An international team of researchers has described the first stem amphibian known to have spent its entire life in the ocean, according to Xinhua. The species, named Gerrothorax maritimus, is reported in a paper published in the journal Nature Ecology & Evolution.
The finding rests on two nearly complete skeletons, each at least 737 millimeters long, recovered from Late Triassic deposits in Guizhou Province in southwest China. The fossils date to roughly 237 million years ago.
According to Xinhua, citing a report in China Science Daily, the researchers used the specimens to build a picture of the animal: a flattened body encased in thick bony armor, a wide flat head, a broad mouth, and strong, well-formed gill arches.
Jiang Dayong, the paper's corresponding author, said the species is the first stem amphibian identified to date as having lived wholly in a marine setting, Xinhua reports.
Why it matters
No amphibian alive today lives only in the sea. Roughly 8,000 species exist, but all depend on freshwater or damp land at some stage of life. The obstacle is skin: amphibian skin is permeable, which lets the animals take in oxygen but also loses water. In seawater, high salinity makes that loss worse and adds further physiological strain.
Some modern amphibians can survive in brackish water where rivers meet the sea, and ancient species have turned up in coastal sediments before. Xinhua notes that earlier evidence was not strong enough to establish that any amphibian had a fully marine existence. The new specimens, the authors argue, close that gap.
The work involved researchers from the Geological Museum of Guizhou, Peking University, and the University of California, Davis.
Anatomy built for the seafloor
Several features of Gerrothorax maritimus appear suited to salt water. Its flattened shape would have let it lie on the seabed and conceal itself in sediment, the researchers suggest.
The heavy bony armor is thought to have limited water loss through the skin. Adults also kept their gills, allowing them to extract oxygen underwater. According to Xinhua, this may have meant they never had to come to the surface to breathe air with their lungs.
The combination is unusual. Gills in adult amphibians are rare among living species, and armor of this kind is more often associated with fish of the era than with amphibians. Together, the traits point to an animal that fed and sheltered on the seafloor rather than one that merely crossed salt water occasionally.
The paper's title, as listed by Nature, places the find in the Late Triassic of China. Coverage of the study has described the animal as an armored bottom-feeder.
What to watch: whether other specimens from the same deposits show the same marine adaptations, which would strengthen the case that this was a lasting way of life rather than an isolated case.
