An ancient amphibian with bony armor and upturned eyes spent its life on the seafloor, eating seafood and drinking seawater, according to a chemical analysis of two fossils published October 26 in Nature Ecology & Evolution. The creature, Gerrothorax maritimus, lived roughly 237 million years ago and is, Science News reported, the first amphibian ever known to have lived without access to land or freshwater.
That may sound like a curiosity from the Triassic. The findings matter to anyone who follows amphibian declines, skin barrier biology, or osmoregulation, the physiological term for how an animal balances water and salts.
In Plain English: The Takeaway
- Modern amphibians drink and breathe through their skin, which is why saltwater kills them; none of the more than 8,000 species alive today lives solely in the ocean.
- G. maritimus dodged that problem with bony body armor that limited water loss and internal gills retained from its larval stage — a fish-like breathing strategy adult amphibians usually lose.
- The evidence is chemical, not guesswork: strontium isotopes in the fossil bones matched the Triassic ocean, meaning the animal actually lived and fed at sea.
Strontium in bone settled the question of where the animal lived
Ryosuke Motani, a paleobiologist at the University of California, Davis, and colleagues examined two well-preserved G. maritimus fossils from the late Triassic Period, which ran from roughly 237 million to 201 million years ago. Each animal measured around 70 centimeters — about the length of a skateboard — and was covered in armor made of bony plates.
Anatomically, the specimens looked nearly identical to their closest relatives, which lived in freshwater. The anomaly was geographic: the fossils sat in rock that was once seabed, roughly 30 kilometers from any ancient land. That left two possibilities. Either the animal lived in the ocean, or it was a land-dweller that drifted out and was buried after death.
The team turned to chemistry. Different forms, or isotopes, of the element strontium occur in different ratios in seawater versus freshwater, and those ratios are recorded in an animal’s bones as it grows. In the fossil bones, the strontium signature matched the Triassic ocean. That, Motani said, means the animal ate seafood and drank seawater — “That means it lived there.”
Why saltwater is lethal to amphibian skin — and how this animal got around it
The same porous skin that lets a modern frog absorb water and oxygen also lets water leak out and salts flood in. Amphibian skin is a living membrane, lined with electrolyte-transporting cells that normally work overtime in freshwater to pump salts inward. Drop that animal into the ocean and osmosis runs in reverse: the body dehydrates into seawater that it cannot drink safely. It is the physiological reason the fossil record shows mammals, reptiles and birds repeatedly adapting to marine life while amphibians, in Motani’s words, “did not really follow the rule.”
G. maritimus skirted the problem with two adaptations. Its bony plates, probably evolved for defense, would have had the side benefit of curbing water loss through the skin. And unlike most modern amphibians, which lose their internal gills after the larval stage, this lineage retained them — a breathing strategy suited to life in deeper, open water rather than at an air-exposed shoreline.
How the two body plans compare
| Feature | Modern amphibians (e.g., frogs, salamanders) | Gerrothorax maritimus |
|---|---|---|
| Habitat | Freshwater or land; none of 8,000+ species lives solely in saltwater | Open marine seafloor, ~30 km from ancient land |
| Skin | Permeable; used for drinking and gas exchange | Armored in bony plates, limiting water loss |
| Breathing in adults | Lungs and skin; internal gills lost after the larval stage | Internal gills retained into adulthood |
| Hunting style | Varied by species | Ambush predator; jaw hinged upward like a toilet seat for suction strikes on fish |
The reaction from outside the team
Lachlan Hart, a paleobiologist at the University of New South Wales in Sydney who was not involved in the study, told Science News the finding marks a first for the group. “This is the first time that we have hard evidence that they were actually adapted for long-term life in the sea,” he said.
The strontium evidence in G. maritimus goes a step further than that earlier, more tentative case.
Where paleontologists will look next
The discovery raises the possibility that other marine amphibians are sitting unrecognized in museum drawers and fossil beds, because researchers hunting these animals have historically searched freshwater and terrestrial deposits. “Because they’re amphibians, we would be looking for them in terrestrial freshwater environments,” Hart said. The seafloor, in other words, may be an under-sampled habitat in amphibian paleontology — and the isotope technique used here is the tool that would confirm any new candidates.
For more than a century, the fossil record has documented saltwater adaptations in mammals, reptiles and birds while amphibians stayed conspicuously ashore. As of October 26, that record has one armored exception.
References
- Motani, R. et al. Nature Ecology & Evolution, October 26, 2026. nature.com/articles/s41559-026-03209-9
- Hart, L. et al.
- Science News, report on Gerrothorax maritimus, 2026.
