New fossil evidence reveals the first fully marine amphibian, Gerrothorax maritimus, which dates to the Late Triassic period nearly 237 million years ago. (Ryosuke Motani/UC Davis via SWNS)
By Stephen Beech
Fossil evidence of the first fully marine amphibian that hunted in the seas 230 million years ago has been unearthed in China.
The "chunky" giant salamander-sized ocean bottom-dwelling creature was almost three feet long with a disc-shaped body and covered in bony armor, scientists say.
And, unlike modern amphibians, the beast - named Gerrothorax maritimus - could tolerate salt water.
The research, published in the journal Nature Ecology and 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.
The research involved scientists from the University of California, Davis, as well as Peking University and the Geological Museum of Guizhou in China.
Study corresponding author Ryosuke Motani of UC Davis said: "This area is part of an ecosystem that we've been looking at and trying to reconstruct.
"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."
An illustrated rendering of Gerrothoroax maritumus, informed by newly discovered fossil evidence of the creature, which is the first fully marine amphibian on record. (ChenGeng/GuizhouUniversity via SWNS)
Of more than 8,000 modern amphibian species, none have been known to live in fully marine environments, and less than 2% exhibit some degree of salt tolerance.
Motani said that 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.
No evidence of a fully marine amphibian existed in the fossil record, until now.
Due to the "pristine" nature of the skeleton specimens, the research team could reconstruct how Gerrothorax maritimus looked and lived in its ocean environment.
They say it had a "flattened" body that was covered in bony armor, a potential defense mechanism against predators such as ichthyosaurs.
Motani said: "If you think about today's amphibians, this animal is unusual because it's big.
"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."
(Ryosuke Motani/UC Davis via SWNS)
Based on Gerrothorax maritimus' bony armor and the shape of its skull, the researchers believe that it was a bottom-dwelling suction feeder.
Motani said: "They would probably just wait at the bottom and then when prey comes around, they would just suck it in."
He noted the feeding style is similar to the wobbegong, a modern, bottom-dwelling carpet shark.
But today's amphibians are unable to live in marine environments due to their salt-intolerant skin.
Motani said: "That skin is permeable, and that allows the animals to use it for multiple purposes.
"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."
He explained that the function of amphibian biology requires them to live in or near freshwater environments.
The animal had a flattened body that was covered in bony armor, a potential defense mechanism against predators like ichthyosaurs. (Ryosuke Motani/UC Davis via SWNS)
While some modern species such as the crab-eating frog show degrees of salt tolerance, they lose weight in brackish waters and need to recover in freshwaters.
The researchers believe that Gerrothorax maritimus circumvented that limitation thanks to its bony armor and more fish-like biology.
They say 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.
As well as the fossils being found in deep sea sediments, Motani said the fact that the specimens were nearly complete skeletons led the research team to believe Gerrothorax maritimus was ocean-dwelling.
He added: "Carcasses at sea, unless they sink to the bottom right away, tend to dry out and lose limbs prior to sinking to the bottom.
"But these animals had their fingers preserved, so that means they probably died near where they lived."
The research team also conducted geochemical analysis of the fossils, which revealed that the proportion of strontium isotopes in them closely resembled those that would be seen in the oceans of the Late Triassic period.