Early tetrapods grew slowly or moderately, like modern amphibians; rapid juvenile growth evolved later with amniotes.

Early tetrapods have often been reconstructed with a broadly amphibian-like life history. Their ancestral growth rate was thought to be slow or moderate, while faster growth and the high metabolic rates associated with it became important later in amniotes.

A fossil called Whatcheeria deltae complicated that picture.

Whatcheeria lived more than 320 million years ago and sits on the stem leading toward modern tetrapod groups. In 2022, researchers examined the microscopic structure of femora from individuals of different sizes. The smallest animals contained fibrolamellar bone, a rapidly deposited tissue associated with fast juvenile growth.

That signal was easy to miss because the bone was heavily remodeled as the animals matured. Adults therefore looked more like the slow-growing stem tetrapods that had shaped the earlier picture.

The finding does not show that all early tetrapods grew quickly. Other stem tetrapods still show slower patterns. It shows that rapid juvenile growth was already part of the group’s evolutionary repertoire and did not first appear with amniotes.