The researchers found “remarkably large” fossilized teeth belonging to a previously unknown species called Yakamys sirindhornae. This prehistoric rodent lived approximately 34 million years ago, during the middle of the Paleogene period. Its remains, consisting of a lower second molar and a lower incisor, were recovered from the Bang Mark Pit at the Krabi lignite mine in Thailand’s Krabi Province.

Although only two teeth have been identified, they offer valuable clues about rodent gigantism, the evolutionary process through which rodents develop unusually large bodies. The discovery suggests that Southeast Asia played a more significant role in the early evolution of giant rodents than scientists had recognized.

Giant Rodents Have a Much Older History Than Expected

“Today, the largest living rodent is the capybara, but very large rodents are not restricted to recent ecosystems,” said Virginie Millien, a biology professor at McGill University and co-author of the study. “Giant rodents evolved repeatedly during the later history of the group, mostly in South America. This discovery suggests that Southeast Asia also played an important role in the early evolutionary history of large-bodied rodents.”

The newly identified animal’s name reflects both its impressive size and its connection to Thailand. Yakamys combines “yak,” the Thai word for giant, with “mys,” the Greek word for mouse or rat.

The second part of its scientific name recognizes an important supporter of scientific research in Thailand.

“The species name, sirindhornae, honors Princess Maha Chakri Sirindhorn, in recognition of her longstanding support for science and paleontological research in Thailand,” Millien said.

How Big Was This 34-Million-Year-Old Rodent?

To estimate the animal’s size, the research team applied seven established methods that use tooth measurements to calculate body mass. Their results suggest that Yakamys sirindhornae weighed approximately 30 kilograms, making it about twice as heavy as other known rodents from the same period, including Hulgana.

That would have made the newly discovered species exceptionally large compared with its contemporaries. However, the researchers emphasize that determining an extinct animal’s weight from a small number of fossilized teeth involves considerable uncertainty.

“This is only a ballpark figure, as there is substantial uncertainty associated with fossil body-mass estimation,” Millien said.

The limited fossil evidence also makes it difficult to reconstruct how Yakamys evolved or what its life was like. Additional discoveries will be needed to better understand its physical characteristics, ecological role, and evolutionary relationships.

International Research Uncovers Thailand’s Prehistoric Past

The fossils were identified through an ongoing international collaboration investigating ancient mammals in Thailand’s Krabi Basin. The project brings together scientists from McGill University, Thailand’s Department of Mineral Resources, and French research organizations, including CNRS and Université de Poitiers.

By revealing evidence of an unusually large rodent in Southeast Asia 34 million years ago, the discovery expands scientists’ understanding of where and when rodent gigantism emerged. Future fossil finds could help establish how these ancient giants lived and how their evolution relates to the enormous rodents that appeared elsewhere in the world.

The research received support from CNRS, Université de Poitiers, Université de Montpellier, McGill University, and the Department of Mineral Resources of Thailand.

The discovery comes from an international team at the University of California, Davis, Peking University, and the Geological Museum of Guizhou. Their findings, published in Nature Ecology & Evolution, are based on two exceptionally well-preserved skeletons dating back nearly 237 million years to the Late Triassic period.

Both fossils were uncovered in China’s Guizhou Province, in a region that was once submerged beneath an ancient ocean hundreds of miles from major landmasses.

“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 finding is especially surprising because amphibians are generally unable to survive in seawater. Among the more than 8,000 amphibian species living today, none are known to spend their entire lives in marine environments, and fewer than 2% have any ability to tolerate salt.

Mammals, reptiles, and birds, by comparison, all include species that can live entirely at sea without relying on land for food or fresh water. Until this discovery, scientists had never identified convincing fossil evidence of an amphibian with the same ability.

A Giant Armored Amphibian Built for the Seafloor

Because the two skeletons are so complete, researchers were able to piece together the animal’s appearance and develop a clearer picture of how it lived.

Gerrothorax maritimus measured nearly three feet long, but its proportions were far different from those of a modern salamander. Rather than having a long, slender body, it was unusually broad and flattened, with a distinctive disc-shaped form.

Its body was also covered in thick bony armor, which may have helped protect it from large marine predators such as ichthyosaurs.

“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.”

The structure of its skull, together with its heavy armor, suggests that this ancient amphibian was an ambush predator. Instead of actively chasing its meals, it likely rested on the seafloor until an unsuspecting animal came within reach.

“They would probably just wait at the bottom and then when prey comes around, they would just suck it in,” said Motani, who noted that the hunting strategy resembles that of the wobbegong, a type of carpet shark that lives near the seafloor and captures prey using suction.

How Could an Amphibian Survive in Salt Water?

One of the biggest mysteries surrounding Gerrothorax maritimus is how it managed to survive in an environment that is hostile to nearly all modern amphibians.

The problem comes down largely to amphibian skin. Unlike the relatively waterproof skin of many other animals, amphibian skin allows water and gases to pass through it. This makes it useful for breathing and maintaining hydration, but also leaves amphibians especially vulnerable to the effects of salt water.

“That skin is permeable, and that allows the animals to use it for multiple purposes,” said Motani. “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.”

As a result, modern amphibians generally depend on freshwater habitats or environments close to them. A few species have developed limited salt tolerance, including the crab-eating frog. Even these animals can lose weight when exposed to brackish water and must eventually return to freshwater to recover.

The ancient Gerrothorax maritimus, however, may have evolved a very different solution.

Researchers suggest that its extensive bony armor restricted the movement of water through its skin. This protective covering may have reduced its dependence on absorbing water directly through the skin, allowing the animal to take in seawater through its mouth instead.

Under this proposed mechanism, water would have been absorbed through the digestive tract, while the animal’s gills could have helped eliminate excess salt. Gills are known to have been present in members of the Gerrothorax genus, giving the researchers reason to suspect that this species possessed a more fishlike approach to managing its salt balance.

Together, these adaptations may explain how an amphibian managed to occupy an ecological niche that remains inaccessible to its modern relatives.

Fossil Evidence Points to a Fully Marine Lifestyle

The researchers also examined whether Gerrothorax maritimus truly lived in the ocean or whether its remains might have been transported there after death.

Several clues strongly favor a marine lifestyle.

First, the skeletons were recovered from sediments deposited in a deep marine environment. Their remarkable completeness also provides important evidence about where the animals died.

“Carcasses at sea, unless they sink to the bottom right away, tend to dry out and lose limbs prior to sinking to the bottom,” said Motani. “But these animals had their fingers preserved, so that means they probably died near where they lived.”

The preservation of such delicate skeletal structures suggests that the bodies were not carried long distances before reaching the seafloor.

To investigate further, the team analyzed the fossils’ chemical composition, focusing on the relative abundance of different forms of strontium, known as isotopes.

The results closely matched the strontium isotope signatures expected in ocean water during the Late Triassic, providing another independent line of evidence that the animals inhabited a marine environment.

Taken together, the fossil preservation, geological setting, and chemical findings make a compelling case that Gerrothorax maritimus spent its life entirely at sea.

The discovery reveals an unexpected evolutionary path for amphibians, showing that at least one ancient species appears to have overcome the biological limitations that keep its living relatives largely confined to freshwater environments.

The research received funding from the National Natural Science Foundation of China.