A team of researchers have revealed how frogs from the Eleutherodactylus genus are considered the oldest Caribbean vertebrates to be found in the US state of Florida after officials unearthed a collection of frog fossils in the Sunshine State.
In explaining the migration shift, researchers cited factors associated with “temporal changes in climate and ecology that operated at different geographic scales.”
The fossils, referred to as Eleutherodactylus genus, were believed to have originated in the Caribbean in the Oligocene Epoch - 33.9 to 23 million years ago, and then colonized Central America in the Middle Miocene Epoch - 16 to 11 million years ago.
But researchers say the Eleutherodactylus (rain frogs or robber frogs) was established in North America by the Late Oligocene, before colonizing Central America in the Miocene Epoch - several million years before their first recorded dispersal in Central America, making them - geologically - the oldest Caribbean vertebrates to be found in Florida.
The oldest-known fossil from this genus belongs to the coquí frog, which has been in the Caribbean forests for a minimum of 29 million years.
Researchers at the Florida Museum have long suspected that some North American frogs first originated in the Caribbean but they weren’t exactly sure how.
"There was a gap in knowledge, but the answer was under our noses the whole time," said María Vallejo-Pareja, first author of the paper. "We already had the fossils, which were collected from the 1970s through the 1990s. We just hadn't worked on them."
Comparing the fossils discovered across northern Florida to the specimen collections of both extinct and living frogs, Vallejo-Pareja found that most of their fossils belonged to the genus Eleutherodactylus, known for having a history of migration.
The genus first originated in the Caribbean from an ancestor that dispersed from South America as early as 47 million years ago during the Eocene Epoch. Then, they diversified on the island through a process called adaptive radiation.
"These fossils are millimeters big," Vallejo-Pareja said, with the smallest fossil measuring just 16 millimeters in length. "So getting to work with them, without breaking or losing them, was a breathtaking moment. And I mean that literally, because if I'm sitting at the microscope with my fossil and I sneeze or breathe too hard, it's gone."
But in order to discover how these frogs traveled a vast expanse of ocean, researchers will have to conduct further studies with more fossils in Central America. However, one theory suggests Florida’s landmass was once connected to the Caribbean, and that a land bridge allowed for these migrations.