Scientists Unearth the Oldest Tadpole Fossil Ever Found, and It’s a 161-Million-Year-Old ‘Giant’

While in search of dinosaur fossils, a team of Argentine and Chinese scientists in Argentina found something else instead: parts of a tadpole’s skull and backbone, preserved in a slab of sandstone. More surprisingly, the amphibian fossil turned out to be 161 million years old, breaking the record for the oldest known tadpole by about 20 million years.

The oldest recorded fossils of adult frogs date back to around 217 million years ago, in the Late Triassic Period, but the oldest preserved tadpoles had been from roughly 140 million years ago, in the Cretaceous. The remains, uncovered in January 2020 and described in the journal Nature on Wednesday, shed light on the history of frog metamorphosis.

“The paleontologists found hundreds of adult specimens of the basal ghost frog, the ancestor of frogs and toads,” says study lead author Mariana Chuliver, a biologist at Maimónides University in Argentina, to Popular Science’s Laura Baisas. “After many days of digging, one team member found a stone with a particular imprint on it, and it was a fossil tadpole!”

Although adult frog fossils are difficult to find, tadpoles are even harder. Fossil frogs tend to be small and fragile to begin with, reports the New York Times’ Asher Elbein. But tadpoles’ bodies are mostly made of cartilage, and they contain a lot of soft tissue, Chuliver tells the outlet.

That made this particular finding an even bigger shock: “It’s not only the oldest tadpole known, but also the most exquisitely preserved,” Chuliver tells Adithi Ramakrishnan of the Associated Press (AP).

A paper in @Nature describes a fossil tadpole from around 161 million years ago, which is the oldest known tadpole reported to date. https://t.co/x4fvENV9PL pic.twitter.com/wPgymSyJCE

— Nature Portfolio (@NaturePortfolio) October 30, 2024

The tadpole, belonging to the species Notobatrachus degiustoi, is an unusual six inches long. According to the study, both tadpoles and frogs of this species are classified as “giants.” Most tadpoles are smaller than mature frogs, but this one was the same length as one of the adults.

“That feature is really, really hard to find in nature today,” Chuliver tells the New York Times. She suggests the large size was most likely due to the tadpoles having no competitors in their seasonal ponds. The young frogs had access to plenty of food resources, allowing them to grow more.

The findings add to the fossil evidence of the life cycle of frogs, showing that they had a tadpole stage for at least 161 million years. “It’s a beautiful confirmation of what many experts had suspected,” says Alexander Haas, a herpetologist at the Leibniz Institute in Germany who was not involved with the study, to National Geographic’s Tim Vernimmen.

Previously, some researchers thought the age gap between the first known frogs and the oldest known tadpoles offered evidence that frogs’ ancestors lacked a tadpole stage and metamorphosis, Chuliver tells the New York Times.

Now, the discovery sheds light on the evolutionary timeline and helps “narrow the timeframe in which a frog becomes a frog,” says Ben Kligman, a paleontologist at the Smithsonian’s National Museum of Natural History who was not involved with the research, to the AP.

For Federico Agnolín, a paleontologist at the Argentine Museum of Natural Sciences who was part of the team that found the fossil, the discovery is testament to the success of the frog life cycle, per National Geographic. It shows that the creatures have maintained their life processes for much longer than thought.

Amphibians are considered indicator species for their ecosystems and tend to be the first ones affected by shifting conditions. Today, more than 40 percent of amphibians are at risk of going extinct. They’re threatened by habitat destruction, invasive species, diseases, climate change and pollution. By depending on both water and land, they are vulnerable to disturbances and changes in either setting.

“The very same metamorphosis that made them successful now makes them more prone to extinction,” Agnolín says to National Geographic.

Alexa Robles-Gil | | READ MORE

Alexa Robles-Gil is a bilingual science journalist based in New York City. Her work has appeared in the Guardian, Undark, Inside Climate News and more.