North and South America haven’t always been connected. South America functioned as a continent-sized island for millions of years following the extinction of the dinosaurs, incubating its own strange assembly of animals such as giant ground sloths, massive armored mammals akin to armadillos and saber-toothed marsupial carnivores. Meanwhile, North America was exchanging animals with Asia, populating it with the ancestors of modern horses, camels and cats, writes Asher Elbein for the New York Times.
Finally, when tectonic activity formed the Isthmus of Panama roughly ten million years ago, a massive biological exchange took place. The many species that had been evolving in isolation from one another on both continents began migrating across the narrow new land bridge. Llamas, raccoons, wolves and bears trekked south, while armadillos, possums and porcupines went north.
It would be reasonable to expect this grand biological and geological event, known to paleontologists as the Great American Biotic Interchange, resulted in equal numbers of northern and southern species spreading across the two land masses; but that’s not what happened.
Instead, many more North American mammal species made homes down south than the other way around. Almost half of living South American mammals have North American evolutionary roots, whereas only around ten percent of North American mammals once hailed from South America. Now, researchers who reviewed some 20,000 fossils may have an answer, according to the Times.
According to the paper, published this week in the journal the Proceedings of the National Academy of Sciences, the asymmetry of immigrant mammal diversity we see today was the result of droves of South American mammals going extinct, leaving gaping ecological holes waiting to be filled by northern species and reducing the pool of potential immigrant species to make the trek north, reports Christine Janis, an ecologist at the University of Bristol, for the Conversation.
"This faunal exchange can be seen as a natural experiment: two continents, each with its own kind of animals were connected by a narrow land bridge, allowing massive migrations in both directions," Juan Carrillo, a paleontologist at the National Museum of Natural History in Paris, fellow at the Smithsonian Tropical Research Institute and first author of the study, says in a statement. "Our study shows how these migrations happened and that South American mammals had more extinctions. The effect of this exchange can be still seen today."
Traditionally, researchers explained the asymmetry of mammal diversity in a few ways. Perhaps more mammal species travelled south than vice versa, or maybe there was a larger pool of northern species to begin with, according to the Conversation. An even more antiquated explanation posited that the “superior” northerners simply out-competed the southerners, a hypothesis that Janis notes smacks of colonialism and racial bias.
But when Carrillo and his team analyzed the fossil record to evaluate the merits of these various hypotheses, extinction was the only one supported by the evidence. He tells the Times that when the land bridge first opened around ten million years ago the exchange of species was “relatively balanced...But what we found was that five million years ago in the Pliocene, there was a disproportionate decline in diversity.”
Some of these extinctions were once explained as being the outcomes of lost ecological battles with northern species. Instead, Janis explains in the Conversation, “the abundance of South American carnivores today, including the fox-on-stilts maned wolf, the otter-cat jaguarundi and the lemur-like kinkajou, is not testament to placental carnivores being superior to marsupial counterparts: rather, their ancestors invaded essentially virgin territory, which the previous occupants had already vacated, and then diversified into wonderous new forms.”
These extinctions prior to the peak years of the Great American Biotic Interchange don’t appear to have a simple explanation, Carrillo tells the Times. Climatic shifts towards drier conditions may have caused forests to retreat in South America, perhaps to the detriment of its wildlife. It’s also not impossible that competition with species of northern origin played a role, but as Carrillo tells the Times, “probably the reason for the extinctions is quite complex, and includes some biological interactions and habitat change.”