Reference Publications

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

South America's initial phase of Cenozoic mammal evolution, from 65 million to 34 million years ago, is known as Stratum 1. The continent was isolated except for limited interchange with North America and Antarctica-Australia. The land was mostly forested, and because the mountains were not as high as they are now, nor the high latitudes as cold and dry, the flora and fauna were probably more uniform across the continent. A diverse suite of "archaic mammals" dominated, including marsupials, at least five different groups of endemic hoofed herbivores (plus the few rare lineages shared with North America), and the edentates. In recent years we have learned that other mammals also were present early in this period, including gondwanatheres, an unusual rodentlike group also found in Madagascar and India.

The only notable mammalian predators were borhyaenoid marsupials: insect and meat eaters that were mostly arboreal and small. Plant eaters included the native ungulates, which arose from much more generalized, rat- to fox-size ancestors. The most diverse group of these hoofed mammals in South America were the notoungulates, eventually comprising hundreds of species. Notoungulates thrived for 60 million years--but all are now extinct. The lighter-bodied, long-legged litopterns were another important group that has since vanished.

An enigmatic group known as xenungulates, whose connection to other families is poorly understood, were represented by a few rare species in both the tropics and Patagonia. Somewhat better known are the pyrotheres and the astrapotheres, animals that could reach the size of rhinos or small elephants. Some evolved tusks and even trunks. Those animals often were specialized for life in wet habitats such as river margins, and therefore were less common than other native ungulates. Rounding out the plant eaters, some edentates and marsupials evolved herbivorous forms.

[ILLUSTRATION OMITTED]

A major shift, heralding the start of Stratum 2, began about 34 million years ago, when a continental-scale ice cap first appeared on Antarctica and the planet's oldest grassland ecosystems arose in South America, some 15 million to 20 million years earlier than on other continents. Recent studies in Andean and tropical regions continue to build evidence that major geologic, climatic, and environmental changes began extremely rapidly and continued across the continent right through to today.

During that period of climatic upheaval, various man> real groups and communities "modernized" in response to new habitats, evolving adaptations to markedly different diets. For instance, grazers developed high-crowned teeth, like those that modern cattle use to grind gritty grass: the enamel, which covers the crowns of most teeth, extends far below the gum line in grazers to provide more hard material to wear down over time.

Edentates reached their apex of anatomical and species diversity during this time. Although only two types of sloths exist today--both extremely slow-moving leaf eaters that live in the trees of tropical forests--their dozens of extinct relatives ranged across all of South America. Most of the vanished ones were fairly large to gigantic, roaming the open plains and woodlands to feed on grasses, shrubs, and leaves [see illustration above]. The armored glyptodonts, extinct relatives of armadillos, were among the most unusual mammals ever to evolve. Bony plates over their backs, covered with keratin (the substance in hair and fingernails), fused to form a solid carapace or shell for protection against predators. Some were huge: Doedicurus weighed about a ton, grew to be as much as twelve feet long, and was armed with a spiky tail club (which was probably used more in mating battles with rivals than for defense).