Roughly between 161 and 168 million years ago, a “giant” tadpole met an untimely end, sinking to the bottom of a pond. After millennia, its fossilized trace was exhumed by researchers in Patagonia (Argentina). The Jurassic specimen, described in Nature on October 30, 2024, sheds new light on the early evolution of amphibians. For good reason, it is the oldest known tadpole – and the best preserved, according to specialists –, surpassing the previous holder of the ancient record by around 30 million years.
Secrets of an extinct “giant” species
The fossil was discovered by chance. Paleontologists then searched the fine sediments and volcanic ashes of the La Matilde formation of the Deseado massif (southern Patagonia, Santa Cruz province). They hoped to stumble upon rare preserved remains of dinosaur soft tissues. Instead, they found those of “giant” frogs measuring between 9 and 15 centimeters, of the extinct species Notobatrachus degiustoi. All specimens were adults, leading some researchers to speculate that during the Jurassic, these amphibians did not experience the tadpole stage of development.
But in January 2020, a member of the team picked up a singular stone: it included the imprint of a tadpole measuring 16 centimeters long from head to tail, including details of the gills, eyes and even some nerves were preserved. A discovery that provides strong evidence that even in their earliest evolutionary stages, frogs and toads experienced the “tadpole” life stage. “It’s like a Mona Lisaa masterpiece of the art of evolution”enthuses to the American Broadcasting Company (ABC) Professor John Long, a paleontologist at Flinders University who did not participate in the research, about the fossil.
Even more, by studying it under a microscope, the study authors discovered that the cartilage supporting the animal’s gills was surprisingly similar to that of modern tadpoles. In the past, it would have sucked in and then exhaled water from the shallow ponds of its tropical habitat through its gills, thus filtering food and absorbing oxygen in one go.
The little one of Notobatrachus degiustoi therefore probably did not feed on the tiny shells, insects and crustaceans found as fossils in the rocks of La Matilde. But rather, from microorganisms and organic debris floating in the water. Even today, the larvae of the bullfrog (Lithobates catesbeianus) scrape the stones from the stones to suck them up, for example.
Explaining the Evolution of Giant Tadpoles
Besides adopting similar feeding strategies, ancient tadpoles probably also resembled some modern species in their “giant” size. The larvae of the current paradoxical frogs (Pseudis paradoxa), also native to Argentina, can reach more than 20 centimeters in length, before shrinking to a “normal” size of barely 5 centimeters once adults. This difference between the two ages, which has intrigued scientists for a long time, could be observable in Notobatrachus degiustoi. However, one thing is wrong: adults of the species are also classified as “giants”.
Scientists therefore suspect that the recently identified 16-centimeter tadpole could, given its well-developed cartilage, have been on the verge of metamorphosis. It probably would not have grown any larger than the adults, whose fossils are about the same length in the formation. The species could therefore not present the same specificity as Pseudis paradoxa. But the latter may still provide clues as to why the tadpoles of Notobatrachus degiustoi have become so big.
Paradox frogs live in shallow temporary pools that dry out and reform with the seasons. They do not face much predation from fish. This allows the young to stay in the tadpole stage longer, making maximum use of the food available in the ponds before metamorphosing into adults and moving on to terrestrial snacks. The discovery of a tadpole more than 160 million years old shows that this evolutionary strategy remained approved for years, helping amphibians survive several mass extinctions.
However, this transition from a larval to adult form now accentuates the difficulties of certain species: their dependence on both terrestrial and aquatic habitats makes them twice as vulnerable to human disturbance. “The same metamorphosis that made them successful today makes them more vulnerable to extinction,” concludes paleontologist Federico Agnolín, whose team is behind the discovery of the informative fossil, to National Geographic.
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