Amber deposits yield oldest evidence of frogs in wet, tropical forests

Tropical rainforests are home to the vast majority of the world’s frog species today. Yet frog fossils from these moist environments have been incredibly rare, largely because the small animals have tiny bones, which make preservation difficult, and the wet conditions usually lead to their quick decomposition. This lack of frog fossil records has made it hard for researchers to build a picture of the earlier ecosystems the amphibians may have occupied.

But now, within amber deposits in northern Myanmar, scientists have found four tiny frogs that they estimate became trapped within sticky tree resin some 99 million years ago. These are the oldest frog fossils known to have been preserved in amber, according to the team led by Lida Xing, an associate professor at the China University of Geosciences.

These fossils, together with other fossils of mosses, bamboo-like plants, aquatic spiders, velvet worms and a dinosaur tail, recovered from these Myanmar deposits, provide the first definitive evidence that the amber-trapped frogs were living in wet, tropical forests alongside dinosaurs some 99 million years ago. The Cretaceous ecosystem they occupied resembles the moist, tropical rainforests of today, the researchers report in a study published in Scientific Reports.

One of the four frogs, which was trapped in sap alongside an unidentified beetle, has a nearly intact skeleton: its skull, forelimbs, part of its backbone, and a partial hind limb are clearly visible in the amber. By using CT scans, the team created a 3D visualization of the frog, based on which they described it as a new, extinct species. They named it Electrorana limoae, its genus name derived from the Latin words electrum, meaning amber, and rana, meaning frog, while they named the species in honor of Mo Li, the woman who purchased the specimens and provided them for the study, the authors write.

“It’s almost unheard of to get a fossil frog from this time period that is small, has preservation of small bones and is mostly three-dimensional. This is pretty special,” co-author David Blackburn, the associate curator of herpetology at the Florida Museum of Natural History, U.S., said in a statement. “But what’s most exciting about this animal is its context. These frogs were part of a tropical ecosystem that, in some ways, might not have been that different to what we find today – minus the dinosaurs.”

Citation:

Xing, L., Stanley, E. L., Bai, M., & Blackburn, D. C. (2018). The earliest direct evidence of frogs in wet tropical forests from Cretaceous Burmese amber. Scientific Reports, 8(1), 8770.