A giant plagiosaurid temnospondyl from Bjørnøya (Svalbard, Norway) and the origin of the plagiosaurid body plan

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Abstract
An articulated temnospondyl skeleton from Middle Triassic deposits of Bjørnøya (Bear Island) in the western Barents Sea is described as a new plagiosaurid genus and species, Arctobatrachus urdensis . With a body length well exceeding 2 m, it represents by far the largest known member of the Plagiosauridae. Large parts of the heavily weathered skull could be reconstructed on the basis of photos taken when the specimen was first discovered in 1948. The skull outline forms a near‐perfect semicircle, with the jaw articulation located well behind the occipital condyles. The palatal and dentary tooth rows consist of large, closely set teeth and intercalated fangs, whereas the maxilla and coronoids bear numerous tiny teeth. The mandible is low but massive, with a prominent, posterodorsally curved retroarticular process. Plesiomorphic character states consist of a rhomboidal interclavicle, a tall and slender cleithrum, short disc‐shaped vertebral centra, and the total absence of gastral scales and osteoderms. Some anterior vertebral centra closely resemble those of large capitosaurs and metoposaurs. A cladistic analysis finds A. urdensis to nest at the base of the plagiosaurids, sharing the extensive pectoral girdle and derived skull features with other taxa of that clade. With its huge size and enlarged gape, A. urdensis represents the first plagiosaurid top predator known so far, and its occurrence in marine deposits is consistent with the euryhaline habits of some other taxa in the clade.

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