A Prehistoric Amphibian That Made Its Home on the Seafloor

Almost every amphibian alive today — frogs, toads, salamanders and the legless ones that look like worms — depends on fresh water or damp land. Their skin does not cope well with salt. That is what makes Gerrothorax maritimus unusual: it is the only amphibian ever found that lived fully in the sea, and it seems to have spent its days resting on the seafloor.

Two skeletons, a long way from shore

The species is known from two nearly complete skeletons found in Guizhou Province, China. The report in Nautilus dates the fossils to around 240 million years ago, during the Triassic period, while the scientific paper describing them gives an age of about 237 million years. Those are two separate estimates for the same Late Triassic discovery. The animal was roughly three feet long and disc-shaped.

The paper calls it a stem amphibian, meaning it sat on an older branch of the amphibian family tree rather than belonging to the groups that survive today. Study author Ryosuke Motani of the University of California, Davis, said in a statement that the area "is part of an ecosystem that we've been looking at and trying to reconstruct," and that while researchers had mostly studied marine reptiles and fish from the site, "now we also have this amphibian and it's a sort of very unusual member of the lineage."

Why four intact legs were the biggest clue

How can anyone tell that an animal lived in the ocean rather than being washed out to sea after it died? Three pieces of evidence point the same way.

• The skeletons were buried in sediments typically associated with a vast prehistoric sea.
• The bones carried a chemical signature — different versions of the element strontium — that matches marine life.
• All four limbs were still attached.

That last point mattered most. The team argues that a carcass washed down to the ocean would likely have lost an arm or a leg on the way, and that even dying in open water would have left telltale injuries. Research on dead sea turtles backs up the idea. Because these two skeletons were intact and far from land, the team concludes the animal lived on the seafloor, where it probably waited for prey to come close and then sucked it into its sizable mouth.

Armor that changed how it breathed

Its most remarkable feature was bony armor across its back, likely a defense against the many ichthyosaurs — extinct marine reptiles — swimming in Triassic oceans.

The armor may also explain how an amphibian managed salt water at all. Amphibians normally drink and breathe through their skin, but a full suit of body armor would block both. Instead, G. maritimus likely used its mouth to take in water and air, relying on its gills and digestive tract to cope with the salt.

Almost no living amphibian can do anything similar. Of the more than 8,000 species alive today, only about two percent tolerate salt water even slightly. The best-known example, the crab-eating frog, survives by holding on to a waste compound that is normally expelled in urine, which stops it losing water in salty conditions — and it still loses weight after too much time in brackish water.