Researchers Unearth Fossil Evidence of First Fully Marine Amphibian

Scientists report fossil evidence of the first fully marine amphibian in a study from the University of California, Davis; Peking University; and the Geological Museum of Guizhou. The amphibian is an armored, giant salamander-sized, bottom-dwelling ocean creature that the researchers named Gerrothorax maritimus. 

The research, published in the journal Nature Ecology & Evolution, is based on the analysis of two nearly complete skeletons that date to the Late Triassic period nearly 237 million years ago. The fossils were discovered in an area of China’s Guizhou Province that was once covered by ocean and located hundreds of miles from large land masses. 

“This area is part of an ecosystem that we’ve been looking at and trying to reconstruct,” said study corresponding author Ryosuke Motani, a professor in the Department of Earth and Planetary Sciences at UC Davis. “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.”   

Of the over 8,000 species of modern amphibians, none have been known to live in fully marine environments, and less than 2% exhibit some degree of salt tolerance. This is odd given that other four-limbed animals — including mammals, reptiles and birds — all have species capable of surviving without food or water from land. Additionally, no evidence of a fully marine amphibian exists in the fossil record. 

Until now.

What did Gerrothorax maritimus look like? 

Thanks to the pristine nature of the skeleton specimens, the researchers could reconstruct how Gerrothorax maritimus looked and lived in its ocean environment. 

Notably, the amphibian had a flattened body that was covered in bony armor, a potential defense mechanism against predators like ichthyosaurs. The animal was almost 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 researchers hypothesize that the creature was a bottom-dwelling suction feeder. 

“They would probably just wait at the bottom and then when prey comes around, they would just suck it in,” said Motani, who noted the feeding style is similar to the wobbegong, a modern, bottom-dwelling carpet shark.      

Why can’t today’s amphibians live in salt water? 

Today’s amphibians are unable to live in marine environments due to their salt-intolerant skin.

“That skin is permeable, and that allows the animals to use it for multiple purposes,” said Motani. “They use it for respiration and water intake, so amphibians don’t drink or breathe through the mouth because they can do both through the skin.”

This function of amphibian biology requires them to live in or near freshwater environments. While some modern species such as the crab-eating frog exhibit degrees of salt tolerance, they lose weight in brackish waters and need to recover in freshwaters.

The researchers propose that Gerrothorax maritimus circumvented this limitation thanks to its bony armor and more fish-like biology. The armor would have prevented water absorption through its skin, requiring the amphibian to drink salt water with its mouth, which it would absorb through its digestive tract. The creature’s gills, which are present in the Gerrothorax genus, would have allowed it to filter out the salt. 

How did researchers determine Gerrothorax maritimus lived in the ocean?

In addition to the fossils being found in deep sea sediments, Motani said the fact that the specimens were nearly complete skeletons led the researchers to believe Gerrothorax maritimus was ocean-dwelling. 

“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.” 

Additionally, the researchers conducted geochemical analyses of the fossils, which revealed that the proportion of strontium isotopes in the fossils closely resembled those that would be seen in the oceans of the Late Triassic. 

The research was supported by the National Natural Science Foundation of China.