Paleontologists Discover the First Known Amphibian That Lived Entirely at Sea: a Flat, Armored Creature From 237 Million Years Ago

Amphibians are known for straddling two worlds, dwelling on land and in water at different points in their lives. In fact, their name means “double life” in Greek. But no amphibian species has ever been known to reside exclusively in a marine environment—until now.

Two nearly complete fossilized skeletons found in southwestern China hint that an armored amphibian lived in the prehistoric salty waters there some 237 million years ago, researchers report in the journal Nature Ecology & Evolution on October 26. The new species, named Gerrothorax maritimus, forces scientists to reconsider how today’s amphibians have adapted to their environments. However, even this frog doesn’t do well in brackish water.

That’s because amphibians have thin, permeable skin that allows them to breathe and consume water. When that skin is exposed to salt water, the animals become dehydrated. As a result, it loses body weight and will have to rest in freshwater to recover.

This key feature prevents amphibians from becoming fully marine.” So how could G. maritimus have accomplished a feat impossible for all amphibians today? The researchers suspect the answer lies in its unique biology and bony armor that would have prevented the animal from absorbing water through its skin. Instead, it had to drink water from its mouth, and its gills filtered out salt.

G. maritimus’ species name means “of the sea” in Latin. G. maritimus was probably a flat, bottom-dwelling creature that sucked in prey that passed by. Multiple signs suggest the roughly 30-inch-long animal resided in the ocean.

The fossils, unearthed in deep-sea sediments from Guizhou Province, China, were largely complete, suggesting the two amphibians died close to where they lived. The species was also found next to an ancient marine reptile called an ichthyosaur. Moreover, Motani and colleagues measured the fossils’ proportions of different forms of the element strontium, a method used to date minerals and water.