First fully marine amphibian relative discovered in 237-million-year-old Chinese rocks

Palaeontologists have identified the first known fully marine member of the amphibian stem lineage, a new species named Gerrothorax maritimus, based on two nearly complete skeletons from Late Triassic rocks of southern China dated to roughly 237 million years ago. The discovery, published in Nature Ecology & Evolution, fills a conspicuous gap in the record of vertebrate life at sea: mammals, reptiles and birds all include species that can survive entirely on marine food and water, but no amphibian, living or extinct, had previously been shown to do so. The researchers attribute this absence to a fundamental constraint of amphibian biology.

Most amphibians have permeable skins that exchange water, gases and salts with their surroundings, which makes them intolerant of seawater. Gerrothorax maritimus, however, descended from a lineage characterised by complete body armour, a covering that would have restricted such skin-based exchanges. According to the authors, this armour plating probably enforced reliance on other organs, namely the gills and the gut, which are more capable of becoming tolerant of salt, and thereby probably prepared the lineage for life in fully marine regimes.

The evidence for a marine lifestyle comes from two independent lines of support. First, the skeletons were found articulated, meaning the animals were buried intact rather than as scattered remains, at a location far from land, making post-mortem transport from freshwater habitats unlikely. Second, the strontium isotope record associated with the specimens indicates that the animals lived in fully marine conditions rather than in brackish or freshwater settings near the coast.

Together, these indicators point to an animal that did not merely visit the sea but lived and died in it. Its new species therefore extends the known ecological range of the amphibian total group into an environment that its living relatives never colonised. The find adds to a long pattern in Earth history, in which land vertebrates repeatedly returned to the sea with varying degrees of adaptation, from partial use of marine resources to complete independence from the land.

The two nearly complete specimens, designated as the holotype GMG 1119 and the paratype GMG 35, preserve enough anatomical detail to establish the species and to place it within the evolutionary sequence of marine adaptations traced by the authors along the group's phylogeny. For the evolution of marine adaptation, the study proposes a specific pathway: rather than gradual physiological tuning of the skin, the decisive step was the loss of skin-based exchange functions through full armouring, which shifted the burden of water, gas and salt regulation to organs that could evolve salt tolerance. This mechanism, the authors suggest, explains how a stem amphibian achieved a lifestyle that no crown amphibian has managed.

The discovery also reframes how biologists think about the limits of amphibian evolution. The barrier that kept crown amphibians out of the open sea was not an unbreakable physiological rule but a trait that could be lost, and its loss opened a route to salt tolerance through entirely different organs. Gerrothorax maritimus thus stands as a rare example of a morphological feature that closed one door to survival while opening another, allowing an armoured stem amphibian to make a living in an environment its living relatives have never entered.

Subject of Research: A fully marine stem amphibian from the Late Triassic of China

Article Title: A fully marine stem amphibian from the Late Triassic of China

Article References: Diao, L., Motani, R., Jiang, D.-Y., Hao, J., & Wang, S. (2026). A fully marine stem amphibian from the Late Triassic of China. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-026-03209-9

Image Credits: AI Generated

DOI: 10.1038/s41559-026-03209-9

Keywords: Gerrothorax maritimus, stem amphibian, marine adaptation, Late Triassic, Carnian, South China, palaeontology, strontium isotopes, body armour, vertebrate evolution, fully, marine