Newly identified 237-million-year-old marine amphibian expands understanding of amphibian evolution

A team of paleontologists has named a new stem-amphibian species, Gerrothorax maritimus, based on two well-preserved skeletons uncovered in 237-million-year-old rocks in Guizhou Province, China. The larger specimen measures about 75 cm, roughly the size of a house cat, and both fossils show heavily armoured bodies and features suggesting a marine lifestyle.

Chemical analysis of the bones matches the composition of ancient seawater, and the fossils were found in sediment deposited at least 30 km from any landmass, indicating the animals lived and died in a fully marine environment. Unlike modern amphibians, which rely on cutaneous respiration and are highly salt-intolerant, G. maritimus possessed internal gills and extensive body armour that likely reduced skin-based water and salt exchange.

“Mammals, reptiles, and birds all have species that survive without food or freshwater from land; however, amphibians…lack such a member partly because of their salt-intolerant skins,” wrote Lipin Diao and colleagues in Nature Ecology & Evolution.

“[G.maritimus] 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,” the authors added.

The animal’s broad mouth suggests it was a bottom-dwelling suction feeder, comparable to modern wobbegong sharks that lie on the seafloor and capture passing prey. While other stem-amphibians have been found in coastal deposits, G. maritimus is the first known amphibian from a fully marine setting, indicating that amphibians briefly colonised the sea during the Triassic.

The discovery broadens the evolutionary picture of amphibians and demonstrates that, like mammals, reptiles and birds, some lineages were capable of adapting to marine habitats, albeit for a limited time.