Amphibians have a characteristic of their own: their life cycle is broken down into two stages, the first in the form of an aquatic larva (tadpole), and the second in a very different adult form, capable of breathing in the open air. Frogs are the order most commonly known to perform this astonishing metamorphosis which involves drastic morphological and physiological changes. But has this biphasic cycle always existed?
No tadpole fossils before the Cretaceous… so far
Fossil records indicate that frogs appeared as early as the Permian, around 270 million years ago. However, no tadpole fossil dating from before the Cretaceous (145 million years) had until now been found, suggesting that older species, particularly during the Jurassic, did not have a larval stage. An idea that turns out to be false, given the recent discovery of a new fossil.
The find was made in Patagonia, somewhat by chance, by a team of paleontologists looking for dinosaur remains. Certainly, the tadpole fossil that was discovered is less impressive than that of a dinosaur weighing several tons, but it is no less exceptional. Because its preservation has allowed a fine and detailed analysis which could help to better understand the evolution of frogs.
The very complete fossil of a long tadpole
The fossil, associated with the species Notobatrachus degiustoiis in fact complete and has soft tissue imprints, including those of the eyes, gills and nerves! An extremely rare discovery, knowing that this type of small aquatic organism is usually very difficult to preserve. The specimen is believed to date from the mid-Jurassic period (161 million years ago), clearly indicating that frogs were already developing from a larval stage. The tadpole found is also at an advanced stage of development, its vertebrae showing signs of ossification. The size of the tadpole is also notable: 16 cm long!
Apart from its size, this specimen seems to have been relatively similar to our current tadpoles at the morphological and physiological level. It obviously fed in the same way, on plankton, algae and various detritus present in the aquatic environment. These results were published in the journal Nature.
