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5d ago

Mexico and Central America Served as a 10‑Million‑Year Holding Pen for Northern Mammals

Stable‑isotope and fossil evidence links the rise of C4 savannas to dietary and trait shifts that primed North American mammals for later southward invasions into South America.

An topographical map shows the green-and-brown Isthmus of Panama against a blue backdrop.
UC Berkeley 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. 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. Illustration by Guillermo Torres Carreño. Reproduced with permission of the Banco de Imágenes Ambientales (BIA), Instituto Alexander von Humboldt.
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Overview

  • New multi‑institutional research shows North American mammals moved into and clustered in southern Mexico and parts of Central America from about 10 million to 7 million years ago, where they diversified and adapted to warmer, drier conditions.
  • The study, published July 30, 2026 in Science, combines nearly doubled Mexican fossil records with stable‑carbon isotope analyses of tooth enamel and paleosols to reconstruct diets and environments.
  • Isotope data indicate C4 savanna grasses appeared in the region around 10 million years ago and became a dominant food source for some herbivores by about 5 million years ago, driving dietary shifts toward grazing.
  • Those dietary and ecological changes favored traits such as larger body size and drought tolerance that gave northern mammals an advantage during the pulsed southward migrations that followed formation of the Isthmus of Panama.
  • The team expanded sampling by mining Spanish‑language and small‑journal literature and museum holdings, and the work points to new field targets in South America and studies of dispersal mechanisms that could clarify the timing of early intercontinental exchanges.