Was this ancient creature a reptile or an amphibian? The answer is both.

Nature · Published 3 October 2026 Swipe more like this →

In short: For decades, paleontologists hailed Seymouria as one of the world's earliest reptiles. Its robust limbs and lizard-like skeleton suggested a life spent entirely on land. Yet, the discovery of fossilized gills in its relatives revealed a surprising truth: this creature began its life as an aquatic, gilled larva.

Living during the Early Permian, roughly 251 to 299 million years ago, Seymouria occupied a strange evolutionary middle ground. When first described in 1904 by Ferdinand Broili, its stocky, terrestrial-adapted body led scientists to classify it as a primitive reptile. It possessed a boxy skull, strong limbs, and a backbone built for walking, leading researchers to group it alongside other "cotylosaurs." However, the 1952 discovery of gills in the related genus Discosauriscus proved that seymouriamorphs underwent an aquatic larval stage, forcing a reclassification of Seymouria as an amphibian.

Despite this amphibian status, Seymouria remains a critical transitional fossil. It exhibits a mosaic of traits: while it retains "primitive" features like an otic notch and labyrinthodont teeth, it also displays advanced, reptile-like characteristics such as specific carotid artery pathways and a specialized stapes for hearing. These features suggest it was closely related to the ancestors of amniotes—the group that includes reptiles, mammals, and birds. Whether it sits on the direct path to amniotes or represents a separate stem-tetrapod lineage remains a subject of ongoing scientific debate.

Physically, these animals were modest in size, reaching about 2 feet (60 cm) in length. They were generalist carnivores, likely feeding on fish, invertebrates, and perhaps even smaller members of their own kind. Their anatomy was a study in compromise; for instance, their swollen vertebrae provided stability for walking but prevented the spinal twisting necessary for speed. Today, the genus is best known from two species: the robust Seymouria baylorensis from Texas and the more widespread Seymouria sanjuanensis, famously found as a pair of articulated skeletons in Germany known as the "Tambach Lovers."

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