Two fossils from China point to the first known fully marine amphibian, revealing an ancient exception to the freshwater dependence of its living relatives.
An armored amphibian nearly three feet long appears to have lived on the seafloor hundreds of miles from a large landmass. How did it survive in salt water when no known amphibian alive today can live entirely at sea?
Two nearly complete skeletons from China offer clues. Researchers from the University of California, Davis, Peking University, and the Geological Museum of Guizhou identified the animal as Gerrothorax maritimus, which lived nearly 237 million years ago during the Late Triassic. Their study, published in Nature Ecology & Evolution, reports the first fossil evidence of a fully marine amphibian.
Preserved fingers point to ocean life
The skeletons were discovered in China’s Guizhou Province, in an area once covered by ocean. Researchers have been reconstructing the ecosystem preserved there through its fossils.
“This area is part of an ecosystem that we’ve been looking at and trying to reconstruct,” said study corresponding author Ryosuke Motani, a professor in the Department of Earth and Planetary Sciences at UC Davis. “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.”
Finding the skeletons in deep-sea sediments helped establish their marine setting. Their remarkable completeness also supported the researchers’ interpretation that the animals had lived there.
“Carcasses at sea, unless they sink to the bottom right away, tend to dry out and lose limbs prior to sinking to the bottom,” said Motani. “But these animals had their fingers preserved, so that means they probably died near where they lived.”
Geochemical analyses provided further evidence. The proportions of strontium isotopes, forms of the same element with different atomic masses, closely resembled those expected in Late Triassic seawater.
Could armor solve the salt problem?
The well-preserved skeletons also revealed an animal with a flattened, disc-shaped body covered in bony armor. That covering may have protected it against predators such as ichthyosaurs, a group of marine reptiles.
“If you think about today’s amphibians, this animal is unusual because it’s big,” Motani said. “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 armor could also help explain how G. maritimus managed life in the ocean. Mammals, reptiles, and birds include species that can obtain the food and water they need at sea. Modern amphibians, however, have skin poorly suited to salt water.
“That skin is permeable, and that allows the animals to use it for multiple purposes,” said Motani. “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.”
This dependence on permeable skin ties modern amphibians to freshwater environments. Fewer than 2% of the more than 8,000 living species show any degree of salt tolerance. Even the crab-eating frog, which can tolerate some salt, loses weight in brackish water and needs to recover in fresh water. Being able to endure those conditions does not amount to independence from fresh water.
The researchers propose that G. maritimus overcame this limitation through its armor and more fish-like biology. Under their explanation, the bony covering would have prevented water absorption through the skin. The animal would therefore have needed to drink salt water through its mouth and absorb water through its digestive tract. Gills, which are present in the genus Gerrothorax, could have filtered out the salt.
Clues to a seafloor ambush hunter
Its armor and skull shape also offered clues to how it fed. The researchers hypothesize that G. maritimus lived on the bottom and captured prey by suction, a feeding style Motani compared with that of the wobbegong, a modern bottom-dwelling carpet shark.
“They would probably just wait at the bottom and then when prey comes around, they would just suck it in,” he said.
Reference: “A fully marine stem amphibian from the Late Triassic of China” by Lipin Diao, Ryosuke Motani, Da-yong Jiang, Jiaxu Hao and Shengyu Wang, 26 October 2026, Nature Ecology & Evolution.
DOI: 10.1038/s41559-026-03209-9
The research was supported by the National Natural Science Foundation of China.
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