Gerrothorax maritimus: first fully marine amphibian found

Researchers from the University of California, Davis, Peking University and the Geological Museum of Guizhou reported fossil evidence of the first fully marine amphibian, according to ucdavis.edu ucdavis.edu. The study, published in Nature Ecology & Evolution, describes two nearly complete skeletons dated to the Late Triassic, nearly 237 million years ago ucdavis.edu. The animal was named Gerrothorax maritimus ucdavis.edu.

The fossils were found in an area of China's Guizhou Province that was once covered by ocean and lay hundreds of miles from large land masses ucdavis.edu. Corresponding author Ryosuke Motani, a professor in the Department of Earth and Planetary Sciences at UC Davis, said the team had mostly studied marine reptiles and fish from this ecosystem, and the amphibian is an unusual member of its lineage.

Why a marine amphibian is unusual

Of the more than 8,000 species of modern amphibians, none are known to live in fully marine environments, and less than 2% show some degree of salt tolerance ucdavis.edu. The fossil record had also contained no evidence of a fully marine amphibian ucdavis.edu. Motani explained that amphibian skin is permeable and is used for respiration and water intake, which ties modern species to freshwater or nearby habitats.

Anatomy and evidence for life in the sea

The well-preserved skeletons allowed the authors to reconstruct the animal: a disc-shaped, flattened body covered in bony armor, almost three feet long ucdavis.edu. By body mass it is probably close to the giant salamander, Motani said, but it was much chunkier. The armor may have defended against predators such as ichthyosaurs. From the armor and skull shape, the researchers hypothesize a bottom-dwelling suction feeder, similar in style to the modern wobbegong carpet shark.

The authors propose that the armor limited water absorption through the skin, so the animal would have drunk seawater through the mouth and absorbed it in the digestive tract. Gills, present in the genus Gerrothorax, would have let it filter out the salt.

Several lines of evidence point to a marine habitat. The fossils lie in deep-sea sediments. The fingers are preserved, whereas carcasses at sea tend to dry out and lose limbs before sinking, so the animals probably died near where they lived, Motani said ucdavis.edu. Geochemical analysis also showed that the proportion of strontium isotopes in the fossils closely resembles that of Late Triassic oceans ucdavis.edu.

UC Davis team describes Gerrothorax maritimus, a fully marine amphibian from 237 million years ago

Why a marine amphibian is unusual

Anatomy and evidence for life in the sea

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