reverse
trek, walking across the land bridge to South America.
Their impact on the native fauna appears to have
been substantial. The list of invaders includes skunks,
peccaries, horses, dogs, saber- toothed and other cats,
tapirs, camels, deer, rabbits, tree squirrels, bears, and an
odd group of elephant- like mammals, the gomphotheres.
Add to this list the field mice, or cricetid rodents—
whose travel route to South America is still debated, butPlate 8- 44. Plate 8- 45.
Plates 8- 44 and 8- 45. Skeletons of a glyptodont (8- 44) and a giant ground sloth (8- 45), both part of a display at the Harvard
Museum of Comparative Zoology, Cambridge, MA. These bulky creatures managed to invade North America during the great
faunal interchange. Photos by John Kricher.
Plate 8- 46. The familiar and highly adaptable Virginia
Opossum, here enjoying the remains of some cat food on
the author’s deck in Massachusetts, first immigrated to North
America about 1.9 million years ago and has been spreading
northeastward ever since. Photo by John Kricher.132 chapter 8 evolutionary cornucopia