Because not all frog species undergo metamorphosis (some instead hatch as juvenile frogs straight from the egg), it has been a matter of debate whether frogs' earliest ancestors were first tadpoles.

Transformation Continues – During the Metamorphosis Stage Once the front legs are developed, we hit part two of the metamorphosis stage. Frogs begin their transition from aquatic to terrestrial.

A 161 million-year-old fossil, linked to a line of extinct frog-like amphibians, is the oldest tadpole ever found. By Asher Elbein The metamorphosis of a frog from baby to adult is one of nature ...

Young frogs and toads, after they first hatch from eggs, are called tadpoles. Over the next two weeks they change dramatically, called metamorphosis. First, tadpoles grow back legs, then front ones.

The specimen, which lived some 161–168 million years in the Jurassic Period, is thought to have failed to complete its metamorphosis into a frog, before being buried in ash and silt.

More information: Amartya T. Mitra et al, Ocular lens morphology is influenced by ecology and metamorphosis in frogs and toads, Proceedings of the Royal Society B: Biological Sciences (2022).