Armored Ancient Amphibian Found to Have Lived Entirely at Sea 237 Million Years Ago

Two skeletons, both unusually complete, came out of rock in China's Guizhou Province. Each belonged to a flattened, armored amphibian about three feet long that lived nearly 237 million years ago, in the Late Triassic. The rock was laid down in deep ocean, hundreds of miles from any major landmass. The animal has been named Gerrothorax maritimus, and its discoverers say it spent its entire life at sea, which no living amphibian is known to do.

The work comes from an international team at the University of California, Davis, Peking University, and the Geological Museum of Guizhou, and it appears in Nature Ecology & Evolution. Ryosuke Motani, a professor in UC Davis's Department of Earth and Planetary Sciences and the study's corresponding author, said his group has been reconstructing this ecosystem for some time. Most of that work has focused on marine reptiles and fish. An amphibian, he said, is a very unusual member of its lineage to turn up there.

The contrast with today is stark. Of more than 8,000 living amphibian species, none is known to spend its whole life in the sea, and fewer than 2% can tolerate salt at all. Mammals, reptiles, and birds each include species that live fully at sea without returning to land for food or fresh water. Until now, scientists had found no convincing fossil evidence that an amphibian had done the same.

The skeletons are complete enough to show the animal's shape. By body mass, Motani said, it was probably similar to a giant salamander, but the proportions differ. Giant salamanders are elongated; this animal was broad and disc-shaped, which he described as chunky. Thick bony armor covered its body and may have protected it from large marine predators such as ichthyosaurs. The skull structure and armor together suggest an ambush hunter. Motani said it likely waited on the bottom and sucked in prey that came close, a strategy like that of the wobbegong, a carpet shark that lives near the seafloor.

The harder question is how it coped with salt water. Amphibian skin is permeable, and living species use it for respiration and for taking in water; they do not drink or breathe through the mouth the way other animals do. That permeability leaves them vulnerable in the sea. A few species, such as the crab-eating frog, have limited salt tolerance, but even they lose weight in brackish water and must return to fresh water to recover.

The researchers propose that Gerrothorax maritimus found another route. Its extensive armor may have restricted water movement through the skin, reducing its reliance on absorbing water that way. It could then have taken in seawater through its mouth and absorbed it through the digestive tract, with gills helping expel excess salt. Gills are known in members of the Gerrothorax genus, which gives the team reason to suspect a more fishlike way of managing salt balance.

The team also tested whether the animals really lived in the ocean or were carried there after death. The skeletons came from sediments deposited in deep marine conditions. Their completeness matters too: Motani noted that carcasses floating at sea usually dry out and lose limbs before sinking, yet these animals still had their fingers, so they probably died near where they lived. Chemical analysis added a third line of evidence. The strontium isotope signatures in the fossils closely matched those expected for Late Triassic ocean water.

Together, the team argues, the preservation, geology, and chemistry make a strong case for a life lived entirely at sea. At least one ancient amphibian apparently overcame the limits that keep its living relatives tied to fresh water. The research was funded by the National Natural Science Foundation of China, and the findings may prompt a reassessment of how amphibians adapted to marine environments.