Massive Fossil Breaks Record as The World's Oldest Tadpole

At more than 160 million years old, a recently discovered amphibian fossil has set a new record for the earliest known tadpole. And it's a doozy, stretching almost 16 centimeters (or 6 inches) in length.

Oh hey there little guy! (Chuliver et al., Nature, 2024)

The impression represents the only known example of a larva from an ancient frog called Notobatrachus degiustoi . Since adults of the species have been found at a similar size, it's hypothesized the chunky pollywog could have been close to its metamorphosis phase.

That makes it unlikely the individual suffered some kind of rare condition like a problematic thyroid that caused it to be larger than its siblings. It's fair to assume all tadpoles of this species may have grown to an impressive length before maturing.

We already know from the fossil record that frogs have been around since at least the late Triassic about 217–213 million years ago, but what was going on with tadpoles that long ago is, at least for now, anyone's guess.

Scientists suspected key characteristics of tadpole evolution arose during a period of time when stem anurans – the evolutionary ancestors of modern frogs and toads – diversified rapidly, at around the cusp of the Triassic and Jurassic periods.

But this estimate was based on living frogs and toads rather than fossil evidence, because, as the study authors point out, "fossil tadpoles are completely absent in the Triassic and Jurassic fossil record."

Until now, that is.

Before this discovery, the oldest tadpoles on record were from around 145 million years ago, so this new find pushes rock-hard evidence of this extreme type of metamorphosis back an extra 15 million years.

"This finding has dual importance because it represents the oldest-known tadpole and, to our knowledge, the first stem-anuran larva," the authors write.

"Its exquisite preservation, including soft tissues, shows features associated with the filter-feeding mechanism characteristic of extant tadpoles. Notably, both N. degiustoi tadpole and adult reached a large size, demonstrating that tadpole gigantism occurred among stem-anurans. "

The tadpole fossil was discovered preserved in Jurassic rock within the La Matilde Formation in Argentina. Analyzing it closely, a team of researchers led by evolutionary biologist Mariana Chuliver from Universidad Maimónides in Argentina mapped out its body in incredible detail.

They could clearly pick out the little guy's head, along with the big dark dot of its eye; most of its body; some nerves; part of its tail; and even a forelimb that helps confirm metamorphosis was well underway before it met an untimely death.

This unprecedented snapshot of prehistoric tadpole anatomy revealed that several key features of modern tadpoles – their filter feeding system, known to scientists as the hyobranchial apparatus, a kind of skeleton tongue, not to mention those awkward little leg stubs – have been a normal part of anuran childhood for at least 161 million years.

"The exquisite preservation of the new specimen, including most of the hyobranchial apparatus and soft tissues, allows us to infer the mode of life and feeding habits of the N. degiustoi tadpole," Chuliver and colleagues write.

"This finding substantially enhances our knowledge of the origins and early evolution of the biphasic anuran life cycle."