Summary
Two nearly complete skeletons from China identify Gerrothorax maritimus, a stem amphibian that lived about 237 million years ago. The fossils’ position far from land and associated strontium isotope evidence suggest it inhabited fully marine environments.
Two nearly complete skeletons from China have led researchers to identify Gerrothorax maritimus, a stem amphibian that lived about 237 million years ago. In a paper published on 26 October 2026 in Nature Ecology & Evolution, the researchers say the fossils’ occurrence far from land, together with strontium isotope evidence, suggests this species lived in fully marine conditions.
The finding adds an unusual branch to the story of vertebrates moving from land into the sea. The researchers describe G. maritimus as the first fully marine species known among stem amphibians—an ancient group outside the lineage containing living amphibians.
Fossils point to life at sea
The species is based on two nearly complete, articulated skeletons. “Articulated” means the bones remain arranged in close anatomical association, rather than being scattered. Their position far from land is one indication that the animal lived in a marine setting. The associated strontium isotope record provides another line of evidence; the researchers interpret the combined evidence as pointing to fully marine conditions.
The fossils date to the Late Triassic, around 237 million years ago. The species name, Gerrothorax maritimus, marks it as a newly described species. The paper’s central claim is not simply that an amphibian could enter salty water, but that a stem amphibian had adapted to a fully marine way of life.
Armour may have helped with salt balance
Amphibians are often constrained in salty environments by their skin, which can exchange water and salts with the surroundings. The researchers propose that G. maritimus’s evolutionary background may have eased this challenge. It descended from a lineage with complete body armour, which would have limited exchanges through the skin.
The authors suggest that this may have favoured greater reliance on the gills and gut, tissues that can become more tolerant of salt. They present this as a likely route to marine adaptation, rather than a directly observed physiological process: fossils preserve anatomy and environmental clues, not the animal’s living salt-regulation mechanisms.
The proposed pathway helps explain how a group generally associated with sensitivity to salt could include a fully marine member. It also shows that the transition from land to sea need not depend on the same adaptations in every vertebrate lineage. Here, the researchers link an armoured body plan with a possible shift in how water and salts were exchanged with the environment.