Frogs and toads begin their lives as tadpoles – and have apparently been doing so for a long time, as a special fossil find from Argentina shows. Paleontologists have discovered the oldest known fossil of a tadpole there. It is 161 million years old. The frog larvae lived in the time of the dinosaurs, but their lifestyle was apparently already very similar to that of today’s tadpoles, as the team reports in “Nature”.
Frogs spend their youth purely aquatically: the spawn develops in water and the frog larvae that hatch from it are equipped with gills. Instead of using their legs, they move by swimming using their powerful tail. Only in the course of larval development do the tadpoles grow legs, the gills are reduced and finally the young frog makes the transition to land. The oldest adult frog fossils date back to 215 million years ago, but it is unclear whether these prehistoric amphibians also underwent this metamorphosis back then. There is a big gap in the fossil record: the oldest known fossils of tadpoles are just 145 million years old.
Tadpole from the time of the dinosaurs
However, an unusual fossil find from Argentina could now shed light on the matter. Paleontologists led by Mariana Chuliver from the Universidad Maimónides in Buenos Aires have just discovered the oldest known tadpole in the Patagonian La Matilde Formation. It is 161 million old and therefore lived during the time of the dinosaurs. The body of the fossil tadpole is almost completely preserved and even contains fossilized soft parts such as the eyes and some nerves. They are each visible as dark imprints. With a length of 16 centimeters, the tadpole is also a real “giant baby”.
The tadpole’s transformation into an adult frog was already very advanced at the time of its death, as Chuliver and her colleagues report. You can even say with some certainty what kind of frog the tadpole would have become: an equally enormous Notobatrachus degiustoi. This is indicated by both the body structure of the tadpole and where it was found, as the team found hundreds of adult Notobatrachus in its environment. The nature of the sediment also suggests that the animals once lived in a flood plain. They shared this habitat with, among other things, various plants, crustaceans, mussels and insects, which also left their fossil remains for posterity.
Way of life was similar to today
Despite living so long ago, the tadpole is surprisingly similar to today’s frog larvae, Chuliver and her team found. “Several anatomical features of the N. degiustoi tadpole suggest that important features have already evolved in the phylum taxa, in particular the structures of the hyobranchial skeleton involved in the feeding mechanism,” explain the paleontologists. The tadpole’s feeding apparatus was already a type of filter system with which it was once able to collect food particles from water currents – just like today’s tadpoles.
“This new discovery shows that a two-phase life cycle with filter-feeding tadpoles inhabiting aquatic environments was present early in the evolutionary history of frogs and toads and has remained stable for at least 161 million years,” the team summarizes.
Source: Nature, doi: 10.1038/s41586-024-08055-y