Fossils Reveal North American Mammals Waited in Mexico Before South America Migration

A new study published in Science on July 30 suggests that North American mammals began migrating southward into Mexico at least ten million years ago, earlier than previously thought. They were confined to a 'biogeographic holding pen' in Mexico, where they diversified until about seven million years ago, when smaller species like rodents and raccoon relatives intermittently crossed into South America. The full-scale interchange occurred after the Isthmus of Panama formed around 2.8 million years ago, leading to competition where North American mammals outcompeted many South American species. The research, led by Jack Tseng of the University of California, Berkeley, and Xiaoming Wang of the Natural History Museum of Los Angeles County, nearly doubled the known mammal fossils from Mexico, spanning 12 million years ago to the present. The findings reshape understanding of the Great American Biotic Interchange and offer insights into modern biodiversity and species resilience. The study also highlights that North American placental mammals may have had advantages over South American marsupials. Scientists hope to expand fossil collection in South America to further refine migration timelines.