How did mammals from North America move into South America millions of years ago

Amazing stuff!

"... researchers have found fossil evidence that animals from different latitudes of North America and South America mixed long before the Panamanian land bridge permanently linked the two continents about 3 million years ago.

Since around 10 million years ago, mammals — primarily small ones like sloths and coatis — found a way across the watery barrier. Only larger mammals had to wait for the permanent land corridor. ...

Biologists thought that only after the isthmus formed did animals surge north and south, what biologists refer to as the Great American Biotic Interchange or GABI. That resulted in the extinction of a lot of South American species as North American species took over.

Yet this picture of mammal movements across continents is too simple, ... paleontologists argue. After assembling and analyzing a new database of fossil mammals discovered in Mexico, they conclude that northern North American animals had already migrated into southern North America by about 10 million years ago, where they waited around in a kind of holding pen until opportunities arose for some of them to continue crossing to the south starting about 7 million years ago. ...

During that pre-GABI time, smaller mammals — heralded by racoon-like coatis and ringtails and small rodents — moved to South America via some kind of intermittent corridor, by island hopping through the Caribbean or by rafting on vegetation. Only one major South American group, the sloths, moved to North America. ..."

From the abstract of the Perspective:

"Whenever plants and animals newly come together, there will be some winners and some losers. This was the case for the Great American Biotic Interchange (GABI), which occurred ~2.8 million years ago (Ma), when the emergence of the Isthmus of Panama connected the continents of South and North America. Although species moved both northward and southward during the GABI, a massive taxonomic asymmetry arose between the two continents, with a much greater proportion of species in South America having a North American origin than the opposite. ... Tseng et al. report that a large fossil dataset from Mexico helps to uncover the events that primed mammals for movement across the Isthmus."

From the editor's summary and abstract:

"Editor’s summary

For millions of years after the breakup of the supercontinent Gondwana, South America was disconnected from other landforms, a time period that has been referred to as its “splendid isolation.”

This isolation allowed for the evolution of mammals with distinctive features, including xenarthrans, sprassodont carnivores, and notoungulates.

The formation of the Panamanian land bridge facilitated connection of North and South American faunas, with a bias from north to south. Tseng et al. looked at fossils from low latitudes in México and found that northern mammals accumulated in the region before the connection (see the Perspective by McGuire and Williams). This allowed for more rapid dispersal south and further supports that the land bridge first formed during the Pliocene rather than the Miocene. ...

Abstract

The movement of taxa between North and South America across the Panamanian land bridge, known as the Great American Biotic Interchange (GABI), established modern-day mammal assemblages in the Americas, heralding the decisive end of what has been called the “splendid isolation” of the South American fauna.

Lacking substantive low-latitude records, previous work on this pattern relied on South American and temperate North American fossils to conclude that pulsed interchanges began about 2.8 million years ago (mya).

Based on new low-latitude fossil data from México, we found evidence for a prolonged, triphasic interchange.

Phase 1 consisted of the initial accumulation of northern higher-latitude mammals in a Mexican “holding pen” beginning 10 mya. 

Dispersal began in phase 2, with a strong north-south gradient and increasing biogeographic connections (7 to 3 mya).

The final phase consisted of the already well-described taxonomic pulses (from ~2.8 mya)."

ScienceAdviser

When mammals crossed between continents (no public access)

North and South America weren’t always connected, but their mammals still found a way to intermingle (original news release) "Before the Panamanian land bridge formed about 3 million years ago, mammals on the two continents were isolated — or so biologists thought. New fossils show that they found ways to mix long before that."

A 10-million-year biogeographic holding pen primed the Great American Biotic Interchange (no public access) "Before the Panamanian land bridge formed about 3 million years ago, mammals on the two continents were isolated — or so biologists thought. New fossils show that they found ways to mix long before that."

This illustration depicts (left) large North American mammals — counterclockwise from top, mammoths, llamas, tapirs, peccaries and saber-toothed cats — waiting in a kind of holding pen in Mexico for the bridge to form.

In South America, the mammalian interchange involved, counterclockwise from top, ground sloths, marsupials, capybaras, armadillos, glyptodonts and, incidentally, terror birds. While North American mammals took over in South America, only about 10% of North American mammals today are derived from mammals that originated in South America.