Frogs are commonly found in wet tropical forests today. More than a third of nearly 7,000 species of frog, in fact, live in wet forests.

Frogs Lived In Wet Tropical Habitat For At Least 99 Million Years

Fossil record, however, has little direct evidence to show a longstanding association between these animals and their most common habitat today.

It is nearly impossible to learn much about early amphibians because of their small size and biology. They do not fossilize easily, and most of the remains of extinct species of frogs are long gone.

Fossil record for amphibians from wet tropical environments have been nearly nonexistent, which leaves paleontologists with few clues on the early evolution of frogs.

A juvenile frog trapped in sticky tree sap about 99 million years ago when the dinosaurs still roamed the Earth however, offers hints where these amphibians lived during the Cretaceous period.

The frog trapped in the amber is an extinct species now named Electrorana limoae. Its remains were one of the four fossils that now provide scientists with the earliest direct evidence that frogs lived in wet tropical forests before the mass extinction event that occurred at the end of the Cretaceous period.

David Blackburn, from the Florida Museum of Natural History, and colleagues who reported the discovery in a new study published in Scientific Reports on June 14 said that the discovery showed that frogs lived in wet tropical forest for at least 99 million years.

"Frogs have been around on earth for approximately 200 million years," said Blackburn. "How long have they been associated with these wet forests? Is it a recent phenomenon or an ancient one? These amber frog fossils indicate that this association extends back to at least 100 million years ago."

Amber Provides Record Of Ancient Ecosystems

The amber that was found during excavations in Myanmar, Southeast Asia, contains the frog's forelimbs, head, part of its spines, and part of one of its hind limbs.

The ribs and bone found in the cartilaginous plate supporting the tongue hint that Electrorana closely resembled some of the modern-day species, such as the midwife toads and fire-bellied toads. Interestingly, these species now live in temperate, not tropical ecosystems.

The amber deposits also provide record of ancient forest ecosystems, as it contains fossil evidence of aquatic spiders, mosses, and bamboo-like plants.

"These amber deposits represent an excellent opportunity for discovering three-dimensionally preserved small vertebrates with a rich associated paleoecological context," Blackburn and colleagues wrote in their study. "These Burmese fossils provide the earliest direct evidence of anurans in a wet tropical forest ecosystem."

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