Mexican Fossils Reveal Ten-Million-Year Evolutionary 'Holding Pen' Before Great American Migration

A team of paleontologists has identified Mexico as a prolonged evolutionary “holding pen” where mammals diversified for a minimum of ten million years prior to the formation of the Panamanian land bridge. By adding new fossil specimens to existing records, they observed a clear north-to-south movement pattern that predates the 2.8-million-year-old connection between the continents. The researchers propose that the progressive narrowing of North America acted like a bottleneck, allowing only certain species to continue southward.

Evidence of a drying climate in Mexico during this interval may have further selected for larger, drought-tolerant mammals that later colonized South America. Lead author Jack Tseng notes that these findings could help predict how modern species might respond to current climate change. The work also raises the possibility that similar bottleneck dynamics could have shaped biotic exchanges on other continents.