Now, scientists have discovered an amber fossil of a juvenile frog in present-day Myanmar dating back about 99 million years.Although the species is extinct, it has now been named Electrorana limoae and is one of four fossils that proves ancient frogs lived in wet tropical forests.

“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,” said Dr David Blackburn.

The best-preserved fossil of the group is of Electrorana limoae. Pic: Lida Xing/University of Geosciences

Dr Blackburn, who is the study’s co-author and the associate curator of herpetology at the Florida Museum of Natural History, added: “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.”

The findings and species description have been published in Nature’s Scientific Reports, alongside a CT scan of the amber fossil.They state that while frogs have been around for at least 200 million years, it’s difficult to know what their early evolutionary forms were like because their fossils do not tend to be well preserved.This means that the fossil record for frogs is skewed towards more robust species which live in arid and seasonal environments, despite the bulk of today’s frog species living in tropical forests.

“Ask any kid what lives in a rainforest, and frogs are on the list,” said Dr Blackburn, “but surprisingly, we have almost nothing from the fossil record to say that’s a longstanding association.”

Fortunately the amber deposits of northern Myanmar in southeast Asia provide a rare glimpse into the ecosystems of ancient forests.

Scientists have found fossil evidence of mosses as well as bamboo-like plants, and more complicated life including aquatic spiders and velvet worms in the amber.Electrorana and the other fossils are the first frogs to be recovered from these deposits and show how frogs inhabited wet, tropical forests during the Cretaceous period.

Although less than an inch long, Electrorana is the most well-preserved of this group of fossils.The frog’s skull is clearly visible through the amber, as are its forelimbs and part of its backbone, as well as a partial hind limb and the unidentified beetle.

But the fossil of the frog raises more questions than it answers, said Dr Blackburn.

Many of the characteristics which herpetologists use to analyse a frog’s evolutionary and life history – the wrist bones, the pelvis, hip bones, the inner ear, and the top of the backbone – are either missing from the fossil or were not yet fully developed in the juvenile frog.

Although the bones that the team are able to see do provide some clues about Electrorana’s possible living relatives, the results are puzzling.Dr Blackburn said that species which have similar features include fire-bellied toads and midwife toads – Eurasian species that live in temperate and not tropical ecosystems.

Gathering CT skeletal data for both living and extinct frogs, one of Dr Blackburn’s long-term projects, is intended to reveal their ancient evolutionary relationships.

@WFS,World Fossil Society,Riffin T Sajeev,Russel T Sajeev