Today, more than half of the 217 mammal genera present in South America have origins in North America. The key to the North American mammals’ success could be an extensive vacation. The scientists pulled together a fossil library of mammals from Mexico, nearly doubling the known mammal fossil record for the area. Instead, the team collected Spanish-language papers, papers from smaller journals and samples from a museum in central Mexico. As mammals in Mexico and Central America were diversifying, new ones were arriving from North America.