NEW YORK (AP) — Scientists have discovered the oldest fossil of a giant tadpole which wriggled more than 160 million years ago.

The new fossilfound in Argentina, is about 20 million years older than the previous ancient record holder.

Parts of the tadpole’s skull and spine are printed into a sandstone slab, along with imprints of its eyes and nerves.

“It is not only the oldest known tadpole, but also the best preserved,” said study author Mariana Chuliver, a biologist at Maimonides University in Buenos Aires.

Researchers know that frogs were hopping 217 million years ago. But it’s unclear exactly how and when they evolved to start as tadpoles.

This new discovery adds some clarity to this timeline. Measuring about 16 centimeters long, the tadpole is a younger version of an extinct giant frog.

“It starts to help reduce the time frame in which a frog becomes a frog,” said Ben Kligman, a paleontologist at the Smithsonian National Museum of Natural History who was not involved in the research.

The results were published Wednesday in the journal Nature.

The fossil looks strikingly similar to today’s tadpoles – even containing the remains of a gill scaffolding system that today’s tadpoles use to filter food particles from the water.

This means that amphibians’ survival strategy has remained proven for millions of years, helping them survive several mass extinctions, Kligman said.

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