NEW YORK — Researchers have made a remarkable discovery of what is believed to be the oldest known fossil of a giant tadpole, dating back over 160 million years. This newly unearthed fossil, located in Argentina, surpasses the previous record-holder by approximately 20 million years, marking a significant advancement in our understanding of amphibian history.

The fossilized remains are embedded in a slab of sandstone, showcasing parts of the tadpole’s skull and backbone, in addition to impressions of its eyes and nervous system. Mariana Chuliver, a biologist from Maimonides University in Buenos Aires and one of the study’s authors, emphasized the rarity of this find, stating, “It’s not only the oldest tadpole known, but also the most exquisitely preserved.”

While it is established that frogs have been present on Earth for at least 217 million years, the specifics of how and when their life cycle transitioned from tadpole to adult form remain somewhat of a mystery. This discovery sheds some light on that evolutionary process. The tadpole, measuring about half a foot in length (16 centimeters), represents a juvenile form of an extinct species of giant frog.

Ben Kligman, a paleontologist affiliated with the Smithsonian National Museum of Natural History, noted that this fossil helps to refine the timeline for the evolution of frogs, commenting, “It’s starting to help narrow the timeframe in which a frog becomes a frog.” The findings were released in a recent publication in the journal Nature.

The fossil exhibits striking similarities to contemporary tadpoles, including remnants of a gill scaffold system, which modern tadpoles utilize to filter food particles from water. This suggests that the survival strategies employed by amphibians have proven effective over millions of years, allowing them to survive various mass extinction events throughout Earth’s history, according to Kligman.