The fossil is one of four that date to the Cretaceous period
At first glance, the oddly shaped splotches darkening a vibrant yellow piece of amber appear to be amorphous blobs. But upon closer examination, a shape emerges.
There are two forelimbs. At the end of each limb are four smaller bones, forming a distinctly handlike shape. Within the largest dark spot, which has a rounded top, eye sockets become distinguishable. It's a skull.
Encased within the smooth chunk of amber is the body of a tiny young tropical frog. Scientists say the diminutive critter, measuring less than an inch long, lived about 99 million years ago before it became entombed in sticky tree sap. At the time, dinosaurs still roamed the Earth.
The fossil is one of four that date to the Cretaceous period, providing scientists with the earliest direct evidence that frogs inhabited wet, tropical forests, according to a statement from the Florida Museum of Natural History. The findings were published Thursday in Nature's Scientific Reports.
"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," the study's co-author David Blackburn, the associate curator of herpetology at the Florida Museum of Natural History, said in a statement. "This is pretty special."
The fossils included one skeleton of a frog that was complete enough for scientists to identify it as a new species, named Electrorana limoae, the study said. The other fossils contain two hands and an imprint of a frog that probably decayed inside the amber.
"Honestly, I'm still astonished that there are even frogs found in amber," Blackburn told Gizmodo. "It is a remarkable experience holding up these small gems and seeing the parts of frogs that look like they might have been left there last month."
While frogs have been around for more than 200 million years, their fossil record is slim and usually skewed toward species from arid, seasonal environments, not tiny rain-forest dwellers, the statement said.
But now, with the discovery of the four small lumps of amber originally excavated from northern Myanmar, Blackburn said scientists know frogs have lived in rain-forest-like environments for at least 99 million years.
"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," he said.
Aside from giving scientists a view into the lives of ancient frogs, the fossils are also the oldest-known examples of frogs preserved in amber. Fossils previously found in the Dominican Republic dated back only about 40 million years.
The fossils were a "miracle" find, Lida Xing, the study's other author and a paleontologist from the China University of Geosciences in Beijing, told BBC News.
"In China, frogs, lizards and scorpions are called three treasures of amber," Xing said.
Beyond the rarity of the discovery, the fossils are critical to helping scientists learn more about the evolutionary history of frogs.
"Our comparisons of the skeleton of these new frog fossils indicate that these amber-preserved frogs were 'true frogs' and may represent one of the most ancient lineages seen today," Blackburn told Popular Science.
Despite everything they have learned from the minuscule remains, Blackburn said, there are still quite a few unanswered questions.
Many characteristics such as wrist bones, the pelvis, hip bones, the inner ear and the top of the backbone are used by herpetologists to figure out details of a frog's life and determine its relation to other frogs, the statement said. Unfortunately, in the recently discovered fossils, those crucial parts were either missing or had not yet fully developed in the young frog.
Blackburn said he continues to hold out hope that other frogs in amber will be discovered.
"We don't have a lot of single-species frog communities in forests," he said. "It seems extremely unlikely that there's only one. There could be a lot more fossils coming."
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