Two of the four frogs found in amber (the specimen on the right shows two views).Image: Lida Xing
The extraordinary discovery of four small frogs preserved in amber is providing the earliest evidence of these now-prolific amphibians living in tropical rainforests.
New research published today Scientific Reports shows that frogs — an animal that first emerged some 200 million years ago — were occupying soggy forested regions at least 100 million years ago. This discovery is a big deal because fossils of forest amphibians are rare, and because scientists haven't been sure when frogs first started to venture into tropical habitats.
"Frogs are common animals to encounter in the wet tropical forests of today, and easily more than a third of the nearly 7,000 species of frogs live in these wet forests," David C Blackburn, a co-author of the new study and the associate curator of amphibians and reptiles at the Florida Museum of Natural History, told Gizmodo. "But being small and living in a tropical forest also means that your likelihood of ending up in the fossil record is pretty low."
Along with Lida Xing from the University of Geosciences in China, Blackburn analysed four exquisite amber fossils found in a deposit in northern Myanmar, formerly Burma. These amber fossils, along with others found nearby, also contained traces of plants, spiders, insects, and marine mollusks. For scientists like Blackburn and Xing, these tiny droplets of preserved tree sap are like a time machine, allowing them to visualise the ecosystems of ancient Cretaceous rainforests.
Artist's interpretation of the Cretaceous Period frog. Illustration: Damir G. Martin
"The forests in which this amber formed was probably a wet tropical forest with freshwater streams or ponds and located somewhere near the beach," said. Blackburn. "We know this through the diversity of invertebrates and plants found in other pieces of amber. In many ways, this tropical ecosystem seems to resemble wet tropical forests that we have today. But important aspects of the life of this frog are unknown, such as what sound it made, if it had a tadpole, or whether it climbed or jumped."
The four frog fossils were not dated directly. Instead, other amber specimens found within the same geological deposits were previously dated directly and indirectly, including traces of volcanic material and fossilized insects. A comparative analysis of the ancient frogs, dubbed Electrorana limoae, with similar species living today revealed more similarities than differences; it seems frogs haven't changed all that much across the millennia.
Frog from Cretaceous Burmese amber.Image: Lida Xing
"Our comparisons of the skeleton of these new frog fossils indicate that these amber-preserved frogs were 'true frogs' but may represent one of the most ancient lineages seen today," said Blackburn. "The presence of ribs and a bones in the cartilaginous plate that supports the tongue suggests an affinity with the species alive today, such as fire-bellied toads and midwife toads."
In addition to providing the earliest direct evidence of frogs in a wet tropical forest ecosystem, these are the oldest records of frogs preserved in amber. Prior, the oldest were from amber deposits found in the Dominican Republic, dated to around 25 million years ago. For Blackburn, it's a discovery that still blows his mind.
"Honestly, I'm still astonished that there are even frogs found in amber!" he said. "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."