Amphibians rely on their permeable skin for the crucial functions of breathing and drinking, a wonderful adaptation in freshwater that is subverted by the salt in seawater. As a result, there has been no known amphibian — extinct or extant — with a fully marine lifestyle. Until now, that is.
Researchers have discovered in China's Guizhou Province two nearly complete skeletons of an amphibian that lived in the sea about 237 million years ago during the Triassic Period, with a flat, disc-shaped and armored body and a wide mouth indicative of a bottom-dwelling suction feeder that ambushed its prey.
The creature, named Gerrothorax maritimus, measured about 30 inches (75 cm) long and predated the dinosaurs.
"The most important finding is that there was an early amphibian that lived in the sea full-time," said paleobiologist Ryosuke Motani of the University of California, Davis, one of the leaders of the study published in the journal Nature Ecology & Evolution.
It was unearthed in rocks that formed from ocean sediment, and was found near fossils of an ichthyosaur — a type of marine reptile — called Guizhouichthyosaurus that was about 23 feet (7 meters) long. Ichthyosaurs were the apex marine predators of the time, and justified the amphibian's bony body armor.
Its use of suction feeding is suggested by the shape of its mouth and the structure of its throat. Suction feeding occurs when an aquatic animal quickly expands its mouth cavity to create a drop in pressure, drawing water and prey inside.
"The skull is flat and wide with a large mouth, having many small conical teeth," paleontologist and study co-author Da-yong Jiang of Peking University in China said, according to Reuters.
"It is a bottom animal, a suction feeder that lived on a muddy to silty seafloor. It would likely capture anything that swam by, such as fish, worms and maybe cephalopods," Jiang said, referring to squid relatives.
The wobbegong, a bottom-dwelling and suction-feeding shark of comparable size, is a good present-day analog, Motani said.
The researchers said Gerrothorax maritimus evolved from armored freshwater ancestors. Fossils of a closely related armored freshwater amphibian were previously discovered.
There are more than 8,000 species of living amphibians such as frogs, salamanders and legless caecilians. Some can withstand brackish water — saltwater and freshwater mixing where bodies of water like rivers meet the sea.
But less than 2% exhibit any salt tolerance, and none live in fully marine environments. In addition, no previous amphibian fossil indicated a marine existence.
An amphibian's skin is multifunctional, acting among other things as the main gateway for breathing and water intake.
"Their skin, however, does not work the same way in seawater," Motani said.
"When a frog swims in seawater, it loses its body water because the seawater sucks out the body water through the permeable skin through osmotic pressure. It cannot drink water either for the same reason. As a result, it loses body weight and will have to rest in freshwater to recover. This key feature prevents amphibians from becoming fully marine."
Gerrothorax maritimus was able to do so through a phenomenon called exaptation, in which a biological trait that evolved for one function is repurposed for a different function.
"There was a freshwater amphibian that happened to have the equipment needed to colonize the sea — the body armor. The armor protected the animal from predators in freshwater but it could also be used to protect the bearer from seawater when the lineage colonized the saltier part of the water system," Motani said, like a person wearing a dive suit for underwater exploring.
The armor would have inhibited seawater absorption through the skin, requiring the animal to drink saltwater through its mouth and then filter out the salt with its gills.
In the fossil record, there are examples of land animals and freshwater aquatic animals whose bodies were carried out to sea after death. But the researchers expressed confidence that this amphibian was a genuine marine creature, in part because its bones exhibited a chemical signature of Triassic seawater, showing the animal developed and grew in the ocean.
"Very odd and charming in its own way," Motani said of the new species. "Its charm is in its unique, somewhat comical appearance and physiological peculiarity. I did not ever imagine that there was an amphibian that shared time and space with ichthyosaurs. That alone made a big impression on me."
