Oldest-known tadpole fossil discovered in Argentina reveals early frog evolution

Scientists have discovered the oldest-known fossil of a tadpole, dating back approximately 161 million years to the Middle Jurassic period

Scientists have discovered the oldest-known fossil of a tadpole, Notobatrachus degiustoi, dating back approximately 161 million years to the Middle Jurassic period. Details of this "exquisitely preserved" 16-centimeter-long larva, discovered in the La Matilde Formation in Patagonia, Argentina, were published in the scientific journal Nature. Lead researcher Mariana Chuliver, a biologist and herpetologist at Maimonides University in Argentina, expressed surprise at the fossil's exceptional preservation, noting it includes soft tissues not usually seen in fossils, such as nerves and eyes.

"This is not only the oldest known tadpole fossil but also the 'finest preserved' among such fossils," Chuliver said, as reported by Xinhua News Agency. The fossil includes parts of the tadpole's skull, backbone, eyes, and nerves, as well as soft organs like cranial nerves and a cartilaginous branchial skeleton. The preservation of these soft tissues provides unique insights into the evolutionary history of anurans, the group that includes frogs and toads, and clarifies aspects of their life cycle evolution.

The tadpole measures approximately 16 centimeters in length, placing it well above the size of most living tadpoles. It is a younger version of an extinct giant frog species, with its skeleton visible and parts of the soft tissues preserved, according to The Guardian. The adult frogs of Notobatrachus degiustoi were also considered giants, growing to between 9 and 15 centimeters in size, and lived in shallow ponds that dried out and reformed with the seasons.

The discovery is significant because, until now, scientists had never unearthed tadpole fossils from before the Cretaceous Period, which began around 145 million years ago, according to Newsweek. This new fossil pushes back the known existence of tadpoles by about 20 million years, as no older tadpole fossils have been found. "The timing of when frog metamorphosis first evolved has been a long-standing puzzle," said Ben Kligman, a paleontologist at the Smithsonian National Museum of Natural History who was not involved with the research. "It's starting to help narrow the timeframe in which a frog becomes a frog," he added, according to The Guardian.

The fossil shows that today's tadpoles have not changed to any significant degree from their ancestors in the Jurassic era. The morphology of tadpoles has remained largely unchanged for millions of years, demonstrating minimal changes in tadpole morphology since that period, according to Reuters. This finding reveals that a biphasic life cycle, with filter-feeding tadpoles inhabiting aquatic ephemeral environments, was already present in the early evolutionary history of anurans and has remained stable for at least 161 million years.

Chuliver explained, "We now know that this filter feeding system was present since the beginning of the group," as reported by The New York Times. The fossil even contains remnants of a gill scaffold system that modern-day tadpoles use to sift food particles from water, indicating that the specialized filter-feeding anatomy had already evolved in this fossilized tadpole, according to ABC News. Agustín Scanferla, a paleontologist and researcher who studied the specimen with Chuliver, added, "The exquisite preservation of the tadpole's hyoid skeleton allows us to confirm that it fed by filtration, trapping food particles from the water, a mechanism executed by the continuous action of a buccal pump, just as occurs in tadpoles of current species," according to La Nacion.

The fossil also provides evidence that both the juvenile and adult stages of Notobatrachus degiustoi reached large sizes, indicating that gigantism in tadpoles was present in the ancestors of anurans. "Gigantism of tadpoles was probably related to a long development and the absence of predators," Chuliver explained. "Moreover, the high availability of food resources, and the absence of competitors such as fish or other anuran species were also key factors for the tadpoles to grow giant," she continued, as reported by Newsweek.

The discovery was made in January 2020 during a joint expedition searching for dinosaur fossils in the La Matilde Formation in Santa Cruz Province, Argentina. Paleontologists found the remains of the tadpole and then took them to the laboratory for further study, according to Infobae. The research team included scientists from Maimonides University, the Argentine Museum of Natural Sciences, and the Chinese Academy of Sciences.

Notobatrachus degiustoi thrived in a warm, tropical climate interspersed with seasonal dry and wet phases during the Middle Jurassic period. The species coexisted with small mammals, dinosaurs, and freshwater shrimp. The fossil was found encased in volcanic ash and silt, which preserved its delicate structures in exceptional detail. "The deposition of these sediments promoted an exquisite preservation of the specimen, which lasted more than 160 million years," Chuliver said, according to Newsweek.

This discovery is a significant breakthrough in understanding the evolution of frogs and toads, as it provides the first evidence for the presence of a tadpole followed by a drastic metamorphosis in the life cycle of anurans from the very beginning of their evolutionary history, according to the Australian Broadcasting Corporation. It helps refine the evolutionary timeline and adds to the understanding of the origins and endurance of the biphasic life cycle seen in frogs today.

"Until now, no tadpoles were known for such ancient frogs, so some researchers postulated that they could have had direct development to the adult," Chuliver said, as reported by La Nacion. "By discovering this new fossil, we demonstrated that was not true," she added, according to VICE. The presence of a tadpole suggests that metamorphosis was present in frogs as we know them today from the beginning.

The researchers hope that this finding will encourage further exploration and discoveries that can shed more light on the early evolution of frogs and their life cycles. "With this information and more specimens, we will be able to learn more about the anatomy and the different stages of development," Scanferla concluded, as reported by La Nacion.

Sources: Primera Hora, VICE, Aftonbladet, Folha de S.Paulo, EL DEBER, Australian Broadcasting Corporation, New York Post, AP NEWS, The Independent, The Guardian, Reuters, The New York Times, The National Geographic Society, La Nacion, Infobae, Clarin, Asharq News, Newsweek, ABC News, Popular Science

This article was written in collaboration with generative AI company Alchemiq