A 161 million-year-old fossil, linked to a line of extinct frog-like amphibians, is the oldest tadpole ever found.

The metamorphosis of a frog from baby to adult is one of nature’s most remarkable transformations. Tadpoles are born with oversized heads and long tails. Soon the tails vanish, swapped out for tiny legs and a more frog-like shape.

This incredible anatomical rejiggering is easy to take for granted. But tracking the evolution of frog metamorphosis has been tricky.

The fossil record of adult frogs is known dating back to the Triassic Period, 217 million years ago, said Mariana Chuliver, a paleontologist at the Universidad Maimónides in Buenos Aires. “But tadpoles are known only from the Cretaceous, 140 million years ago. So there was kind of a question regarding the origin of tadpoles.”

On Wednesday, Dr. Chuliver and her colleagues announced the discovery of a 161 million-year-old fossil tadpole, the oldest yet found. More than that, this tadpole was long enough to fit in your typical hot dog bun. The find, which relates to a close relative of our contemporary frogs, was described Wednesday in the journal Nature.

Adult frog fossils can be difficult to find, because the animals tend to be small and fragile. But tadpoles pose an even greater challenge.

“They have these really soft bodies with a lot of soft tissue, and most of its body is made of cartilage, so it’s really hard to find preserved tadpoles because of these characteristics,” Dr. Chuliver said.

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