The Exotic Animals of Prehistoric South America





Isolated from the rest of the world for tens of millions of years, the animals of prehistoric South America evolved down their own path.

In 1492, Christopher Columbus discovered the Americas, heralding the exchange of crops and livestock between the Old World and the New. However, while the exchange can be credited with bringing everything from turkeys to tomatoes to our dinner plates, it also killed countless millions. The arrival of the smallpox virus alone (which, mercifully, was declared eradicated in 1980) sealed the fate of around 95% of the native American population.

The above undoubtedly represents one of the greatest tragedies of human history, but biotic exchanges like these have occurred for natural reasons ever since life first appeared on Earth. Long before the human impact became a thing, nature also had its way of dramatically changing the course of evolution: Shifting landmasses create and destroy bridges between continents and islands, leading to biotic exchanges and isolation respectively.

Sometime at least 3 million years ago, the volcanic Isthmus of Panama rose from the water, permanently closing the Central American Seaway that had kept the Americas apart throughout the Cenozoic Era and before. Until then, South America’s ecosystem had evolved in near complete isolation from the rest of the world for at least 100 million years. With the formation of a complete land bridge, the foundations for the greatest biotic exchange since the extinction of the dinosaurs were in place.

Marsupial Origins

Australia is well-known for its unique ecosystem which, like that of South America, is the result of millions of years of isolation. However, both continents have something very important in common. Until around 34 to 50 million years ago, both continents were connected to one another via Antarctica which, at the time, wasn’t covered in ice. Recent genetic research suggests that marsupials, such as the iconic kangaroo or koala, can trace their roots all the way back to South America.

Marsupials belong to a slightly more inclusive class of mammals known as metatherians, which have been around for much longer than the placental mammals that tend to dominate most terrestrial ecosystems today. They appeared around 130 million years ago in what is now South America, spreading across to North America and Australasia sometime later. Sporting an entirely new environmental niche to exploit, Australia proved the perfect environment for marsupials to diversify.

Matters were quite different in South America during Pliocene Epoch between 5.3 and 2.8 million years ago. As the formation of a more permanent Central American land bridge heralded the peak of the Great American Interchange, the fate of many endemic metatherians was sealed. Entire mammalian orders went extinct in short order as their populations were decimated by the arrival of competing animals making their way southward from North America.

Rom-diz

Among the first casualties of the interchange were the carnivorous metatherians, such as the bizarre sparassodonts. I mentioned these in last week’s list article as a perfect example of convergent evolution, owing to their uncanny similarity to the completely unrelated feline lineage. However, just as the native civilizations of South America were outgunned by the arrival of Europeans, sparassodonts failed to compete with North American carnivores like canids and felids.

Terrifying Birds

Sparassodonts were not the only apex predators dominating Pliocene South America. While they were fast and agile, filling much the same ecological niche as today’s cougars, the predatory abilities of the so-called terror birds were perhaps even more formidable. These enormous flightless birds grew up to 9.8 feet (3.3 metres) high, and they thrived across South America for some 60 million years until their final disappearance 1.8 million years ago.

Phorusrhacids, as they are known in more scientific circles, were more successful than the sparassodonts in terms of surviving the Great American Interchange. They were the only large predators to successfully cross into North America at the time as well, albeit only as far as what are now Texas and Florida. Other genera declined drastically in numbers, ultimately going extinct as they failed to compete with North American carnivores.

Bernard Dupont

Relatives of the terror birds survive to this day in the form of the South American seriemas. These long-legged birds are the sole surviving member of the cariamiformes, the order that the phorusrhacids belong to. Although seriemas don’t exactly look formidable, their hunting and eating habits have given us insights into those of the terror birds: Their feeding style involves picking up their prey and smashing it repeatedly into hard surfaces to break bones and tenderise meat.

Giant Ancestors

Xenarthrans are placental mammals that were once endemic to South America, but moved northward during the interchange. Their extant representatives include tree sloths, armadillos and anteaters. However, these unusual animals have nothing on their ancestors, which thrived and diversified throughout millions of years of isolation. Early xenarthrans, by contrast, were among the largest terrestrial mammals to live in South America since the extinction of the non-avian dinosaurs.

While modern sloths are exclusively arboreal and grow to about the size of a medium-sized dog, their ancestors were often enormous. The largest of all of them was the eremotherium, an elephant-sized ground sloth weighing over five tonnes. Although all ground sloths are now extinct, they managed to spread throughout the Americas during the biotic interchange. Incredibly, they only disappeared 11,000 years ago, making it possible that humans played a role in their extinction.

Like sloths, armadillos had also reached enormous sizes by the Pliocene Epoch. Known as glyptodonts, they were close relatives to modern armadillos, but they weighed up to two tonnes. Heavily armoured with almost impenetrable shells, some genera also sported club-like tails much like the ankylosaurian dinosaurs. They disappeared around the same time as the ground sloths, which also happened to coincide with the arrival of people in the Americas.

Xenarthrans weren’t the only giant herbivores endemic to South America before the interchange. The continent was also home to a unique group of ungulates, a diverse group of mostly large herbivorous mammals that includes everything from horses to camels. Due to convergent evolution, many of these creatures looked vaguely similar to their counterparts on other continents. The 1.4-tonne toxodon, for example, looked a bit like a stocky rhinoceros, albeit without a horn.

Robert Bruce Horsfall

Another mysterious mammalian order from South America was macrauchenia, a group of animals that looked vaguely like llamas, albeit with short trunks. Nonetheless, they were not related in any way to llamas and other camelids. Disappearing some 10,000 years ago, they were yet more casualties of the Holocene extinction event.

Out of all the exotic mammals of prehistoric South America, perhaps none were more bizarre than the astrapotherians. These lived between 59 and 12 million years ago, disappearing long before the Great American Interchange for reasons unknown. Since first being described in 1894, these animals, which looked like something of a cross between a rhinoceros and a miniature mastodon, have long confounded palaeontologists who still aren’t agreed on their taxonomic ranking.

•

There’s little doubt that the Age of Mammals would have turned out very differently had it not been for the Great American Interchange and the much later Holocene extinctions. Unfortunately, however, for South America’s native megafauna in particular, both natural and human history have not been kind. If you can think of any exotic taxons I haven’t mentioned here, feel free to discuss them in the comments!

Further Reading