The oldest tadpole fossil ever has been found—and it’s a big one
The discovery in Patagonia shows that frogs have had a tadpole stage for at least 160 million years. Here’s how the ancient frogs compare to those of today.
A newly discovered frog fossil represents the oldest tadpole ever found, and it looks remarkably like the tadpoles you’re probably familiar with except for one thing—it was a giant.
Paleontologist Federico Agnolín and his colleagues discovered the fossil by accident. The team had been scouring the same quarry at Estancia La Matilde in the Santa Cruz province of Argentina for dinosaurs. They hoped that the fine sediment and volcanic ash that formed the quarry’s Jurassic-period rock may have preserved the imprint of soft tissues never discovered before. Yet instead of dinos, they kept finding frogs.
The frogs were all adults from the same extinct species, Notobatrachus degiustoi. That led some researchers to speculate that perhaps in those days, frogs didn’t have a tadpole stage yet. Yet in January 2020, a team member picked up a rock during a break and found a more than six inches long imprint of a tadpole revealing exquisite details of its gills, eyes, and even some nerves.
The researchers who described the fossil in Nature today estimate it to be between 161 to 168 million years old, beating the previous record holder by around 30 million years. The find provides solid evidence frogs have had a tadpole stage for at least that long. “It’s a beautiful confirmation of what many experts had suspected,” says herpetologist Alexander Haas of the Leibniz Institute in Bonn, Germany. Reconstructing tadpole evolution based on their diversity today, Haas and others previously predicted that tadpoles would have existed this early on.
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Since he wasn’t an expert in frogs himself, Agnolín sought help from biologist Mariana Chuliver, who like him is based at the Fundación Félix de Azara in Buenos Aires, Argentina, and had studied tadpole development in frogs before. When looking at the fossil under the microscope, she found the cartilage supporting its gills to be surprisingly similar to that of tadpoles alive today.
Like its present-day brethren, tadpoles of this extinct species would have sucked in water and then pushed it out across their gills, filtering out food and absorbing oxygen all in one go. This suggests it probably wasn’t feeding on the small shellfish, insects and crustaceans also found as fossils in these rocks, but rather on micro-organisms and organic debris floating in the water, says Chuliver.
A ribbiting paradox
The ancient tadpoles probably resembled some modern species in size and feeding strategy. Bullfrog tadpoles scrape algae of stones and then suck them in, and they also occasionally grow to a large size. Meanwhile, the tadpoles of so-called paradoxical frogs (Pseudis paradoxa), an intriguing species also from Argentina, grow to nearly eight inches long, while adults barely exceed two inches.
Most other frogs tend to be largest in the adult phase of their lives. Why exactly N. degiustoi tadpoles grew and stayed so large is unclear. “I’m surprised by the exceptional preservation of this tadpole,” says zoologist Marissa Fabrezi of the Universidad Nacional de Salta in Argentina, who has studied paradoxical frogs. “It’s difficult to explain the size of giant tadpoles. This is important for understanding their evolution.”
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Given the well-developed cartilage and even some bone structures imprinted on the rock, the fossilized tadpole was close to metamorphosis, says Chuliver. So it probably would not have grown bigger than the adults, which tend to be roughly the same length as the tadpole in the quarry’s fossil record. Yet modern paradoxical frogs may still provide clues to why N. degiustoi’s tadpoles grew so large.
Like the fossilized frogs, paradoxical frogs live in temporary pools that dry up when there isn’t enough rain, which means they don’t face much competition or predation from fish. This allows the tadpoles to stay in the tadpole stage for longer and grow larger before they metamorphose into adults, making maximum use of the food available in the pools before moving on to terrestrial snacks.
The discovery of this more than 160-million-year-old tadpole is a testament to the success of the frog’s way of life, says Agnolín. Today, however, many frogs are struggling, as their dependence on both land and water habitats makes them twice as vulnerable to human disturbance. “The very same metamorphosis that made them successful now makes them more prone to extinction,” says Agnolín.
