First seagoing amphibian fossil rewrites ancient sea life

Two nearly complete skeletons from Guizhou Province in China have given researchers the first clear case of a seagoing amphibian fossil, an animal that spent its life in the sea rather than in freshwater. The creature, Gerrothorax maritimus, lived about 237 million years ago, in the Triassic Period. According to Reuters, paleobiologist Ryosuke Motani of the University of California, Davis, called the full-time marine life the most important finding of the study, which appeared in the journal Nature Ecology and Evolution. The discovery matters because living amphibians are tied to freshwater in ways that make salt water hostile, so a marine member of the group redraws a long-standing boundary in the fossil record.

Modern amphibians breathe and drink partly through permeable skin, a trait that works well in ponds and streams and poorly in the ocean. In seawater, that same skin can let body water escape, and drinking does not solve the problem in the same way it does on land. A small share of living species tolerate brackish water where rivers meet the sea, but none live entirely in marine water. That gap in the record is what makes this seagoing amphibian fossil stand out: the animal seems to have crossed a barrier that still holds for every living relative.

A flat hunter on the seafloor

Gerrothorax maritimus was built low and wide, with a flat, disc-shaped body covered in bony armor and a broad mouth lined with many small conical teeth. Researchers put its length at about 75 centimetres, a size that would have made it a substantial presence on a muddy or silty seafloor. Paleontologist Da-yong Jiang of Peking University described the skull as flat and wide with a large mouth, a shape that points toward ambush feeding rather than open-water pursuit. The animal probably waited on the bottom and drew in prey that came within reach.

Suction feeding works by a rapid expansion of the mouth cavity, which drops the pressure inside and pulls water and prey inward. The throat structure of this seagoing amphibian fossil fits that method, according to the researchers, and the comparison they drew was with the wobbegong, a bottom-dwelling shark that feeds in a similar way. The two animals are not close relatives, but the shared feeding style helps explain how a slow, armored animal could catch moving prey without chasing it. Armor, in this reading, was part of the hunting plan as well as protection.

The case for a life spent at sea

Finding a fossil in marine rock does not by itself prove that the animal lived in the sea, since a carcass can drift offshore or be carried by a river. The research team therefore looked for signs that these animals died near where they lived. The skeletons came from rocks formed from ocean sediment in Guanling County, in a basin that was then connected to the Tethys Ocean, and the site lay far from major land. The larger specimen still had hand bones connected in their anatomical positions, a delicate arrangement that would be hard to preserve after long transport at sea.

Chemistry added a second line of evidence for this seagoing amphibian fossil. The team measured strontium isotopes in the bones and found values close to the estimated signature of seawater in the early Late Triassic, while the surrounding rock carried a stronger signal from nearby land. That contrast supports the view that the animals took up a marine signal while alive, rather than picking one up only after burial. The researchers also set aside damaged areas of one specimen from the analysis, so the case rests on the best-preserved anatomy rather than on every visible feature.

Armor, gills and the salt problem

The armor may have done more than fend off predators. Ichthyosaurs, large marine reptiles that shared those seas, would have made the seafloor a dangerous place, and bony plates across the back and underside gave this animal a hard shell against attack. According to Reuters, Motani argued that armor which first protected a freshwater ancestor from predators could later serve a different purpose in salt water, by limiting how much seawater passed through the skin. That shift in function is the kind of reuse that lets an old trait open a new habitat.

With less water crossing the skin, the animal would have needed another way to take in water and shed salt. The researchers propose that it drank through its mouth and used internal gills to filter out excess salt, a pattern closer to marine fish than to a modern frog. The proposal remains an inference, since soft tissues such as gills do not survive as fossils, and the study authors are clear that the precise physiology cannot be measured from bone alone. What the bones do show is a body plan that could support that arrangement, and that is enough to change how scientists think about amphibian limits.

Questions remain, and reproduction is the largest of them. Eggs and young would also have needed to tolerate salt water, and the fossils do not show whether this species laid salt-tolerant eggs or gave birth to live young. Further specimens could show how widespread the marine lifestyle became and whether related animals shared it. For now, this seagoing amphibian fossil gives the fossil record a creature that lived where amphibians were thought unable to live, and it leaves researchers with a sharper question about what else an armored body plan might have made possible. Reuters reported the discovery from the study published in Nature Ecology and Evolution.