Four fossils of small frogs found in Myanmar provide the earliest direct evidence of anurans (the order of animals that includes frogs and toads) in a wet tropical forest. The findings are presented in Scientific Reports.

Frogs originated about 200 million years ago but the fossil record for these early amphibians is relatively sparse. Today, many different species of frog inhabit tropical forests around the world. They likely originated in the Paleogene, about 66 to 23 million years ago. However, there is little direct evidence of when frogs first started living in tropical habitats.

Lida Xing and colleagues describe four fossil frog specimens - called Electrorana limoae - from the mid-Cretaceous period (approximately 99 million years ago). The specimens were preserved in amber deposits in Kachin State, Myanmar, and provide a record of a forest ecosystem that existed during the Cretaceous. The authors provide detailed three-dimensional anatomy, which is usually unavailable for toads and frogs from this time period. The associated fauna preserved with the frogs includes plants, spiders, insects and marine mollusks. The research provides evidence that the frogs lived in a humid, warm, tropical forest ecosystem that contained at least some freshwater habitats.

The authors also compared the fossils to CT-scans of living species and published descriptions of extinct species of frog. They observed physical similarities between Electrorana and certain species that inhabit temperate regions today, which suggest that the ancestors of today’s frogs may have occupied a larger diversity of habitats.