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

The three major groups of mammals surviving today arose as those major events unfolded: monotremes (the small group of egg layers such as the platypus and spiny echidnas); marsupials (species whose young undergo much of their development as sucklings, usually in a pouch, such as kangaroos, koalas, and opossums); and placentals (mammals whose developing young are nourished for a long time in the womb through a placenta, such as cats, cows, humans, and whales).

Monotremes split from the ancestor of marsupials and placentals (the mammal group Theria) at least 165 million years ago, and their oldest fossils indicate they began to diversify in the ancient southern continents in the later stages of the Mesozoic, by 120 million years ago. Today monotremes live only in Australia and New Guinea, but a fossil platypus found not long ago in South America proves that this group once ranged more widely across the Southern Hemisphere.

Marsupials originated in the north--the most ancient fossils known so far have been found in China and date back 125 million years--then dispersed through North America and across ephemeral north-south island chains into South America, Antarctica, and Australia, perhaps around 70 million years ago. Those three southern continents still retained some land connections with each other but at that time had broken away from Africa.

Placentals diversified on a number of different continents, but only a few lineages were present on the "southern three" during the end of the Mesozoic (the so-called Age of Dinosaurs) and early part of the Cenozoic (around 65 million years ago). In South America, and to a more limited extent Antarctica, archaic placental lineages included the edentates (sloths, armadillos, and anteaters) and a rich variety of native ungulates, or hoofed animals. At least one or two ancient lineages of ungulates from North America also made it to South America.

South America was already largely an island by about 90 million years ago, the time of its final separation from Africa. Its separation from Australia occurred between 60 million and 55 million years ago and from Antarctica between 40 million and 34 million years ago, during final fragmentation of Gondwana. The continent then remained isolated until its (geologically speaking) very recent reconnection to North America, around 3.5 million years ago. The continent's relentless westward drift created the 5,000-mile-long Andes Mountain chain. That massive western spine was born from volcanic eruptions and compressive forces as tectonic plates underlying the Pacific Ocean were subducted beneath the continental margin. Beyond those dramatic events, mammal evolution in South America also proceeded hand in hand with local geological and environmental transformations and global climate changes.

In the 1950s Simpson proposed dividing the continent's mammal history into three time intervals, known as "strata," based largely on fossil finds from high latitudes [see illustration at right]. While his scheme remains a reasonable framework, subsequent research has greatly increased our knowledge of faunas from other parts of the continent, such as the tropics and the Andes. For example, my Chilean, U.S., and French colleagues and I have scoured the Chilean Andes for fossil mammals, yielding thousands of well-preserved specimens, including the oldest rodents from the continent, the oldest very complete New World monkey skull, and more than twenty-five new species that help to document the earliest grassland habitats on the planet. A more refined stratum picture is thus emerging.