Scientists have discovered the oldest fossil of a tadpole that lived more than 160 million years ago.
A fossil found in Argentina is older than the previous record holder by about 20 million years.
Parts of the tadpole's skull and spine, along with impressions of eyes and nerves, are embedded in the sandstone slab.
"It is not only the oldest known tadpole, but also the best preserved," said Mariana Čuliver, a biologist at Maimonides University in Buenos Aires.
About 16 centimeters long, the tadpole fossil is a younger version of the extinct giant frog.
Researchers know that these frogs were around 217 million years ago. But exactly how and when they evolved is unclear, and the new discovery adds clarity to that timeline.
"It helps narrow down the time frame in which a frog became a frog," said Ben Kligman, a paleontologist at the Smithsonian National Museum of Natural History in Washington.
The fossil is strikingly similar to present-day tadpoles, and has the remains of the gill system that modern tadpoles use to extract food particles from water.
This means the survival strategy of amphibians has remained unchanged for millions of years, helping them survive several mass extinctions, Kligma said.
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