Nestled deep within the Haitian forests resides the blue-eyed La Hotte glanded frog (Eleutherodactylus glandulifer), a creature that eluded scientific observation for a staggering 20 years.
This frog is part of a diverse genus native to the Caribbean, which includes the more widely known coquí frog (Eleutherodactylus coquí), celebrated as a cultural symbol in Puerto Rico. Now, a recent analysis of fossils reveals that frogs from the Eleutherodactylus genus represent the oldest Caribbean vertebrates found in Florida, challenging prior assumptions about their arrival in North America.
María Vallejo-Pareja, a graduate student at the University of Florida and the paper’s first author, unearthed this insight by delving into understudied fossil collections.
“We had a knowledge gap, but the solution was right under our noses,” Vallejo-Pareja commented. “The fossils we needed were already in our possession, gathered between the 1970s and 1990s. We just hadn’t examined them closely.”
The evolutionary history of frogs remains partially documented. Scientific analyses demonstrate that frog families experienced rapid diversification after the Cretaceous-Paleogene mass extinction, famously eliminating the dinosaurs 66 million years ago. Subsequently, frogs continued diversifying over several million years. While they first appear in Florida’s fossil records during the Oligocene Epoch (approximately 34 to 23 million years ago), these records remain sporadic.
The field of frog paleontology is comparably underexplored compared to other vertebrate groups, presenting challenges when researchers at the Florida Museum stumbled upon a trove of frog fossils in paleontological sites across Florida, including Brooksville 2 and Live Oak SB-1A. These fossils, collected between the 1970s and 1990s when frogs weren’t the primary focus, remained unstudied until Vallejo-Pareja’s investigation.
Vallejo-Pareja compared the Florida site fossils with existing collections containing specimens of both extinct and extant frogs, including the Florida Museum’s samples of the La Hotte glanded frog. Her analysis revealed that most of the gathered fossils belonged to the Eleutherodactylus genus, commonly known as rain frogs or robber frogs.
Rain frogs have a migratory history, originating in the Caribbean from an ancestor that dispersed from South America around 47 million years ago during the Eocene Epoch. After reaching the islands, the ancestral population rapidly diversified into multiple species via adaptive radiation, analogous to Charles Darwin’s documented finches in the Galapagos Islands, where one migrant species swiftly evolved into at least 13 different species, occupying various new feeding niches.
Presently, rain frogs are distributed across the Caribbean, Central, and North America. The oldest known fossil from the genus belongs to the coquí frog, present in Caribbean forests for at least 29 million years. In the 1970s and ’80s, it unintentionally arrived in Florida and Hawaii through nursery plants and is now considered an invasive species in both states.
DNA analysis previously suggested that Caribbean frogs in the Eleutherodactylus genus initially reached Central America during the middle Miocene Epoch, 16 to 11 million years ago, before spreading to North America. However, fossils from this study demonstrate that rain frogs were already in Florida during the late Oligocene, several million years before their documented dispersal into Central America.
Rain frogs seemingly possess an adept ability to migrate, yet it remains unclear how they reached Florida. While overwater dispersal on flotsam or buoyant debris seems plausible, most of the Florida peninsula was submerged when the frogs supposedly arrived. The increased distance between landmasses would have made their journey longer and more perilous than it would be today.
It’s plausible that there were multiple dispersal events, but Vallejo-Pareja emphasizes that this hypothesis requires testing through the discovery of more fossils in Central America. However, tracking frog dispersal poses challenges due to their small size and high mobility.
“These fossils are minuscule,” Vallejo-Pareja remarked. The tiniest fossil frog measured only 16 millimeters from snout to rear end, smaller than a U.S. penny. “So, handling them without breaking or losing them was breathtaking. And I mean that literally, because if I’m at the microscope with my fossil and I sneeze or breathe too hard, it’s gone.”
Despite the current widespread distribution of rain frogs throughout North and Central America, these findings suggest that Florida served as their initial habitat, alongside a diverse array of now-extinct animals at sites like Live Oak SB-1A and Brooksville 2, where rain frog fossils were abundant. These animals included bear-dogs, bone-crushing dogs, a weasel-like carnivore, squirrels, beavers, and rabbits.
Eleutherodactylus marks the earliest documented Caribbean vertebrate arrival in Florida. While evidence supports the migration of rodents and salamanders from North America to the Caribbean during the Oligocene and Miocene, data on movement from the islands to Florida remains scarce. Caribbean toads, snakes, and lizards likely traversed to Florida during the subsequent epoch, the Miocene, but these records remain inconclusive and require further investigation.
Vallejo-Pareja anticipates that the methodology and data from her paper will invigorate frog paleontology research, lauding the existing work in the field while emphasizing the need for more. She has created digital 3D models of the fossil bones used in the study, providing additional reference material for paleontologists.
In forthcoming endeavors, Vallejo-Pareja aims to employ methods developed in this study to understand how frogs adapt to environmental changes. Despite enduring numerous extinction events, frogs exhibit high responsiveness to variables like temperature and precipitation.
“What occurred with frogs during a glacial maximum?” she pondered. “Did they shrink or grow larger? Did their diversity decrease or increase? Did they endure? Examining the past to understand how frogs responded would be fascinating.”
The study’s co-authors include the Florida Museum’s Edward Stanley, Jonathan Bloch, and David Blackburn.
Header Image Credit : Florida Museum of Natural History photo by Kristen Grace