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.
“It had decomposed mildly, but you could even observe the very good skeletal structure with the naked eye,” Xing says.
An illustration shows how the ancient frog may have become trapped in tree resin that went on to create the amber fossil.
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.
And a Beetle, Too
Given the quality and variety of fossils in these amber deposits, there might be future opportunities to study the diets of these frogs to see how they differ from living relatives, he adds.
Tantalizingly, the lump of amber encasing Electrorana also holds a beetle, hinting that it may have been a prey animal for the dino-era amphibian. (Also see the first know tick wrapped in spider silk found trapped in amber.)
The more complete frog was a juvenile with soft bones that didn’t fully fossilize, so the scientists are missing many aspects of the skeleton that might have given more insight into its behavior and ecology, such as the hip joints related to hopping and the inner ear bones.
However, Blackburn, who is one of the study authors, hopes that as more fossils are collected, they will find even better-preserved samples and be able to compare them with living frogs. That would allow scientists to ask more sophisticated questions about the way these ancient frogs lived and evolved.
“I can only hope that there are more spectacular fossils to come,” he says. “In today's tropical forests, there is a rich diversity of living frog species. So, there might be many more species to discover still in the Cretaceous amber from Myanmar.”
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