Two nearly complete fossilized skeletons unearthed in southwest China’s Guizhou Province have revealed the existence of Gerrothorax maritimus, identified by researchers as the first fully marine amphibian on record. While more than 8,000 amphibian species exist today, none live entirely in the sea due to permeable skin that causes rapid water loss in high salinity, making the Late Triassic discovery an unexpected find, China Science Daily reported.
Anatomy and Defense Mechanisms of Gerrothorax Maritimus
Living approximately 237 million years ago during the Late Triassic period, Gerrothorax maritimus measured at least 737 millimeters long, or nearly three feet long, making it an unusual, giant salamander-sized creature. The specimens exhibit a flattened body covered with heavy bony armor, a broad and flat head, a wide mouth, and sturdy, well-developed gill arches. Ryosuke Motani noted that the animal’s large, disc-shaped body is unusual compared to modern amphibians. The heavy armor likely served as a defense mechanism against predators such as ichthyosaurs inhabiting the ancient ocean.
The creature’s physical adaptations allowed it to thrive on the seafloor, hundreds of miles from a large landmass. According to research findings published in Nature Ecology & Evolution, the flattened body helped the amphibian hide in seafloor sediments. Furthermore, adult specimens retained their gills and could breathe underwater, potentially eliminating the need to surface for air.
Geochemical and Geological Evidence from Guizhou Province
The fossils were recovered from deep-sea sediments in Guizhou Province more than 30 kilometers from any possible nearby islands. Researchers from the Geological Museum of Guizhou, Peking University, and the University of California, Davis analyzed the skeletal remains and determined they had not drifted far after death. Because carcasses at sea typically lose limbs and dry out before sinking, the presence of intact fingers on the specimens indicates the animals died near where they lived.
Researchers have been reconstructing the ecosystem preserved there through its fossils. 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.
To confirm the marine habitat, the team conducted geochemical analyses on the bones. Strontium isotope proportions in the fossils closely matched the chemistry of Triassic seawater rather than the surrounding rock, supporting the conclusion that the animals grew up drinking seawater.
Modern Amphibian Constraints Versus Ancient Marine Survival
Today, no amphibian can survive exclusively in marine environments. Modern amphibian skin is permeable, serving multiple biological functions including respiration and water intake. 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 skin permeability creates severe physiological challenges in saltwater.
While some living species tolerate brackish water and ancient fossils have appeared in coastal sediments, undisputed evidence of a fully marine lifestyle remained missing until the discovery of Gerrothorax maritimus. Less than 2% of modern amphibians exhibit even a minor degree of salt tolerance, highlighting how this ancient amphibian diverged from the physiological limitations of its modern relatives.
Frequently Asked Questions About the Discovery
Where exactly were the Gerrothorax maritimus fossils found?
The two nearly complete skeletons were discovered in southwest China’s Guizhou Province, in marine sediments situated more than 30 kilometers from any possible nearby islands.
When did Gerrothorax maritimus live?
The species lived approximately 237 million years ago during the Late Triassic period.
Which institutions participated in the research?
Researchers from the Geological Museum of Guizhou, Peking University, and the University of California, Davis conducted the study, with financial support from the National Natural Science Foundation of China.
How big was the newly discovered marine amphibian?
The more complete fossil specimen measures at least 737 millimeters along its preserved midline, making it roughly the size of a giant salamander.