Predatory amphibians swam the seas 237 million years ago
Until now, it was thought that the squishy, permeable skin of amphibians was unsuited for marine life because it would absorb too much salt from sea water – but a fossil found in China upends that view
The Late Triassic seas contained a multitude of marine reptiles, but lurking on the sea floor was something different: a marine amphibian. A fossil of the large, bottom-dwelling ambush predator discovered in China is the first evidence that amphibians once found a way to adapt to life in the ocean.
Named Gerrothorax maritimus, the near-perfect and exquisitely preserved 74-centimetre-long fossil was found by farmers in what is now a national geopark in Guanling county, China.
The researchers who studied the fossil think that its secret to survival in salt water, which is normally fatal to amphibians because of their permeable skin, probably lay in its strong body armour.
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“It was a flat animal that lived on the sea floor,” says Ryosuke Motani at the University of California, Davis, who was part of the team that analysed the fossil. It was probably an ambush feeder, he says, as its wide mouth and the strong hyoid bone in its neck suggest it used suction feeding, as is seen in some modern bottom-dwelling predators of a similar size that use an ambush strategy, such as wobbegong sharks.
As soon as the researchers began to study the fossil, they knew it was neither fish nor reptile. It had to be an early amphibian because of its tell-tale anatomy, which included its four legs and gills.
There is, however, no evidence that the creature underwent metamorphosis, as many modern amphibians do, such as when tadpoles become frogs, says Motani.
To prove that the animal was indeed marine and not just a land-based creature washed into the sea after death, the researchers studied the strontium isotope ratios of the fossil bones.
They found that the remains of the new species had a clear signature of sea water, meaning G. maritimus grew to adulthood while in the ocean. Palaeogeographic reconstructions also suggest that, 237 million years ago in the Late Triassic Period, the location in which the fossil was discovered was at least 30 kilometres away from land.
The fossil was also recovered alongside that of a large marine reptile: an ichthyosaur called Guizhouichthyosaurus, which reached lengths of 7 metres.
Interestingly, the closest relative to G. maritimus lived in freshwater environments. A fossil of this amphibian, Gerrothorax pulcherrimus, was first described nearly a century ago.
This has led the researchers to suggest the marine lifestyle of G. maritimus was an accident of evolution. “There was a freshwater amphibian that happened to have the equipment needed to colonise the sea – the body armour,” says Motani.
The armour originally protected the Gerrothorax genus from predators in fresh water. But because it formed an impermeable barrier, it could also help the amphibians survive in seawater, he says.
Although it is commonly believed that “amphibious” means the ability to be on land or in the water, this is a misconception, says Motani. There are many modern newts that never leave the water, yet they are amphibians, he says.
“My take is that it is similar to calling a snake a tetrapod, when it does not have limbs,” says Motani.
John Long at Flinders University, Australia, says the fossil is a first, given that none of the 8000 or so living species of amphibians today is fully adapted for life in the sea.
“If not for the marine invasion by many groups of reptiles at the same time, they might have even had a chance at diversifying in this niche, but that clearly didn’t occur,” he says. “It shows us that amphibians closely related to modern forms were able to adapt to different environments, like seawater, in ways we never thought possible.”
Nature Ecology & Evolution DOI: 10.1038/s41559-026-03209-9
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