An aquatic predator measuring about 2.7 metres long lived in the Arctic more than 200 million years ago. Its almost complete skeleton was discovered in 1948 on Bjørnøya, Bear Island, in Norway. A new study shows that it belonged to an unknown species, named Arctobatrachus urdensis.
The amphibian belonged to the temnospondyls, now-extinct group of four-limbed vertebrates. Its branch, the plagiosaurids, included fully aquatic animals with a highly distinctive appearance.
Evolution of the pectoral girdle in plagiosaurids. Comparison of the skull, chest bones and, where known, the forelimbs of four species, including Arctobatrachus urdensis.
To picture Arctobatrachus, imagine a low, broad and strongly flattened animal. Its head was also very wide, with large eye sockets and a vast mouth. Its short limbs completed a body shape poorly suited to rapidly pursuing prey.
Plagiosaurids probably lived mostly near the bottom. They could wait for a fish or another small animal to pass nearby. Their wide mouth could then open suddenly and suck in water along with the prey. This hunting method better fits their heavy, flattened bodies than a long pursuit.
The length of 2.7 metres is particularly remarkable within this family. Other temnospondyls reached several metres, but known plagiosaurids were generally smaller. Gerrothorax pulcherrimus, one of the group’s best-known representatives, measured about 70 cm, for example.
Type specimen of Arctobatrachus urdensis. Photographs of the skull and skeleton taken in 1948, compared with their condition in 2025, after severe deterioration. The drawing reconstructs the appearance of the fossil when it was discovered.
Scale: 10 cm.
The fossil’s history goes back to a British expedition conducted in 1948. The skeleton was found in rocks on Mount Misery, near the summit of Urd. As early as 1949, a short publication in Nature reported the discovery and assigned the rocks to the Late Triassic.
For decades, the Bjørnøya specimen was considered close to the European genus Plagiosternum. A study published in 2025 had already indicated that it was actually a different animal. The new analysis by Rainer Schoch and his colleagues now makes it possible to assign it its own genus and species.
This revision also shows that plagiosaurids could reach a wider range of sizes than previously thought. The Arctic fossil provides a particularly spectacular example at 2.7 metres. One difficulty nevertheless remains: not the entire skeleton discovered in 1948 was recovered, and some elements are known only from documents produced at the site.