Splendid isolation: South America was an island for millions of years, fostering an evolutionary explosion of unique mammal species

Natural History, June, 2009 by John J. Flynn

Ungulate lineages continued to undergo remarkable changes. Litopterns radiated in a number of different anatomical directions, with horselike forms evolving extreme specializations for running, such as losing all but one long toe on each foot. Others vaguely resembled a pastiche lover's cross between camels, giraffes, and elephants, with robust bodies, broad-hoofed feet, long necks, and prominent trunks. With hundreds of species, the notoungulates provide textbook examples of evolutionary convergence in mammals. Teeth, skulls, and body shapes comparable to those of rats, rabbits, primates, hyraxes, sheep, horses, and rhinos all evolved.

Misled by those resemblances, the pioneering Argentine paleontologist Florentino Ameghino believed that most major mammal groups originated in South America, as suggested by the prosaic names he applied to many species, such as Notohippus ("southern horse"), Archaeohyrax ("ancient hyrax"), Notopithecus ("southern ape"), Homunculus ("little man"), and more. But we now know that many of the species were notoungulates, and none were closely related to any of the groups on other continents for which they were named. Still other South American mammals, because of their distinct ancestry and unique adaptations, resembled nothing found on the other continents. Such extreme examples of convergence and divergence epitomize evolution in isolation.

[ILLUSTRATION OMITTED]

Although South America did not witness the rise of many large native predators, during Stratum 2 some borhyaenoid marsupials evolved into bearlike animals with powerful jaws and crushing teeth, such as Borhyaena, and even into a saber-toothed killer, Thylacosmilus [see illustration on page 27]. Other native predators were the Phorusrhacidae, or "terror birds": at least one 15-million-year-old species of these carnivorous ground birds towered nine to ten feet tall and sported a sharp, hooklike beak as part of its two-foot-long head.

The other major mammal events during this time were the arrivals of rodents and primates in South America. But from where? The existing fossil record shows they were absent for all of Stratum 1: the earliest well-dated rodent fossil on the continent is 32 million years old, and the earliest primate fossil is about 28 million years old--the oldest well-preserved skull of a New World monkey is our team's fossil find, 20-million-yearold Chilecebus [see illustration belou,]. Close relatives of both groups lived on other continents well before that. Long-standing arguments exist about where South America's rodents and primates originated. But the characteristics of the earliest South American fossils indicate that the first immigrants arrived from Africa, by at least 32 million years ago in the case of the rodents. They could have crossed the Atlantic Ocean--still a relatively narrow barrier--on rafts of vegetation.

The final interval, Stratum 3, began inconspicuously enough about 10 million years ago, but major changes were precipitated when the Isthmus of Panama joined North and South America, about 6.5 million years later. The new barrier between the tropical Atlantic and Pacific Oceans dramatically changed ocean currents, helping to initiate a series of alternating ice ages and interglacial periods. Grassland and forest environments grew or shrank in rapid pulses in response to the accompanying fluctuations in moisture and temperature. In addition, the bridge between North and South America was the basis for the Great American Biotic Interchange, with species crossing into new regions and radically altering faunal competition, adaptation, and extinction.