Artistic rendition of Gerrothorax maritimus. Credit: Chen Geng.

The ocean is a terrible place for amphibians. Saltwater pulls moisture through their permeable skin, threatening to dehydrate them even while they’re surrounded by water.

But 237 million years ago, a creature built like a flattened tank apparently made the seafloor its permanent home.

Meet Gerrothorax maritimus, an armored temnospondyl belonging to an ancient branch of the amphibian family tree. Gerrothorax seems to have done what no living amphibian can: spend its life at sea. Not even the most salt-tolerant frogs alive today can manage that.

The question is how this ancient oddball overcame a problem that still defeats its modern relatives. Researchers suspect its secret was hidden in its armor.

An Ambush Predator in a Suit of Armor

Local fossil collectors excavated two skeletons in China’s Guizhou Province in 2004. The larger (the reference fossil, called holotype) preserves nearly 74 centimeters (29 inches) of the animal, with part of its tail missing. The smaller (the paratype) preserves about 49 centimeters of the skeleton, including its head and torso, but lacks the tail and parts of its limbs.

The holotype. Credit: Ryosuke Motani
The paratype. Credit: Ryosuke Motani

Gerrothorax belonged to an extinct group called temnospondyls. Picture something closer to an unusually broad salamander, with a pancake-flat body and overlapping bony plates covering its back and belly, that’s largely what the group looks like.

Those plates may have helped protect it against ichthyosaurs, formidable marine reptiles that prowled the same Triassic seas.

Rather than chasing down its prey, Gerrothorax may have waited on the seafloor for unsuspecting fish to approach.

A 2013 anatomical study had already suggested that another Gerrothorax species fed by sucking prey into its mouth. The newly discovered species had even sturdier bones supporting the throat, potentially giving it a stronger underwater gulp, while the shape of its collarbones allowed its throat to expand rapidly, drawing prey into its enormous mouth.

Artistic rendition of G. pulcherrimus, the freshwater cousin from the 2013 study. Credit: Wikimedia Commons

Researchers suspect it hunted much like today’s wobbegong sharks, which ambush prey from the ocean floor.

“They would probably just wait at the bottom and then when prey comes around, they would just suck it in,” said Ryosuke Motani, a paleontologist at the University of California, Davis, in a statement.

But catching fish at sea is one thing. Living there without ever returning to fresh water is another.

How Did It Beat the Salt?

The fossils came from sediments deposited at least 30 kilometers (19 miles) from any possible island, and roughly 200 kilometers from substantial land.

Could two freshwater amphibians simply have washed out to sea?

Their condition argues against it. Both skeletons retain much of their original arrangement; one preserves delicate finger bones that would likely have fallen away during a long journey as a floating carcass.

The preserved fingers of the holotype. Credit: Ryosuke Motani

The researchers also tested strontium isotopes, chemical signatures preserved in the bones. Their values were close to those of Triassic seawater but different from the surrounding sediment. The researchers even propose that they routinely drank seawater.

Their survival strategy (or rather, the strategy researchers suspect they had) was unusually ingenious.

Most amphibians have thin, permeable skin that allows water to pass through it easily. In the ocean, this becomes a serious problem: salty seawater draws moisture out of their bodies, causing dehydration.

Some living amphibians, including the crab-eating frog, can tolerate salty or brackish conditions. But none are known to survive their entire lives in the open ocean.

But G. maritimus had something its modern relatives lack: an almost continuous covering of bony plates across its back and belly.

An Armor to the Rescue

This armor may have reduced the exchange of water and salts through its skin. That could have pushed the animal toward a different way of regulating its internal chemistry, relying more heavily on its digestive system and gills.

Under this hypothesis, it swallowed seawater, absorbed water through its gut, and expelled excess salt using specialized physiological processes associated with its gills.

However, soft tissues did not survive, and researchers cannot determine exactly how the animal managed its salt balance.

They also cannot tell how it reproduced. The unshelled eggs of most living amphibians are poorly suited to seawater, raising the possibility that Gerrothorax evolved more resistant eggs or even gave birth to live young. For now, neither possibility can be confirmed.

Earlier evidence of ancient amphibians in coastal environments had never established a fully marine lifestyle. In a 2021 review of marine adaptation, Motani and colleague Geerat Vermeij identified amphibians as conspicuously absent from the most advanced stages of ocean colonization.

These fossils offer something much stronger: an animal that apparently crossed a barrier its living relatives never have.

“This is potentially a revolutionary discovery,” Steve Brusatte, a paleontologist at the University of Edinburgh, who was not involved in the study, told Nature. “If this Triassic amphibian, Gerrothorax, really was fully marine, then it is a game changer.”

The findings don’t mean amphibians suddenly became good at living in saltwater. They suggest that one remarkable evolutionary lineage may have found a workaround — and that its protective armor helped make it possible.

For one strange chapter in Earth’s history, at least, the oceans had amphibians, too.

The study was published in the journal Nature Ecology & Evolution.