First fully marine amphibian discovered in China, challenging evolutionary assumptions

Researchers have identified Gerrothorax maritimus, the first known amphibian to live exclusively in the ocean. Published in Nature Ecology and Evolution, this discovery overturns the long-held belief that no amphibian lineage has ever fully colonized marine environments. The species lived during the Late Triassic, approximately 237 million years ago, in waters covering what is now Guizhou Province, China.

Physical Adaptations

Unlike modern amphibians with permeable skin that cannot tolerate salt, Gerrothorax maritimus possessed a unique bony armor. This carapace likely restricted water loss and facilitated respiration through gills and the digestive tract, allowing it to survive in high-salinity conditions. The animal was robust and disc-shaped, measuring between 73 and 75 centimeters in length. It employed a suction-feeding strategy similar to the modern wobbegong shark, lying in wait on the seabed for prey.

Evidence and Methodology

The conclusion that this was a fully marine creature rests on rigorous analysis of two nearly complete skeletons. Researchers confirmed the marine origin through:

• Strontium isotopes in the bones, which matched specific Late Triassic ocean signatures Source: ABC.
• Preservation integrity, as delicate finger bones remained intact, suggesting the body did not drift far from its burial site Source: Gizmodo.
• Geological context, placing fossils at least 30 kilometers from ancient coastlines Source: El Mundo (ES).

Evolutionary Significance

Lead author Ryosuke Motani of UC Davis notes that the adaptation was likely accidental; ancestral freshwater amphibians already had bony plates for defense, which coincidentally protected them from desiccation in saltwater. While soft tissues are absent, preventing confirmation of exact physiological mechanisms, the fossil record clearly shows an amphibian thriving where none exist today.