Millions of years ago, a 16-centimetre tadpole met an untimely end, sinking to the bottom of a pond where it would stay for millennia.
Research on the Jurassic amphibian, which has today been published in Nature, found that the resulting fossil from Patagonia, Argentina, is the earliest-known tadpole specimen
It has been dated to around 161 million years old, close to the start of the earliest frog and toad species discovered.
The finding sheds new light on early frog evolution, suggesting that even at their earliest evolutionary stages, frogs and toads had a tadpole life stage, according to John Long, a palaeontologist at Flinders University who was not involved in the new research.
"Frogs metamorphose from tadpoles … that's one of the most dramatic transformations in the life history of any backboned animal on the planet," Professor Long said.
"And this new fossil is just spectacular because it proves that frogs were transforming from tadpoles way back in the early Jurassic."
Giant tadpoles
The tadpole is a juvenile of the extinct species Notobatrachus degiustoi, and likely lived in shallow ponds which would dry out and reform with the seasons.
The adult frog would have looked extremely similar to frogs today.
They likely ate insects and lived along the tadpole ponds, according to Mariana Chuliver Pereyra, palaeontologist from Maimónides University and the lead author of the new paper.
"The adults were bulky toads, inhabiting ephemeral ponds in a typical tropical habitat," Dr Chuliver Pereyra said.
Fossils of the frogs found nearby in earlier digs showed that the creature grew to between 9 and 15cm in size.
But the newly found tadpole is even bigger. The specimen measures 16cm from tip to tail, and the team suggest it may have died just before its frog metamorphosis.
Giant tadpoles aren't particularly unusual. Paradoxical frog tadpoles — yes, that's actually the name of a type of frog that still lives in South America today — grow up to 27cm, double the size of the fossil tadpole, before shrinking back down to a normal-sized frog.
But the adult frogs of N. degiustoi are also classed as "giant", and this together is unusual. The team aren't yet sure how this might have affected the evolution of the species.
"Having a giant tadpole might have had consequences for adults," Dr Chuliver Pereyra said.
The tadpole specimen was extremely well preserved, with even some soft tissues being imprinted into the fossil in dark marks on the rock.
"It's like a Mona Lisa. It's a masterpiece of evolution's artistry," Professor Long said.
"We get this snapshot of a delicate creature with beautifully preserved soft tissues. Not just the skeleton, but there's nerves, there's eyes."
The researchers analysed these soft tissues and were able to confirm that many features of tadpoles today — like being filter feeders — existed even during the Jurassic period.
Before this discovery it was assumed that frogs millions of years ago also started life as tadpoles, but this is the first proof from such an ancient frog species.
"One of the most groundbreaking aspects of this finding is that it provides the first evidence for the presence of a tadpole, followed by a drastic metamorphosis … from the very beginning of the evolutionary history," Dr Chuliver Pereyra said.
A new frog fossil site
The team discovered the tadpole fossil and adult frog fossils from the La Matilde Formation in the Santa Cruz Province of Argentina.
[La matilde Formation]Scientists have found other fossils from the Middle and Late Jurassic period in the formation, but until now most of the fossils discovered were dinosaurs and plants, not frogs.
"A team of palaeontologists from [Argentina and China] were excavating in a quarry looking for small dinosaurs," Dr Chuliver Pereyra said.
"During this excavation they found several adult specimens of the fossil frog N. degiustoi and after many days of digging, one team member found a stone with a particular imprint on it, and it was a fossil tadpole!"
For Professor Long, the location is one of the most exciting parts of the research, as most of the best fossil finds come from a small number of sites.
"I haven't heard of this site before, so it's really exciting to see that there are other sites with such incredible preservation," he said.
"Hopefully there'll be more exciting discoveries coming out of that site in the future."
