There is a group of vertebrates, called anurans, which includes toads, escuerzos and frogs. They are animals with a life cycle in very different stages: an aquatic larva, or tadpole, and a generally terrestrial adult. But what happens when you want to study these processes in species dating back millions of years?
A research group from the Argentine Museum of Natural Sciences (MACN), the Azara Foundation in Argentina and the Chinese Academy of Sciences has made a paleontological discovery that helps to understand the evolution of the life cycle of frogs and toads. The fossil in question corresponds to a 165 million year old tadpole (from the Jurassic period). It was discovered at Estancia La Matilde, located in the north-eastern sector of the province of Santa Cruz, about 100 kilometers from Puerto Deseado, Argentina.
“The species in question belongs to an ancestor of anurans, Notobatrachus degiustoi. The scarcity of tadpoles in the fossil record has made the origins and initial evolution of the larval phase enigmatic,” Federico Agnolín, co-author of the work and independent researcher at the National Council for Scientific and Technical Research (CONICET), explains to CTyS. -UNLaM Agency of Argentina. The specimen found is so well preserved that the contours of the body, eyes, nerves and even the hyobranchial apparatus can be observed.
The tadpole measured about 16 centimeters total at the waist, well above the size of most living tadpoles. Furthermore, it was almost the same length as the adults of the species. This allows researchers to say that both stages of development have reached large sizes. Gigantism in tadpoles, apparently, was also present in the ancestors of anurans.
“This specimen has a double significance. On the one hand it corresponds to the oldest evidence of a fossil tadpole in the world. On the other hand, it stands out for its exceptional conservation. Tadpoles are animals with a soft and poorly ossified body, which makes their fossilization very difficult,” explains Marian Chuliver, researcher at the CCNAA-Azara Foundation and first author of the research, in a press release.
One of the key contributions of this work is data on the evolutionary lineage of these species. The analysis of the family relationships of the larvae of current and fossil amphibians has allowed us to place the fossil tadpole of Notobatrachus very close to the group that includes all current anurans. The analyzes could be carried out thanks to the use of a supercomputer from the High Performance Computing Center of the National University of Córdoba in Argentina.
Fossil remains investigated. (Image: Research Team)
“The kinship relationships found for the Notobatrachus tadpole were as expected if we consider the anatomy of adults. What was a big surprise was the great resemblance the new specimen bears to some of the tadpoles living today. These analyzes show that the shape of the larval body of anurans has undergone relatively few changes over the last 160 million years,” explains Martín Ezcurra, one of the authors of the study and researcher at MACN.
The species Notobatrachus degiustoi is a distant precursor of anurans. It has been known since 1957, from the description of numerous skeletons of adult individuals also found at the La Matilde ranch.
“Studies on Notobatrachus began at the end of 1950, by the great Argentine paleontologist Osvaldo Reig. At the time, his discovery overturned everything that was thought about the evolution of frogs. Furthermore, it demonstrated that South America was a key stage in the early evolution of the group” explains Agnolín.
How did the discovery of the new specimen happen? In a somewhat random way. As detailed in the research team members’ press release, in January 2020, a working group led by researchers Fernando Novas (CONICET) and Xu Xing of the (Chinese Academy of Sciences) had begun carrying out explorations in Santa Cruz province at search for fossils of “feathered dinosaurs”.
Even though no dinosaurs were found, this great discovery was made: paleontologist Matías Motta, a postdoctoral fellow at CONICET, discovered a slab with a very particular imprint. It was a complete tadpole of Notobatrachus degiustoi which preserved the body with remains of the skull, much of the postcranial skeleton and part of the tail.
The discovery was presented publicly through the academic journal Nature. (Source: Nicolás Camargo Lescano / CTyS-UNLaM Agency)
Time.news Interview: Unraveling the Secrets of Ancient Tadpoles
Interviewer: Welcome, everyone, to this special edition of Time.news. Today, we have the pleasure of speaking with Dr. Federico Agnolín, an independent researcher at CONICET and co-author of a groundbreaking study on a 165-million-year-old tadpole from Argentina. Federico, thank you for joining us!
Federico Agnolín: Thank you for having me! I’m excited to share our discoveries with you.
Interviewer: Let’s dive right in. Can you explain to our viewers what makes this fossil discovery so significant in understanding the evolution of frogs and toads?
Federico Agnolín: Absolutely. The fossil we uncovered belongs to a species called Notobatrachus degiustoi, an ancestor of modern anurans. What’s fascinating is that this tadpole is the oldest evidence of its kind found so far. Generally, tadpoles are underrepresented in the fossil record because their bodies are soft and less likely to fossilize, making our find particularly rare and valuable.
Interviewer: That’s remarkable! What can you tell us about the size of the tadpole? It seems it was quite large for its time.
Federico Agnolín: Indeed! This tadpole measured about 16 centimeters in length, which is significantly larger than most contemporary tadpoles. In fact, it was almost the same size as the adult form of the species. This suggests that gigantism in tadpoles was a trait present in the ancestors of anurans, a feature that’s pretty unexpected.
Interviewer: Fascinating! Moving on to the preservation of this fossil—how did that play a role in your analysis?
Federico Agnolín: The exceptional preservation of the specimen allows us to observe intricate details such as the body contours, eyes, nerves, and even the hyobranchial apparatus. This clarity has enabled us to make more accurate comparisons between ancient and modern amphibians, shedding light on their evolutionary pathways.
Interviewer: That’s impressive! You mentioned earlier that this fossil helps place the Notobatrachus tadpole within the lineage of current anurans. Can you elaborate on that?
Federico Agnolín: Certainly. Through our analyses, particularly using advanced computational tools from the High Performance Computing Center at the National University of Córdoba, we were able to determine the evolutionary relationships between this fossil tadpole and modern amphibians. The results showed that this tadpole shares many characteristics with today’s anurans, which reinforces our understanding of their lineage and developmental stages.
Interviewer: It sounds like this discovery is revolutionizing our understanding of amphibian evolution. Were there any surprises during your research?
Federico Agnolín: Yes, there were! One of the most exciting findings was how closely the fossil tadpole resembles some of the tadpoles we see today. We had anticipated certain kinship relationships based on adult anatomy, but the surprising physical similarities highlighted a remarkable continuity in form that has persisted across millions of years.
Interviewer: This is certainly a breakthrough for paleontology! Looking ahead, how do you see this research impacting future studies in this field?
Federico Agnolín: I believe this discovery will encourage further exploration of the fossil record, particularly of tadpoles and other underrepresented life stages. It could lead to new insights into the evolutionary history of amphibians and perhaps even other vertebrate groups.
Interviewer: Thank you, Federico, for shedding light on this incredible discovery and its implications for our understanding of evolution. We look forward to seeing how your research progresses!
Federico Agnolín: Thank you! I appreciate the opportunity to share our work.
