The oldest known fossil tadpole was a big baby

Tadpoles have been wriggling in the world’s ponds for at least 161 million years.

A newly detailed fossil finding pushes the record for earliest known tadpoles back an additional 30 million years, researchers report October 30 in Nature. The petrified pollywog shows that the modern filter-feeding, puddle-dwelling characteristics of tadpoles had already evolved in some of the earliest frogs.

In 2020, a team of scientists from Argentina and China went to Argentinian Patagonia searching for dinosaur fossils, but instead found hundreds of fossilized frogs of the extinct species Notobatrachus degiustoi. Among them was a tadpole fossil in a sandstone slab.

Evolutionary biologist Mariana Chuliver at the Félix de Azara Natural History Foundation in Buenos Aires and her colleagues identified the fossil as the same species as the adult frogs thanks to shared features of its vertebrae. The tadpole was far along in its development, and some of its hind legs and forelegs had formed. It was also remarkably well-preserved, says Chuliver, with soft tissues, including eyes and nerves, set in the stone.

The N. degiustoi tadpole lived between about 168 million and 161 million years ago, in the middle of the Jurassic Period. While that’s about 20 million years after the first frog in the fossil record, the findings out of Patagonia represent “the oldest tadpole found to date,” says Chuliver. Before now, the oldest known tadpoles belonged to Shomronella jordanica, a frog that lived in Israel about 130 million years ago, during the Cretaceous Period. Other fossilized amphibian larvae date back even further, but none bear the characteristics of tadpoles, which go through a uniquely extreme metamorphosis.

The amphibian wasn’t just ancient; it was huge, measuring about 16 centimeters from snout to tail tip. Today, many species’ tadpoles are a couple centimeters or less in length. Such giant tadpoles aren’t common today, says Chuliver, and when they do occur, they usually grow into relatively small adults compared with other frog species. Instead, N. degiustoi was big throughout its life, much like the modern American bullfrog (Lithobates catesbeianus).

The fossil tadpole also appears to have fed like modern tadpoles do, sucking and straining food particles out of the water. The tadpole fossil’s throat skeleton was preserved, showing it had the same crucial filter-feeding apparatus as its modern counterparts (SN: 9/25/17). This suggests that filter-feeding tadpoles have been a winning evolutionary strategy for a very long time, says Chuliver.