Eryopids were 2 m large relatives of modern amphibians that formed top predators in the Paleozoic. They are found in aquatic deposits and apparently preyed on fishes. Their life cycle is preserved in a single species, Onchiodon labyrinthicus, from eastern Germany. It existed in a stressed ecosystem, and the broad variation in the studied sample correlates with seasonal fluctuations. Eryopids evolved into increasingly large, heavily built inhabitants of streams and seasonal water bodies.