Look at any frog or newt and the ocean seems like forbidden territory. Not one of the roughly 8,000 amphibian species alive today spends its life in salt water, and fewer than 2% can cope with even a little of it. A pair of fossils from southern China now suggests things were different deep in the past.
Writing in Nature Ecology & Evolution, a Chinese-American team, with scientists in Davis, Beijing and Guiyang, describe Gerrothorax maritimus, a temnospondyl that lived some 237 million years ago. Both skeletons are almost complete. They show a broad, disc-shaped animal around 74 centimetres long, encased in bony plates from head to tail. Co-author Ryosuke Motani says it weighed roughly as much as a giant salamander but was built very differently: “really chunky” rather than long and slender.
The problem every amphibian faces in the sea is its skin, which is thin and porous because it doubles as a lung and a drinking straw. In brine, that turns into a liability. The team thinks the armour sealed Gerrothorax off from the water around it, so it swallowed seawater instead, handled it through the gut and used its gills to dump the excess salt.
Why believe it was a true ocean dweller? The rocks were laid down far from any coastline of the time, and the bones still include delicate fingers, a sign the bodies did not drift far before sinking. Strontium isotopes in the skeletons also match Late Triassic seawater.
Its lifestyle was probably that of an ambush hunter on the seabed, sucking in passing prey much like a modern wobbegong shark. Edinburgh palaeontologist Steve Brusatte, who was not involved, called the find a potential “game-changer”.
