_Gerrothorax maritimus_, a fully marine amphibian from the Late Triassic Period 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. 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.
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.
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