What Caused the Largest and Most Gruesome Massacre in Earth History?
The horrifying slaughter of an entire continent
Roughly 2.7 million years ago, something happened that led to massive extinction rates of the mammals of an entire continent—roughly 50 to 70% of its furry genera. And the wide-scale death was not caused by any of the usual suspects—erupting volcanos, rising seas, expanding deserts, creeping glaciers, atmospheric changes, spreading disease, or asteroid strike. No, as paleontologists discovered, this was a continent-wide crime scene—and the agents of the carnage used teeth and claws.
In the previous post, we read about the stark differences between the Northern and Southern mammals. When they are brought together, the former often out-compete—or more typically, devour—the latter.
One of the main reasons for this hemispheric bias in mammalian success is that Eurasia remained linked to North America via the Bering Bridge throughout the Cenozoic—and was connected to Africa as well. The resulting massive terrestrial platform promoted the interaction and competition among a far greater number of individuals and species—a tri-continental evolutionary arms race among animals that promoted speed and guile and countless other helpful adaptations. The herbivores had to develop new talents and abilities to out-compete other plant-eaters and evade all the carnivores that were raiding their realm from every direction. This, in turn, pushed the evolution of the predators.
Each of the southern continents had far fewer individuals playing the genetic lottery, so they had fewer opportunities for their mammals to happen upon peculiarly profitable adaptations. The competition was among smaller groups and proceeded in a more leisurely fashion.
An extraordinary biogeographical event—what today is known by the relatively innocuous name, “the Great American Interchange”—helps dramatically demonstrate these differences between Northern and Southern mammals.
The Bloody Fall of South America
From about 35 million years ago to three million years ago, that is, throughout much of the ascendancy of the mammals, South America was an isolated continent. And from about 65 million years ago, it was a continent that was hard to reach. Thus, it developed a rather peculiar endemic mammalian fauna from relatively few invasions from three geographical sources. First, the Xenarthra—its sloths, anteaters, and armadillos—are sister to the Afrotheria, both groups derive from ancestral populations likely separated by continental break-up and drift, perhaps 100 million years ago. Second, hoofed mammals (ungulates), the ancestors of an extinct but once-diverse group of South American herbivores, likely entered from North America around 60 million years ago. These took many forms, superficially resembling llamas, hippos, and pigs, though they left no living descendants.
Third, marsupials—most closely related to Australian forms—appeared in South America, presumably via Antarctica, which at the time was connected to both continents and not yet icebound. Then, around 30 million years ago (or slightly earlier), South America was invaded by two African groups: monkeys and caviomorph rodents (including the ancestors of capybaras and chinchillas). These colonizers likely rafted across a narrower, younger Atlantic—more of a seaway than an ocean at the time.
At this point, this trickle of mammals into South America stopped for tens of millions of years, and the resulting isolation produced many peculiar South American forms—as isolation always does. The radiation among the Xenarthra produced the lumbering, elephant-sized ground sloths and the spectacular and seemingly-Mesozoic glyptodonts—giant armored armadillos, as big as a small car, with spikes along their tails. The caviomorph rodents also flourished, leading to the capybara, porcupines, chinchilla, guinea pigs, and the now extinct “Ratzillas”—Phoberomys pattersoni, a bison-sized guinea-pig like creature, and its close relative, Josephoartigasia monesi, which weighed a metric ton and is the current record-holder for largest rodent ever. Some sizable predators evolved as well—the sabre-toothed panther-like marsupial, Thylacosmilus (“pouch sabre”), and the giant flightless terror birds, the phorusrhacids, some of which stood as large as ten feet tall. These birds did not become extinct until just a few million years ago and had they lived until the present day, it is likely the notion that birds evolved from dinosaurs would have occurred to biologists much earlier. The surprising size of a number of the South American plant-eaters was probably an effective enough adaptation against the relatively small number of predators there, so they remained slow and untroubled.
Thus, up until a few million years ago, South American flora and fauna were quite as distinctive as the Australian and New Zealand biota seem today—and just as vulnerable. But, in one of those colossal transformational episodes that so often grips this world, the Isthmus of Panama emerged from the sea, and South America suddenly became acquainted with the cunning, speed, teeth, and claws of the Laurasian biotic realm.
As noted, biogeographers refer to the event as an “interchange,” as plants and animals moved in both directions between the continents. But it is certainly clear that North America had the upper hand. Perhaps, the most obvious result we find in the fossil record is that North American meat-eaters flooded southward into this new-found rich and fleshy landscape and discovered a wealth of large and lumbering herbivores to rend and claw. As the biologist Stephen Wroe described it, “At that point, suddenly, wham, the carnivore diversity in South America goes absolutely stratospheric… You go from having a handful of not particularly big mammalian carnivores to having arguably the most extraordinary range of big carnivores in the world."1
These predators include massive sabre-toothed cats, bears, at least six species of canines, and a 400 kg lion that was four times larger than the marsupial lion. The gargantuan sizes attained by relatives of the South American guinea pigs, armadillos, and sloths would not have been enough to protect them from the northern marauders. And the rise of the Panamanian link may be properly described as a South American disaster, not as devastating as the one that befell the Antarctic fauna during its great freeze, but certainly far gorier. It produced what perhaps was the bloodiest spectacle since the fall of the tyrannosaurs.
Northern herbivores, like the now-extinct American camels, moved south too, which is why we find llamas, alpacas, and other camel-like relatives in South America today. Camels also roamed across the Bering Bridge into Mongolia, Arabia, and Africa—creating an apparent disjunction of the South American and central Asian forms that would have seemed quite mysterious had we not discovered fossils of their ancestors in North America. The distributional history of tapirs is similar, moving into South America and over the Bering Bridge into Southeast Asia. They too have since gone extinct in North America.
Some South American taxa also moved northward during this great biological swap, opossums and porcupines, for example. The giant sloth, the glyptodont, and the terror bird also all made it into North America as far as Florida, but all have since gone extinct.
In general, the biological current was strongest toward the south. Roughly half of existing South American species are derived from North American forms – especially the cats, rabbits, bears, raccoons, deer, foxes, squirrels, and weasels. In contrast, only 10% of North American mammals are descended from South American ancestors. Because of the interchange, all the ancient South American hoofed animals were replaced with northern ungulates (the deer, tapirs, and camel-relatives) from the north, and nearly all species of South America’s old diverse marsupial fauna have now disappeared—leaving only three forms, the wide-ranging opossums, the rat-like “shrew opossums,” and the diminutive, tree-climbing “monito del monte,” which looks like a mouse.2
Focusing more carefully on the ancient South American survivors, we find an even sharper biogeographical pattern. By and large, the pre-invasion southern mammals that were able to outlast the onslaught all shared one feature in particular. And we will discuss those survivors in a future post.
As quoted by Larua Spinney in “The Lion King.” See also S. Wroe (2002) “A review of terrestrial mammalian and reptilian carnivore ecology in Australian fossil faunas,” Australian Journal of Zoology, 50, 1-24.
This post was adapted from passages in Here Be Dragons: How the Study of Animal and Plant Distributions Revolutionized Our Views of Life and Earth (Oxford University Press, 2009.)
FYI, you’ve mistakenly swapped a former for a latter.
Otherwise 11/10!
“In the previous post, we read about the stark differences between the Northern and Southern mammals. When they are brought together, the *latter* often out-compete—or more typically, devour—the *former*.”