The uplift of the Isthmus of Panama 2.6 million years ago formed a land-bridge long thought to be the crucial step in the interchange of animals between the Americas, with armadillos and giant sloths moving up into North America and relatives of modern horses, rabbits, foxes, pigs, cats, dogs, and elephants moving down into South America.
But researchers writing in the Journal of Vertebrate Technology say fossil crocodilians found in Panama shed a surprising new light on the history of interchange and animal distributions between the Americas when they were still separated by seas.
Partial skulls of two new species of caiman -- relatives of alligators, who live exclusively in South America today -- were discovered in rocks dated from 19.83 and 19.12 million years old, they said.
"These are the first fossil crocodilian skulls recovered from all of Central America. They fill a gap in evolution between the alligators of North America and the caimans of South America," lead author Alex Hastings, a fossil crocodilian specialist at Georgia Southern University, said.
The fossils suggest caimans dispersed north from South America by the early Miocene era, more than 10 million years earlier than the spread of mammals, researchers said.
"Somehow, they were able to cross over from South America when it was completely isolated by seaways -- this is one of the mysteries that will drive future inquiry and research in this region," said study co-author Jonathan Bloch, a vertebrate paleontologist at the Florida Museum of Natural History.