Among those animals were the gigantic ancestors of armadillos and groundhogs that migrated northward, while relatives of horses, rabbits, foxes, pigs, cats, dogs and elephants moved southward.

Now researchers from the Smithsonian Tropical Research Institute and scientists from the University of Florida have discovered crocodilian fossils that shed new light on the interchange and distribution of animals in the Americas. The fossilized parts of skulls are of two previously unknown species of caimans, relatives of lizards, which now live only in South America. “These are the first crocodilian skull fossils found in all of Central America,” said Alex Hastings, the lead author of the study who is also a specialist in crocodilian fossils at Georgia Southern University.