The discovery of a fossil tadpole from 165 million years ago provides clues about the evolution of frogs

There is a group of vertebrates, called anurans, which include toads, escuerzos and frogs. They are animals with a life cycle in very different phases: aquatic larva, or tadpole, and a generally terrestrial adult. But what happens when you want to study these processes in species from millions of years ago?

A research team 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, precisely, 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 northeastern sector of the province of Santa Cruz, about 100 kilometers from Puerto Deseado, in Argentina.

“The species in question belongs to an ancestor of the anurans, Notobatrachus degiustoi. The scarcity of tadpoles in the fossil record made the origins and early 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 the CTyS-UNLaM Agency. from Argentina. The specimen found is so well preserved that the outline of the body, the eyes, nerves, and even the hyobranchial apparatus can be observed.

The tadpole measured about 16 centimeters in total in life, well above the size of most living tadpoles. Furthermore, it was almost the same length as adults of the species. This allows the researchers to affirm that both stages of development reached large sizes. Gigantism in tadpoles, apparently, was also present in the ancestors of the anurans.

“This specimen has a double relevance. On the one hand, it corresponds to the oldest record of a fossil tadpole worldwide. On the other hand, it stands out for its exceptional preservation. Tadpoles are animals with a soft, poorly ossified body, which makes their fossilization very difficult,” details 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 the data on the evolutionary line of these species. An analysis of the kinship relationships of current and fossil amphibian larvae allowed us to place the fossil Notobatrachus tadpole 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 the adults. What was a great surprise was the great similarity that the new specimen has with some of the tadpoles that live today. These analyzes show that the larval body shape 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 the MACN.

The species Notobatrachus degiustoi is a distant precursor of the anurans. It has been known since 1957, from the description of numerous skeletons of adult individuals also found at the La Matilde ranch.

The species, researchers say, is also of great importance because it preserves “primitive” traits that do not exist in living frogs and toads. Furthermore, there are a large number of very well preserved adult individuals. These include not only the articulated skeleton but also imprints of muscles and other soft tissues.

“Studies on Notobatrachus began at the end of 1950, in the hands of the great Argentine paleontologist Osvaldo Reig. At that time, its discovery overturned everything that was thought about the evolution of frogs. Furthermore, it showed that South America was a key stage in the early evolution of the group” explains Agnolín.

How did the discovery of the new specimen occur? In a somewhat random way. As detailed in the press release by the members of the research group, in January 2020, a work team led by researchers Fernando Novas (CONICET) and Xu Xing from (Chinese Academy of Sciences) had begun to carry out explorations in the province. of Santa Cruz in search of fossils of “feathered dinosaurs”.

Although no dinosaur finds were made, this great discovery was made: paleontologist Matías Motta, a CONICET postdoctoral fellow, discovered a slab with a very particular imprint. It was a complete tadpole of Notobatrachus degiustoi that preserved the body with remains of the skull, most of the postcranial skeleton and part of the tail.

The finding has been presented publicly through the academic journal Nature. (Source: Nicolás Camargo Lescano / CTyS-UNLaM Agency)