Armored fossil reveals first fully marine amphibian from Triassic seas

Scientists have identified the first amphibian known to have lived fully in the ocean, an armored, bottom-dwelling creature from the Late Triassic that the researchers have named Gerrothorax maritimus. The finding, published in Nature Ecology & Evolution by a team from the University of California, Davis; Peking University; and the Geological Museum of Guizhou, is based on two nearly complete skeletons dated to nearly 237 million years ago. The fossils were unearthed in China’s Guizhou Province, in an area that was once covered by ocean and sat hundreds of miles from large land masses. Of the more than 8,000 species of modern amphibians, none are known to live in fully marine environments, and less than 2 percent exhibit some degree of salt tolerance. No evidence of a fully marine amphibian had previously existed in the fossil record, making the new specimens an unusual member of the lineage. “This area is part of an ecosystem that we’ve been looking at and trying to reconstruct,” said study corresponding author Ryosuke Motani, a professor in the Department of Earth and Planetary Sciences at UC Davis. “We’ve been mostly studying marine reptiles and the fish from this place, but now we also have this amphibian and it’s a sort of very unusual member of the lineage.”

The pristine condition of the skeletons allowed the researchers to reconstruct how Gerrothorax maritimus looked and lived in its ocean environment. The animal was almost three feet long with a disc-shaped, flattened body covered in bony armor, which the researchers hypothesize may have served as a defense mechanism against predators such as ichthyosaurs. In body mass, the team estimates it was probably about the same as a giant salamander, but its proportions were strikingly different. “If you think about today’s amphibians, this animal is unusual because it’s big,” Motani said. “It’s probably, body mass-wise, about the same as the giant salamander, but what’s different is that giant salamanders are elongated whereas this animal is really chunky.” Based on the bony armor and the shape of its skull, the researchers hypothesize that the creature was a bottom-dwelling suction feeder. “They would probably just wait at the bottom and then when prey comes around, they would just suck it in,” Motani said, noting that this feeding style is similar to that of the wobbegong, a modern bottom-dwelling carpet shark.

The discovery raises the question of why today’s amphibians cannot tolerate salt water, and the answer lies in their permeable skin. “That skin is permeable, and that allows the animals to use it for multiple purposes,” Motani said. “They use it for respiration and water intake, so amphibians don’t drink or breathe through the mouth because they can do both through the skin.” This function of amphibian biology ties them to freshwater environments, since salt water would disrupt the balance their skin depends on. Some modern species, such as the crab-eating frog, exhibit degrees of salt tolerance, but they lose weight in brackish waters and need to recover in fresh water. This constraint makes the absence of marine amphibians especially odd, given that other four-limbed animals, including mammals, reptiles and birds, all have species capable of surviving without food or water from land.

The researchers propose that Gerrothorax maritimus circumvented the salt-water limitation thanks to its bony armor and more fish-like biology. In this hypothesis, the armor would have prevented water absorption through the skin, requiring the amphibian to drink salt water with its mouth and absorb it through its digestive tract. The creature’s gills, which are present in the Gerrothorax genus, would then have allowed it to filter out the salt. The team emphasizes that this is a proposed mechanism rather than a directly observed one, but it offers a testable explanation for how a stem amphibian could have tolerated a fully marine lifestyle that no modern relative shares.

Several independent lines of evidence led the researchers to conclude that Gerrothorax maritimus lived in the ocean. The fossils were found in deep sea sediments, and the fact that the specimens were nearly complete skeletons suggested the animals died where they lived rather than drifting at sea. “Carcasses at sea, unless they sink to the bottom right away, tend to dry out and lose limbs prior to sinking to the bottom,” Motani said. “But these animals had their fingers preserved, so that means they probably died near where they lived.” In addition, the researchers conducted geochemical analyses of the fossils, which revealed that the proportion of strontium isotopes in the bones closely resembled what would be expected in the oceans of the Late Triassic. Together, the sedimentary context, the preservation of delicate skeletal details and the strontium isotope data point consistently to an animal that inhabited the sea itself rather than a freshwater habitat on distant land. The research was supported by the National Natural Science Foundation of China, and the team notes that the Guizhou ecosystem, previously studied mainly through its marine reptiles and fish, now includes a fully marine amphibian among its reconstructed members.

Subject of Research: Fossil evidence for the first fully marine amphibian, Gerrothorax maritimus, from the Late Triassic of China

Article Title: Researchers unearth fossil evidence of first fully marine amphibian

Article References: Researchers unearth fossil evidence of first fully marine amphibian. Original publication (Bibliographic details unavailable.)

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Keywords: paleontology, amphibians, Gerrothorax maritimus, Late Triassic, marine ecosystems, fossils, China, evolution, Researchers, unearth, fossil, evidence