Haiyiyuan Named as Possible First Fully Marine Amphibian

What changed

Chinese and US researchers have identified two nearly complete fossil skeletons from Guizhou as a new amphibian, Haiyiyuan. The fossils date to about 237–235 million years ago, measure 737 millimetres long, and may be the first known amphibian capable of living entirely in the sea.

Why it matters

Haiyiyuan changes the shape of the amphibian family tree. More than 8,000 amphibian species live today, but none is fully marine. These fossils suggest that, at least once, an amphibian crossed that boundary and stayed there.

The location is a key part of the case. The fossils were found at least 30 kilometres from a possible island, their preservation suggests the animals died nearby, and the chemistry of their bones closely matches seawater. Together, those clues point to a life spent in the ocean rather than a brief trip from land or freshwater.

Its body also looks built for the problem. A flattened form and heavy armour may have limited water loss, while its gills and digestive system may have helped manage salt and swallowed seawater. Its mouth suggests suction-feeding from the seafloor, giving the animal a picture-book role: a shelled amphibian vacuuming up prey in an ancient sea.

The cautious conclusion is also the most interesting one. The marine lifestyle is inferred rather than directly observed, and soft tissue did not survive, so Haiyiyuan’s physiology and reproduction remain unknown. Even so, the discovery shows that an evolutionary route once existed for amphibians to become entirely marine, despite the challenges of permeable skin and salty water.