Frogs, like butterflies and salamanders, have what is called a "bi-phasic life cycle." They undergo metamorphosis to grow from their larval stage, as a non-reproductive tadpole¹ that swims underwater, to their adult stage, as a frog that can jump on land.

This drastic transformation² is considered by scientists to be a marvel³ of nature — and now, experts know a little more about its history.

This week, a paper published in the journal Nature detailed⁴ a tadpole fossil that proves that tadpoles⁵ existed at least 161 million years ago, in the mid-late Jurassic period. This is 20 million years earlier than scientists previously⁶ thought.

Well-preserved tadpole fossils like this one, found in Argentina, are especially hard to come by. Tadpoles are delicate creatures. They're made of much more cartilage than a frog and this larval phase only lasts a few months before they transform.