Amphibians are traditionally defined by their ability to inhabit both land and water, a trait reflected in their name which means "double life" in Greek. However, no species has ever been documented living entirely within a marine setting until the discovery of Gerrothorax maritimus. Researchers reported in Nature Ecology & Evolution on October 26 that two nearly complete fossilized skeletons found in southwestern China indicate this armored amphibian inhabited prehistoric salty waters approximately 237 million years ago. This finding challenges existing understandings of how amphibians have adapted to their environments throughout history.
Steve Brusatte, a paleontologist at the University of Edinburgh not involved in the study, described the discovery as potentially revolutionary. He noted that if this Triassic amphibian was indeed fully marine, it represents a significant shift in scientific understanding. Today, none of the more than 8,000 living amphibian species reside in marine environments, and only about 2 percent possess any level of salt tolerance. Modern amphibians typically have thin, permeable skin used for breathing and water absorption, which causes rapid dehydration when exposed to salt water due to osmotic pressure. Consequently, they cannot drink seawater and must return to freshwater to recover.
The researchers suggest that G. maritimus overcame these biological limitations through unique adaptations. Its bony armor likely prevented water absorption through the skin, forcing the animal to drink water through its mouth while using gills to filter out salt. The species name, meaning "of the sea" in Latin, reflects this marine lifestyle. Evidence suggests the roughly 30-inch-long creature was flat and bottom-dwelling, feeding by sucking in passing prey. The fossils were unearthed from deep-sea sediments in Guizhou Province, China, and their largely complete state implies the animals died near where they lived.
Further analysis supports the marine hypothesis. Ryosuke Motani, a paleobiologist at the University of California, Davis and co-author of the study, explained that carcasses at sea usually dry out and lose limbs before sinking unless they descend immediately. Additionally, the fossils were found alongside an ancient marine reptile known as an ichthyosaur. Chemical analysis of strontium isotopes in the fossils matched the proportions found in Late Triassic oceans. Anatomical comparisons indicate G. maritimus is closely related to G. pulcherrimus, a freshwater amphibian from the same period with similar armor and body shape.
The study concludes that G. maritimus's marine existence was likely an evolutionary accident, where a freshwater species possessed the necessary equipment, specifically its body armor, to colonize the sea. Gareth Hopkins, a zoologist at Western Oregon University not involved in the research, stated that these findings highlight the complex and ancient relationship between amphibians and salt water. He emphasized that understanding these historical adaptations is critical for conservation efforts as rising sea levels and freshwater salinization threaten modern amphibian populations.
Sources: Smithsonian Magazine.