Media release

From: Springer Nature

The oldest-known tadpole

A fossil tadpole from around 161 million years ago is the oldest known tadpole reported to date. The discovery, described in this week’s Nature, sheds light on the evolution of frogs and toads.

Frogs and toads belong to a group of tailless amphibians called the anurans. They have a characteristic two-stage life cycle, in which the aquatic tadpole larva metamorphoses into the adult form. Adult frogs are represented in the fossil record back to the Late Triassic (around 217–213 million years ago), but tadpoles are undocumented before the Cretaceous (around 145 million years ago).

Mariana Chuliver and colleagues describe a well-preserved fossil tadpole (Notobatrachus degiustoi) from the Middle Jurassic of Patagonia (around 168–161 million years ago). The head, most of the body and part of the tail are visible, as are the eyes, nerves and a forelimb, which suggests the tadpole was in the late stages of metamorphosis. Collectively, they show that key features of today’s tadpoles, such as their filter feeding system, had already evolved in early anurans around 161 million years ago. With an estimated length of almost 16 cm, the tadpole was also a giant. Many adult N. degiustoi frogs, also considered giants, have been previously documented at the same location. Gigantism has evolved multiple times in anuran history, but the study shows that this is one of the few species to have both giant tadpoles and giant frogs.

The discovery of this new specimen reveals that key features of the tadpole body plan were already present early in anuran evolution, indicating that a two-stage life cycle with a drastic metamorphosis was already present in anurans around 161 million years ago, the authors conclude.