A flat body, wide mouth and bony armor have been preserved in the fossils of an animal that probably spent its entire life at sea. Two almost completely preserved skeletons discovered in China's Guizhou province show the first known amphibian with a fully marine lifestyle.
The species has been named Gerrothorax maritimus. It lived about 237 million years ago, during the Triassic period, and reached a length of about 75 centimeters. The animal lived on the seabed before the dinosaurs.
A predator of the seabed
The shape of its body and skull suggests that Gerrothorax maritimus probably hid its prey in the mud and sucked it in. Its wide mouth was well-suited for swallowing fish, worms, and possibly cephalopods that passed by.
🚨NEW STUDY DROP🚨 NEW AMPHIBIAN - G. maritimus A new species of Gerrothorax has been described from Late Triassic China based on two nearly complete skeletons showing this species was adapted to marine habitats! 🎨: @JoschuaKnuppe https://t.co/ptj017YjYD
— EDGE Science (@EDGE_Science_) October 26, 2026
The remains were found in rock layers formed by ocean sediments, along with fossils of Guizhouichthyosaurus - a marine reptile that reached about seven meters in length. At that time, ichthyosaurs were among the top predators in the marine ecosystem. The bony armor likely provided additional protection for the smaller animal.
„The most important discovery is that there was an ancient amphibian that spent its entire life in the sea,“ says paleobiologist Ryosuke Motani of the University of California, Davis, one of the study's leaders.
How an amphibian survived in salt water
Amphibians usually rely on their permeable skin for vital functions such as breathing and water intake. However, sea salt can draw fluids through the skin and cause osmotic stress. That's why today's amphibians don't live entirely in the sea, with only some of them tolerating semi-saline waters.
According to Motani, it was the armor of Gerrothorax maritimus that changed that boundary. It probably limited the penetration of seawater through the skin and allowed the animal to rely more on its gills and digestive system. Thus, a scar that arose in freshwater ancestors as protection from predators acquired a new function - it helped the species adapt to marine life.
“This is a bottom-dwelling animal that fed by suction and lived on the seabed covered with silt or sediment. It probably caught anything that passed nearby - fish, worms, and perhaps cephalopods“, explains paleontologist and co-author of the study Da Yun Jiang of Peking University.
The study is published in the journal Nature Ecology & Evolution. It expands the understanding of amphibian evolution by showing that this group managed, at least once in the distant past, to overcome the obstacle that today prevents any of its representatives from living entirely in a marine environment.