A 237-million-year-old fossil discovered in China has been identified as the first fully marine amphibian, Gerrothorax maritimus, a flat, armored creature that lived its entire life in the ocean, according to research published in Nature Ecology & Evolution.
A Unique Marine Amphibian
Scientists have unearthed evidence of Gerrothorax maritimus, a prehistoric amphibian that defied the norm by thriving in saltwater environments. Unlike modern amphibians, which rely on permeable skin for respiration and water intake, this species developed adaptations to survive in the ocean. The discovery, led by Ryosuke Motani of the University of California, Davis, and colleagues from Peking University and the Geological Museum of Guizhou, challenges previous assumptions about amphibian evolution. The study was published in Nature Ecology & Evolution with DOI: 10.1038/s41559-026-03209-9.
The two nearly complete skeletons, found in Guizhou Province, reveal a creature about 75 centimeters long, with a disc-shaped, heavily armored body. Its flat form and bony plates likely protected it from predators like ichthyosaurs, while its gills and digestive system may have helped manage salt intake. This area is part of an ecosystem that we've been looking at and trying to reconstruct,
Motani said, highlighting the significance of the find. The fossils were discovered by farmers and are now housed at the Geological Museum of Guizhou in Guiyang.
The amphibian’s survival in the ocean hinged on its unique physiology. Unlike modern amphibians, which struggle with saltwater due to their permeable skin, Gerrothorax maritimus may have relied on its armor to prevent water loss. That skin is permeable, and that allows the animals to use it for multiple purposes,
Motani explained. 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 creature’s bony armor likely reduced osmotic exchanges, while its gills filtered salt from seawater. Its mouth shape suggests it fed by suction, drawing prey from the seabed.
Adaptations for a Marine Life
Chemical analysis of the fossils showed strontium isotope ratios matching Late Triassic seawater, confirming the creature’s marine lifestyle. 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 noted. But these animals had their fingers preserved, so that means they probably died near where they lived.
The skeletons were found at least 30 kilometers from land, and their articulation suggests they did not drift long distances after death. This aligns with palaeogeographic reconstructions indicating the site was 30 km from land during the Late Triassic.
The study also referenced earlier work by Farish Jenkins of Harvard University, who described in 2008 how the creature lifted its skull instead of dropping its lower jaw. The amphibian’s close relative, Gerrothorax pulcherrimus, was previously found in freshwater environments, suggesting the marine adaptation was an evolutionary accident. There was a freshwater amphibian that happened to have the equipment needed to colonise the sea – the body armour,
Motani said.
Evolutionary Implications
The discovery reshapes understanding of amphibian evolution. Gerrothorax maritimus belongs to the temnospondyl group. Motani said: If you think about today’s amphibians, this animal is unusual because it’s big.
The species’ armored body and specialized feeding habits—likely using suction to capture prey—set it apart from its land-dwelling relatives. The fossil was found alongside remains of Guizhouichthyosaurus, a 7-meter-long ichthyosaur, indicating a shared marine ecosystem.
John Long at Flinders University called the find “a first,” noting no modern amphibian is fully marine. If not for the marine invasion by many groups of reptiles at the same time, they might have even had a chance at diversifying in this niche, but that clearly didn’t occur,
he added. Modern amphibians like the crab-eating frog lose weight in brackish water and require freshwater recovery, but Gerrothorax maritimus’ adaptations allowed it to thrive in saltwater.
Unanswered Questions
>