Fossil From China Reveals the First Amphibian Built for Life in the Open Ocean

Paleontologists have described the first amphibian known to have lived entirely in the open ocean, a discovery that overturns a long-standing assumption about one of the four major groups of land vertebrates. The animal, a bony-armored, flat-bodied creature named Gerrothorax maritimus, was identified from two nearly complete skeletons unearthed in southern China and dated to the Late Triassic, roughly 237 million years ago.

The find, published in the journal Nature Ecology & Evolution, was led by Ryosuke Motani, a paleontologist in the Department of Earth and Planetary Sciences at the University of California, Davis, working with colleagues at Peking University and the Geological Museum of Guizhou. The team argues that the fossils, their burial setting and their chemistry together show the animal spent its life at sea rather than venturing there occasionally from fresh water.

A skeleton far from any shoreline

Gerrothorax maritimus was almost three feet long, with a flattened, disc-shaped body encased in bony armor plating, built more like a tank than a frog. The two skeletons were recovered from rock in Guizhou Province that was, in the Late Triassic, open ocean hundreds of miles from the nearest large landmass. That distance from shore is part of the case for a marine lifestyle: both specimens are articulated, meaning their bones are still arranged as they were in life, including small finger bones that tend to scatter quickly if a carcass drifts or decays before burial. The paper's authors also measured strontium isotope ratios in the fossils, a chemical signature that varies between fresh and ocean water, and found values consistent with a Late Triassic marine environment rather than a river or lake.

"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," Motani said, in comments carried alongside coverage of the study. The genus Gerrothorax itself is not new to science; previous species have turned up in freshwater lake deposits in Greenland and Europe, which is part of what makes a fully marine relative so unexpected.

Why amphibians stay out of salt water

The discovery matters because no living amphibian, and until now no fossil amphibian, is known to have managed a fully marine existence. Mammals, reptiles and birds have each produced lineages, from whales to sea turtles to penguins, that live entirely on marine resources without needing fresh water or land. Amphibians have not, and the reason is physiological: frogs, salamanders and their relatives breathe and absorb water through permeable skin, a trait that works well in fresh water but leaves them unable to tolerate salt. Of the roughly 9,100 living amphibian species catalogued by the research database AmphibiaWeb, the study's authors note that none live in a fully marine environment and fewer than 2 percent show any salt tolerance at all.

Gerrothorax maritimus appears to have solved that problem by not relying on its skin. The researchers propose that its heavy, unbroken armor plating blocked the water and salt exchange that ordinarily happens across amphibian skin, effectively sealing it off. In place of that route, the animal would have needed to drink seawater outright and process it through its gut, while its gills, inherited from an aquatic ancestry, likely handled the job of filtering out excess salt. The same armor, the team notes, may have doubled as defense against the marine reptiles that shared its habitat, including ichthyosaurs that could grow past 10 feet long.

"They would probably just wait at the bottom and then when prey comes around, they would just suck it in," Motani said of the animal's likely hunting style, comparing it to the ambush feeding of a modern wobbegong shark.

The paper classifies the new species as a "stem amphibian," a term paleontologists use for extinct lineages that branch off before the common ancestor of today's frogs, salamanders and caecilians, rather than a direct relative of any living group. Guizhou's Late Triassic marine rocks are already known among paleontologists for a rich record of marine reptiles, including the small, long-necked Keichousaurus and several ichthyosaur species, which is part of why Motani's lab has spent years working the region. Gerrothorax maritimus is the first amphibian described from that same marine fauna, suggesting it shared the water column with predators that had already fully committed to ocean life millions of years earlier.

A bottom-dwelling ambush hunter

The researchers interpret Gerrothorax maritimus as a bottom-dweller rather than an active swimmer, its flattened body suited to lying still on the seafloor before drawing in prey with a rapid gape, a strategy known as suction feeding. That lifestyle fits the rest of the evidence: a heavy, low-slung body is a poor shape for open-water pursuit but a good one for ambush, and it matches how the genus's freshwater relatives are thought to have hunted in lake-bottom settings.

The work is a reminder that the fossil record still holds surprises even in animal groups considered well understood, as early coverage of the discovery has observed. Land vertebrates have returned to the sea repeatedly over hundreds of millions of years, and each time the return trip required shedding or modifying traits built for life on land. Amphibians were assumed to be permanently excluded from that pattern by their skin alone; this fossil suggests that assumption held only because no one had yet found the exception.

What comes next

Guizhou's Triassic marine beds have already yielded extensive fossils of marine reptiles and fish, and Motani's group says it expects more amphibian material to turn up as excavation continues in the region. Open questions include how widespread a marine lifestyle was within the broader Gerrothorax lineage, whether other armored Triassic amphibians made similar transitions, and how the gill- and gut-based salt handling proposed for Gerrothorax maritimus would have worked in practice. Those questions will likely be taken up through further fieldwork in Guizhou and comparative study of the genus's other, freshwater species, along with additional geochemical testing on any new specimens recovered from the same marine beds.