Newly Identified Triassic Amphibian Shows Full Marine Adaptation
Scientists have described Gerrothorax maritimus, a 30-inch armored amphibian that lived in the sea 237 million years ago, marking the first known fully marine amphibian.
Researchers in China have uncovered two nearly complete skeletons of an amphibian that inhabited the ocean during the Triassic Period, about 237 million years ago. The species, named Gerrothorax maritimus, measured roughly 30 inches in length and possessed a flat, disc-shaped, armored body with a wide mouth suited for suction feeding on the seafloor.
"The most important finding is that there was an early amphibian that lived in the sea full-time," said paleobiologist Ryosuke Motani of the University of California, Davis, a lead author of the study published in Nature Ecology & Evolution.
The fossils were recovered from marine sedimentary rocks in Guizhou Province and were found alongside remains of the ichthyosaur Guizhouichthyosaurus, a 23-foot marine reptile that dominated the ecosystem at the time. The amphibian’s bony armor likely offered protection against such predators.
"The skull is flat and wide with a large mouth, having many small conical teeth," noted paleontologist Da-yong Jiang of Peking University. "It is a bottom animal, a suction feeder that lived on a muddy to silty seafloor. It would likely capture anything that swam by, such as fish, worms and maybe cephalopods."
Motani compared the extinct creature to the modern wobbegong shark, another bottom-dwelling suction feeder of similar size. The study suggests that Gerrothorax maritimus evolved from armored freshwater ancestors, repurposing its protective armor to limit seawater absorption through its skin—a process known as exaptation.
"Their skin does not work the same way in seawater," Motani explained. "When a frog swims in seawater, it loses body water through osmotic pressure. This key feature prevents amphibians from becoming fully marine."
Chemical analysis of the bones indicated a seawater signature, supporting the conclusion that the animal lived and grew in a marine environment rather than being a post-mortem transport from freshwater habitats.
"Very odd and charming in its own way," Motani added, describing the species’ distinctive appearance and its coexistence with ichthyosaurs.