Fossils in China Reveal First Known Fully Marine Stem Amphibian, Gerrothorax maritimus

Researchers say they have identified the first known fully marine stem amphibian in the fossil record, based on two nearly complete fossils from Late Triassic marine rocks in southwestern China. The study, published Oct. 26 in Nature Ecology & Evolution, describes the species as Gerrothorax maritimus, an extinct early amphibian relative that appears to have lived in the open sea about 237 million years ago.

The claim matters because amphibians are generally associated with freshwater. Their permeable skin and eggs usually make life in full-strength seawater difficult, even though some living species can tolerate brackish or salty habitats. No fully marine amphibians are known today, making strong evidence for a sea-dwelling member of an early amphibian lineage especially unusual.

The new paper, “A fully marine stem amphibian from the Late Triassic of China,” is by researchers including Lipin Diao, Ryosuke Motani of the University of California, Davis, and Da-yong Jiang of Peking University and the Laboratory of Orogenic Belt and Crustal Evolution. The fossils — two nearly complete, articulated skeletons from Guizhou Province — come from marine deposits in the Guanling/Xiaowa Formation region, a famous Late Triassic fossil site.

The researchers argue that several lines of evidence point to a fully marine lifestyle rather than an animal washed out from land or freshwater. The skeletons are articulated and nearly complete, down to small distal elements, which suggests they were not transported far after death. They were preserved in a deep-marine depositional setting far from contemporaneous shorelines. And strontium isotope ratios from the bones match Late Triassic seawater values, further supporting the idea that the animal lived in the sea.

Gerrothorax maritimus was about 737 millimeters, or roughly 29 inches, long. The study describes it as flattened and heavily armored, with a wide, flat skull consistent with a bottom-dwelling suction feeder. In other words, it appears to have been adapted to life on the seafloor, snapping up prey by rapidly drawing water and food into its mouth.

For general readers, the animal was not a modern frog or salamander. The authors identify it as a stem amphibian in the plagiosaurid temnospondyl lineage — an extinct branch of early amphibian relatives. That makes the find notable not because it rewrites the history of modern amphibians, but because it expands the known ecology of a much older group. Earlier Gerrothorax species had been found in freshwater or brackish settings, so this marine species stands out within its own genus as well.

The setting adds weight to the case. The Guanling/Xiaowa beds in Guizhou are a well-known Late Triassic marine Lagerstätte, or site of exceptional fossil preservation, famous for marine reptiles and fishes. Finding these amphibian fossils there, in sediments that represent a marine ecosystem, fits the broader picture the researchers have been assembling from the site.

“This area is part of an ecosystem that we’ve been looking at and trying to reconstruct. We’ve been mostly studying marine reptiles and the fish from this place, but now we also have this amphibian and it’s a sort of very unusual member of the lineage,” Motani said in a UC Davis press release on Oct. 26.

The study’s conclusions are narrower than they are dramatic. The fossils support the idea that G. maritimus was the first known fully marine species of a stem amphibian, but major questions remain. Researchers still do not know how it coped physiologically with salt, where or how it reproduced, or whether life in the sea represented a lasting marine lineage or a short-lived ecological experiment.