A fully marine stem amphibian from the Late Triassic of China

Land vertebrates colonized the sea repeatedly through geologic time with various levels of marine adaptation, from partial use of marine resources to fully marine lifestyles. Mammals, reptiles and birds all have species that survive without food or fresh water from land; however, amphibians, including both crown and stem species, lack such a member partly because of their salt-intolerant skins. Here we report the first fully marine species of a stem amphibian, Gerrothorax maritimus n. sp., based on two nearly complete skeletons from the Late Triassic (about 237 million years ago). Its articulated occurrence far from land, together with associated strontium isotope record, suggests life in fully marine regimes. It descended from a lineage with complete body armour, which would limit skin-based water, gas or salt exchanges typical of amphibians and instead enforced the use of the gill and gut that can become more tolerant of salt-this probably prepared them for marine adaptation.