Armored amphibian from 237 million years ago is the first known to live entirely at sea

The discovery is surprising because no amphibian alive today is known to live entirely in the sea. Of the more than 8,000 living amphibian species, fewer than 2% tolerate salt at all, and even those, such as the crab-eating frog, must return to freshwater to recover. The new fossils come from a region that was submerged beneath an ancient ocean hundreds of miles from major landmasses.

Gerrothorax maritimus measured nearly three feet long, but its proportions differed sharply from a modern salamander. It had a broad, flattened, disc-shaped body covered in thick bony armor, which may have protected it from large marine predators such as ichthyosaurs. "If you think about today's amphibians, this animal is unusual because it's big," said study corresponding author Ryosuke Motani, a professor in the Department of Earth and Planetary Sciences at UC Davis. "It's probably, body mass-wise, about the same as the giant salamander, but what's different is that giant salamanders are elongated whereas this animal is really chunky."

The structure of its skull and heavy armor suggest it was an ambush predator that likely rested on the seafloor and sucked in passing prey, a strategy Motani compared to the wobbegong, a carpet shark that hunts near the seafloor. “They would probably just wait at the bottom and then when prey comes around, they would just suck it in,” he said.

The research team, from UC Davis, Peking University and the Geological Museum of Guizhou, also proposed how the amphibian may have survived salt water. Amphibian skin is permeable, allowing water and gases to pass through, which leaves them vulnerable to salt. “That skin is permeable, and that allows the animals to use it for multiple purposes,” Motani said. “They use it for respiration and water intake, so amphibians don't drink or breathe through the mouth because they can do both through the skin.”

The researchers suggest the animal's extensive bony armor may have restricted water movement through its skin, possibly reducing its dependence on absorbing water that way. Instead, it may have taken in seawater through its mouth, absorbing it through the digestive tract while its gills may have helped eliminate excess salt. Gills are known to have been present in members of the Gerrothorax genus, supporting the idea of a fishlike approach to salt balance.

Several lines of evidence point to a fully marine lifestyle. The skeletons were recovered from deep marine sediments, and their completeness is telling. "Carcasses at sea, unless they sink to the bottom right away, tend to dry out and lose limbs prior to sinking to the bottom," Motani said. "But these animals had their fingers preserved, so that means they probably died near where they lived." Chemical analysis of strontium isotopes in the fossils closely matched the signatures expected in Late Triassic ocean water, providing an independent check. The research received funding from the National Natural Science Foundation of China.