Fossil remains unearthed in southwest China reveal Gerrothorax maritimus, a 237-million-year-old armored stem amphibian that hunted on the Late Triassic ocean floor. Described in Nature Ecology & Evolution, the discovery marks the first known instance of a fully marine amphibian in the fossil record.
Paleontologists describing two nearly complete skeletons from Guizhou Province have identified an extinct prehistoric creature that breaks biological rules for four-limbed vertebrates. Among more than 8,000 recognised species of living amphibians, including frogs, newts, salamanders, and worm-like caecilians, none inhabit the open ocean full-time. The transition across the ecological barrier between fresh water and the sea remained an uncrossed boundary until now.
Fossil Discovery in Guizhou Province
Local farmers in southwestern China’s Guizhou Province originally found the pair of analyzed skeletons. A regional fossil repository recently obtained them. The reference specimen is kept at the Geological Museum of Guizhou, in Guiyang.
Researchers from Peking University examined these two nearly complete southern Chinese fossil specimens. The Geological Museum of Guizhou also provided researchers for the project. The University of California, Davis in the United States completed the team.
The specimens, originally excavated by local fossil collectors in 2004 and subsequently acquired by a regional fossil repository, date back roughly 237 million years to the Late Triassic period. Researchers estimate the specimen reached about 75 centimeters in length, roughly three feet long, matching the mass of a modern giant salamander.
Unlike modern salamanders, which possess slender builds, Gerrothorax maritimus featured a disc-shaped, heavily flattened body encased in a protective shell of bony plates. This armor likely defended the creature against predators of the Triassic seas, such as ten-foot-long ichthyosaurs that hunted in the same waters.
If you think about today’s amphibians, this animal is unusual because it’s big. 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.
Ryosuke Motani, vertebrate paleontologist at the University of California, Davis
Isotope Ratios Suggest Amphibian Lived as Seafloor Predator
The animals did not simply drift into marine environments after death. The skeletons were recovered from strata deposited at least 30 kilometers away from any ancient landmass or islands, showing little evidence of transport damage. Strontium isotope ratios measured on the bones and surrounding sediment closely match the chemical signature of Late Triassic seawater.
Based on cranial architecture and the distribution of its bony plating, researchers propose that the amphibian operated as a bottom-dwelling ambush predator. Its hunting strategy likely mirrored that of the modern wobbegong, a camouflaged carpet shark that rests motionless on the seafloor.
They would probably just wait at the bottom and then when prey comes around, they would just suck it in.
Ryosuke Motani, study corresponding author
Overcoming the Salt Barrier in Extinct Stem Amphibians
Living amphibians struggle in saline environments because their soft, permeable skin functions as a respiratory membrane and water-absorption organ. In seawater, osmotic pressure pulls internal moisture outward through the skin, causing rapid dehydration. While a tiny minority of modern species—such as the crab-eating frog—tolerate brackish water temporarily, they quickly lose weight and must return to freshwater.
The research team suggests that Gerrothorax maritimus bypassed this restriction through a combination of its heavy bony armor—which prevented unwanted absorption across the skin—and a more fish-like biology.
Ryosuke Motani is a vertebrate paleontologist at the University of California, Davis. He noted that this is odd given that other four-limbed animals, including birds, reptiles and mammals, all have species capable of surviving without food or water from land, whereas no modern amphibian species among more than 8,000 are known to live in fully marine environments.
Paleontologists plan to examine the dorsal sides of the newly recovered fossils—currently preserved belly-up—to refine reconstructions of the animal’s ecology and determine whether fully marine amphibians represented a wider, hidden phenomenon in ancient oceans.