Frogs preserved in amber are the oldest known that lived in tropical forests

Electrorana limoae
Electrorana limoae

An article published in the journal “Scientific Reports” describes a research that brings evidence of the oldest presence of frogs in the tropical forests of today’s Myanmar. A team of paleontologists examined four specimens of a 99 million year-old frog of a species that was called Electrorana limoae which were preserved in amber.

The study of the origins and evolution of frogs is complex because they’re small and delicate animals whose fossils have problems to preserve well. For this reason, the species that lived in arid environments, the most robust, are better known even if today they’re present mainly in tropical forests. Basically, we know that frogs existed for at least 200 million years but we only know a part of their history.

In 2015, four specimens of frogs preserved in amber were purchased in northern Myanmar. Fossil frogs were found preserved in amber in two cases in the Dominican Republic. In Myanmar many fossil insects were discovered thanks to the fact that they were preserved in amber, such as the Linguamyrmex vladi, but in this case they were frogs, even if they have the size of insects because they’re long about a couple of centimeters (less than an inch).

Amber made it possible to preserve those frogs, which were submitted to an X-ray micro-CT scan, a version of the scan normally used in the medical field adapted to studies such as paleontology. The holotype – meaning the specimen on which the original description of the species is based – of Electrorana limoae probably hadn’t finished its growth.

The image (courtesy Xing et al, doi: 10.1038/s41598-018-26848-w) shows the holotype of Electrorana limoae in the photo A and three other specimens in the other photos. The specimens shown in the photos B and D are presented in two different views. In the photo D the oval surrounds the fossil frog specimen.

The examination of the results of the micro-CT scans suggested a possible relationship of Electrorana limoae with living species such as the common midwife toad (Alytes obstetricans). This raised perplexity in the researchers because this modern frog lives in temperate climates such as the European ones, not in tropical climates.

Probably there are still too many holes in our knowledge of the evolution of frogs to understand the correct relationships among the various species of frogs, both extinct and current ones. Precisely for this reason, each new discovery is important and the specimens preserved in amber are often the best to get new information.

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