During the Late Cretaceous/Early Tertiary, 1 group of ungulates and 1 group of edentates migrated to South America.  From these ungulates evolved 6 ungulate orders (including horselike and camel-like forms) which were unique to South America and left no living descendants.
Toxodon was one of the most common mammals in the South American Pleistocene.
toxodon toxodon


Edentates were one of the earliest placental orders and they possess a number of traits which make them unique such as neck vertebrae which vary in number from 6 to 9 (as opposed to the standard seven), ribs whose bony portions reach the sternum, additional points of articulation between back vertebrae, a small and simple brain, the loss of enamel, inefficient thermoregulation and low body temperature, and a long sacrum (Kurten, 1980).

Armadillos probably represent the ancestral edentate form. They are known from South America since the Paleocene and the two groups, the modern dasypodines and the extinct pampatheriines, had diversified in South America long before they migrated to North America. From the Miocene through the Pleistocene, many pampatheriines gradually increased in size. Glyptodonts migrated to North America in the Pliocene and, unlike some South American species, lacked a tail club. The earliest glyptodont in North America was small measuring 1.3 meters and weighing up to 55 kg (Kurten, 1980).

    The first edentates to migrate to South America evolved into a great diversity of species which were unique there.  The edentates included armadillos and anteaters.  The giant armadillo Glyptotherium had a skeleton 2.5 meters long and possessed a tail club. 

A variety of armadillos existed in North America after the Interchange including a four foot long species, giant 10 foot long glyptodonts, and a rhino-sized armadillo belonging to an extinct group called the pampatheres. Glyptodonts evolved in South America from armadillos with movable bands. The later forms possessed a single, inflexible, shell-like dome made of about 2,000 bony scutes. Glyptodonts could weigh a ton with lengths of 10 feet and a height of 6 feet. They also possessed hoof-like nails and a small trunk (Kurten, 1988).


The skulls of two modern anteater species are depicted below.


Giant fossil armadillos are depicted in the following images.
glyptodont glyptodont
Modern armadillos are depicted in the following images.
armadillo 1 armadillo 2
     A third group of edentates were the sloths. The fossil record includes a variety of ground sloths which ranged in size from that of a pig to the elephant-sized Megatherium which possessed huge claws. One species of ground sloth (Megalonyx jeffersonii) even migrated as far as the Beringia land bridge which connected Alaska to Siberia. The largest ground sloths of the genus Eremotherium reached 20 feet in length and weighed almost as much as elephants. The Jefferson sloth Megalonyx jeffersoni was first described by Thomas Jefferson (Kurten, 1988).
sloth sloth 2
sloth 3 sloth 4


     South America was isolated from North America from the Late Cretaceous to the Late Pliocene, about 3 million years ago.   The South American fauna was unique and many of the primitive groups of mammals survived there without pressures from advanced placental predators.  When the isthmus of Panama formed to fuse the two American continents 3 million years ago, at least 16 genera of mammals migrated north and at least 23 genera migrated south. 

      The effect on North America was minimal.  Glyptodonts and giant sloths did colonize North America (as did at least one kind of terror bird, Titanis) but they eventually became extinct, perhaps because of being hunted by early humans.  Today, the only North American survivors of this interchange are the opossum, a porcupine, an armadillo, and 2 species of rodents.

    The effect on the South American fauna was enormous.  The North American immigrants were very successful although not all of the migrating species survived to modern times (such as a few three-toed horses, bears, and elephants).  The South American ungulates and terror birds became extinct as did many of the edentates and marsupials, in part because of the advanced placental predators.  More than half of the modern South American mammalian genera are descended from the North American species of this interchange.

     At least some of this unique South American fauna survived until early human groups arrived, such as the giant sloths.  The mythological figures of native Paraguayans include a fierce creature known as the Ao-Ao.  Although this seems a bit odd given that Ao-Ao is the guarani word for sloth and modern sloths are anything but fierce, it is quite possible that this legend has its origin in the hunting experiences of the early native South Americans which encountered the much larger relatives of the modern sloth.  Below is a Paraguayan representation of the mythological Ao-Ao.

ao ao