Ancient North American Mammals May Have Gathered in Mexico and Diversified Before Moving to South America, Priming Them to Thrive There

An analysis of fossils from the country hints that animals from the north stayed in a “holding pen” between seven million and ten million years ago. The timeline might help explain why so many South American mammals have North American ancestry, but not vice versa

Sarah Kuta | Daily Correspondent

August 4, 2026 10:00 a.m.

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Today, North and South America are connected by a narrow strip of land called the Isthmus of Panama. But that wasn’t always the case. For 60 million years, the two continents were separated by water.

Scientists have long thought that animals were confined to either North or South America until roughly 2.8 million years ago, when the Isthmus of Panama formed and allowed them to move freely between the two huge landmasses, a phenomenon known as the Great American Biotic Interchange. But that may not be the whole story.

A new analysis of fossils from Mexico hints that mammals living in North America had already started heading south by at least ten million years ago, researchers report in a study published July 30 in the journal Science. Unable to proceed any further, the creatures waited in a “biogeographic holding pen” and diversified until about seven million years ago, the team writes in the paper, when opportunities arose for some smaller species to continue their journey south into South America. Later, when the Panamanian isthmus formed, the “floodgates opened,” and mammals moved both north and south, study co-author Jack Tseng, a vertebrate paleontologist at the University of California, Berkeley, says in a statement.

The findings not only reshape scientists’ understanding of how mammals spread between North and South America. They also offer fresh insights into modern biodiversity on both continents, Tseng says in the statement. The new picture may help researchers and conservationists understand “which species are more likely to persist and survive future perturbations in the environment, given what they’ve already experienced for the past ten million years.”

Historically, the mammal fossil record from Mexico has been limited. But in recent years, the researchers behind the new study and their colleagues have been building a more comprehensive database. Now, they’ve nearly doubled the number of known mammal fossils found in the country, which span from 12 million years ago to the present day.

When the team took a closer look at this new fossil library, they noticed that many of the mammal fossils from Mexico—including some from as far back as ten million years ago—were very similar to those found farther north in North America. Analysis of these fossils helped build the new intercontinental mammal movement timeline.

During the holding pen phase between seven million and ten million years ago, large-bodied mammals from North America adapted to their new local climate and continued to diversify. Starting around seven million years ago, some species of smaller North American mammals, such as rodents and relatives of raccoons, intermittently made their way to South America, though scientists don’t know exactly how they got there. They might have floated on rafts made of vegetation or swam between islands, for example.

Once the land bridge formed and mammals began moving in both directions, those from North America outcompeted those from South America—and many South American species died out.

Scientists say that’s to be expected. “Whenever plants and animals newly come together, there will be some winners and some losers,” Jenny L. McGuire and Claire M. Williams, paleobiologists at Georgia Tech who were not involved with the research, write in a commentary accompanying the new paper.

The effects of this uneven success are still apparent today: Only four modern mammal genera in South America have North American origins, while more than half of the 217 mammal genera in South America have North American ancestry, the researchers write in the paper.

Northern mammals weren’t necessarily “biologically superior” to their southern counterparts, Alberto Boscaini, a paleontologist at the University of Buenos Aires in Argentina who was not involved with the study, tells Science News’ Bethany Brookshire. They may simply have acquired “preadaptations that became useful during a moment of profound tectonic change,” he says.

For example, scientists think that during the holding pen phase, northern mammals competed with one another for limited resources, so only the fittest survived to make the move into South America. Another possible advantage: North America was dominated by placental mammals, which give birth to fully developed offspring that quickly become independent, whereas South America had more marsupials, which give birth to relatively underdeveloped young that continue growing in pouches.

Moving forward, scientists hope to collect and investigate more fossils from South America to better understand which specific species migrated southward and when.

“Our findings suggest that as our fossil records improve, particularly related to finer resolution of the age relationships and their geographic locations, we are able to see nuances previously missed,” study co-author Xiaoming Wang, vertebrate paleontology curator at the Natural History Museum of Los Angeles County, says in a separate statement.

Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.