More than a third of the 7,000-odd living species of frogs and toads are found in rain forests around the world. But the fossil record for amphibians from these kinds of wet, tropical environments has been almost nonexistent, leaving paleontologists with few clues to their early evolution. Now, lumps of amber dating back to the Cretaceous period have revealed a set of four tiny tropical frogs that lived alongside the dinosaurs, making them the oldest frog fossils of their kind. The specimens include the remains of an ancient frog complete enough to be described as a new species, called Electrorana limoae.

“It was exhilarating to hold these small fossils up to the light to reveal the frogs within,” says David Blackburn, a paleontologist at the Florida Museum of Natural History in Gainesville. “We have few small and intact fossil frogs, and the primary specimen of Electrorana is a rare find.”

In life, all of these frogs would have been less than an inch long, according to a paper describing the fossils today in Scientific Reports and led by National Geographic Explorer Lida Xing of the China University of Geosciences in Beijing. (Recently, scientists found the origins of the disease that’s wiping out modern amphibians.)

“Lizards and frogs in amber are certainly not unheard of, but ones this old are exceptional,” says Marc Jones, an expert on fossil frogs based at the Natural History Museum in London, U.K. “The frog fossil record remains biased and patchy but does include the occasional gem, like this, that helps us appreciate what we are missing.”

The 99-million-year-old frogs come from the same amber deposits in northern Myanmar that have produced many exquisite fossils, including a dinosaur tail, a couple of baby birds, intact bird wings, and countless insects. Bits of bamboo, velvet worms, and aquatic spiders also found in this amber suggest that the Cretaceous environment was a rain forest, since similar species are commonly found in wet tropical forests today.

The Dexu Institute of Paleontology in Chaozhou acquired the frog specimens as donations from private Chinese fossil collectors. The institute had three of the fossils for some years, Xing says, but they contained only frog forelimbs and the impression of a headless body missing its skeleton. A “miracle” donation of a larger and more complete specimen in 2010 made their latest research possible.

CT scans revealed much more about the three-dimensional structure and internal anatomy of the fossils, including evidence that Electrorana was similar to modern frogs in many ways. The animal appears to be an ancient member of one of the oldest lineages of living frogs, represented by modern species such as fire-bellied toads and midwife toads.

“Whilst Electrorana doesn’t preserve much soft tissue, unlike some amazing lizard specimens from the same deposits, its well-preserved skeleton represents the oldest record of a frog from a tropical forest, which is a very important modern habitat for frogs,” says Michael Pittman, a paleontologist at the University of Hong Kong.


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