Paleontologists don’t always get what they’re looking for. In January 2020, a team from the Argentine Museum of Natural Sciences and the Chinese Academy of Sciences hoped to find some feathered dinosaurs in what is now Patagonia. Although they didn’t discover any dinosaurs, they instead stumbled upon the oldest known fossil tadpole.
“The paleontologists found hundreds of adult specimens of the basal ghost frog, the ancestor of frogs and toads,” Mariana Chuliversays a biologist from the Universidad Maimónides in Buenos Aires Popular science. “After many days of digging, a team member found a rock with a certain print on it, and it was a fossil tadpole!”
This 161 million year old specimen sheds light on the evolution of frogs and toads. It is described in a study co-authored by Chuliver and published Oct. 30 in the journal Nature.
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The frogs and toads on Earth belong to a group of tailless amphibians called anurans. The two-stage life cycle – where the aquatic tadpole transforms into an adult form – is one of the key features of anurans. Adult frogs are represented in the fossil record from about 217 to 213 million years ago in the Late Triassic. However, tadpoles have not been seen before the Cretaceous period (about 145 million years ago).
In the new studythe team describes this well-preserved fossil tadpole. It belongs to the species Notobatrachus degiustoi and dates to the Middle Jurassic about 168 to 161 million years ago. Most tadpoles are only about an inch long, so at nearly 6 inches long, the fossil tadpole is considered a giant.
In evolution, gigantism is the result of species evolving large body sizes relative to their small ancestors. Lots of adults Notobatrachus degiustoi frogs are also considered giants, and gigantism has evolved several times in anuran history. According to the research, this is one of the few species that has both giant tadpoles and frogs.
“Adult Notobatrachus degiustoi They were bulky toads that probably had insectivorous diets and were closely associated with water bodies,” says Chuliver. “The habitat in which this species lived was probably a shallow pond, which dried out occasionally as the climate varied.”
It lived in these shallow waters among various plants, bivalves, brachiopodsand insects. Notobatrachus degiustoi tadpoles were also filter feeders, pumping water into their side chamber and then filtering the food particles. This feeding system is still present in living tadpoles and the fossil is an important piece of evidence that it had already evolved in early anurans. about 161 million years ago.
Most of the body and part of the tail are visible in the fossil, in addition to the eyes, nerves and a forelimb. This suggests that the tadpole was in a late stage of metamorphosis.
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“The most surprising part of this study was when we placed the tadpole under a binocular microscope and observed the beautiful preservation of the gill skeleton and other soft tissues, such as nerve prints,” says Chuliver.
Some of them they could see The main features of the tadpole’s body plan were already present this early in the evolution of anuran.
“It was incredibly surprising that this fossil also preserved several features commonly used to stage extant tadpoles, which is why we were able to stage this tadpole,” says Chuliver.