Extreme Stories: Mammals from Another World
It's safe to say that the mammals of South America marched to their own evolutionary beat for the better part of the last 80 million years or so. During much of mammalian evolution, South America was an island continent.
One way that new species evolve is when a population becomes isolated. For land animals, there's no more extreme form of isolation than being completely surrounded by an ocean. The isolated continent produced unique species ranging from giant sloths and tank-like armadillos to tiny hopping marsupials.
An array of massive plant-eaters evolved on South America. Some even developed giant tusks and long trunks. These hefty herbivores included a pair of lumbering giants like the Pyrotherium ("fire beast") and Astrapotherium ("lightning beast"). What becomes incredibly prevalent, especially on an isolated continent, is the effect environment can have on very different species to make them appear somewhat related.
Adaptations that help ensure survival in a specific environment tend to evolve again and again. For instance, suppose a habitat contains tough, abrasive grasses that can be chewed only by animals with hard, continuously growing, grinding teeth. That type of tooth may evolve separately in several distantly related groups. This pattern is called "convergent evolution."
During its 80 million years of isolation, South America had no elephants, horses, camels or rabbits. Instead, adaptations like trunks, hooves, long necks or chiseling teeth evolved independently in different members of a single mammal group, the notoungulates, or "southern hoofed plant eaters."
One example of a notoungulate was Protypotherium, which lived in what is now Argentina 24 to 29 million years ago. This small plant-eating mammal had gnawing incisor teeth, deep pockets on the jaws for chewing muscles, specialized ankles, and long legs like a rabbit. But it's not a rabbit; it's a notoungulate, a completely different group of placental mammals.
Just when you thought the island version of South America couldn't offer us any more insight to some of the more amazing twists and turns of mammalian evolution--add some burning hot lava to the mix.
An erupting volcano can destroy any living thing in its path. But on rare occasions, cooler volcanic debris swiftly engulfs and suffocates animals without incinerating them. Millions of years later, volcanic rocks formed from this ash can produce perfectly preserved fossils.
A collection of such fossils from the Andes Mountains of Chile--among the most well-preserved ever found of mammals that lived in South America between 10 and 40 million years ago--has completely changed our understanding of this continent's early mammals, ancient climates, and even the formation of the Andes themselves.
One monkey skull discovered is the oldest ever found in South America. For decades, scientists had debated whether New World monkeys originated in Asia or Africa. This monkey fossil's teeth and skull share many specialized features with primates from Africa, indicating an African origin. Its ancestors may have floated across the ocean on huge "rafts" of matted vegetation.
Those "rafts" may also have delivered the earliest known rodents to reach South America at least 33 million years ago. Evolving in isolation, some evolved to massive sizes, including the 130-pound capybara and now-extinct species that were as big as a bear.
Another set of fossils shed some light on the early development of grasslands in South America.
Chewing abrasive grass all day requires high-crowned teeth--otherwise the grit from grazing would rapidly wear them down. On northern continents, high-crowned teeth first appeared about 15 million years ago. But in South America, startling new fossils from the Andes of Chile reveal that high-crowned teeth evolved in many different mammal groups more than 30 million years ago--indicating that grasslands arose much earlier in South America than on other continents.
From giant rodents to a host of notoungulates, the many millions of years that the mammals of South America spent in extreme isolation and the myriad of special characteristics they developed along the way give a distinct feeling-- these are mammals from another world.
And in a way, they were.