Earth’s first fully-marine amphibian stalked the seafloor with a chunky, bony body

Nearly 237 million years ago, a spiky and toothy bottom dweller skulked the Late Triassic oceans that once covered present-day China. What sets this newly discovered giant salamander-sized prehistoric amphibian apart is not its impressive body–it’s that it lived in the ocean. The newly described Gerrothorax maritimus is the first example of a fully marine amphibian in the fossil record, according to a study published today in the journal Nature Ecology & Evolution.

There are currently over 8,000 species of living amphibians, and none have been known to live in fully marine (salt water) environments. Less than two percent like crab-eating frogs (Fejervarya cancrivora) and some spotted salamandars have any degree of salt tolerance. Other four-limbed animals, including birds, reptiles, and mammals, all have species who can survive without any food or water from land. There was also no evidence of a fully marine amphibian in the fossil record. Until now.

In this study, a team from the University of California, Davis in the United States and Peking University, and the Geological Museum of Guizhou in China analyzed two nearly complete skeletons found in southern China’s Guizhou Province. The fossils were found in an area once covered by ocean and hundreds of miles from any large land mass. 

“This area is part of an ecosystem that we’ve been looking at and trying to reconstruct,” Ryosuke Motani, a study co-author and paleontologist at UC Davis, said in a statement. “We’ve been mostly studying marine reptiles and the fish from this place, but now we also have this amphibian and it’s a sort of very unusual member of the lineage.”   

The well-preserved skeletons helped the team reconstruct what Gerrothorax maritimus looked like and offer clues to its marine life. Notably, it had a flattened body covered in bony armor. This armor likely defended it against predators like the 10-foot-long ichthyosaur that stalked these seas millions of years ago. Gerrothorax maritimus itself was roughly three feet long with a disc-shaped body.

“If you think about today’s amphibians, this animal is unusual because it’s big,” Motani said. “It’s probably, body mass-wise, about the same as the giant salamander, but what’s different is that giant salamanders are elongated whereas this animal is really chunky.”   

Based on Gerrothorax maritimus’  bony armor and the shape of its skull, the team believes that it was a bottom-dwelling suction feeder, similar to the wobbegong, a family of 12 bottom-dwelling carpet shark species alive today. 

“They would probably just wait at the bottom and then when prey comes around, they would just suck it in,” said Motanti.

Modern amphibians can’t live in marine environments because their skin does not tolerate salt. The skin is permeable, and allows them to use it for multiple purposes like respiration. Amphibians don’t drink or breathe through the same mouth opening since they can do both just using their skin. 

Due to this salt-intolerant biology, they must live in or near freshwater. While some modern species like the crab-eating frog have some degree of salt tolerance, they will lose weight in brackish waters and must recover in freshwater.

The researchers propose that bony armor and a more fish-like body helped Gerrothorax maritimus circumvent this limitation. The armor would have kept its skin from absorbing water, meaning that it needed to drink salt water with its mouth, and then absorb it through its digestive tract. The creature’s gills would have allowed it to filter out the salt. Other species in the Gerrothorax also have gills. 

According to the team, the fossils’ location within deep sea sediments and the fact that they were nearly complete led them to believe that Gerrothorax maritimus was an ocean-dweller. 

“Carcasses at sea, unless they sink to the bottom right away, tend to dry out and lose limbs prior to sinking to the bottom,” said Motani. “But these animals had their fingers preserved, so that means they probably died near where they lived.”