A large temnospondyl humerus from the Rhaetian (Late Triassic) of Bonenburg (Westphalia, Germany) and its implications for temnospondyl extinction

Temnospondyls are a group of basal tetrapods that existed from the Early Carboniferous to the Early Cretaceous. They were characteristic members of Permian and Triassic continental faunas around the globe. Only one clade, the Brachyopoidea (Brachyopidae and Chigutisauridae), is found as relics in the Jurassic of eastern Asia and the Cretaceous of Australia. The other Late Triassic clades, such as Plagiosauridae, Metoposauridae, and Cyclotsauridae, are generally believed to have gone extinct gradually before the end of the Triassic and putative Rhaetian records are stratigraphically poorly constrained. Temnospondyl humeri all show a similar morphological pattern, being stout, short, with widened ends, and with a typical torsion between the proximal and distal heads. Based on these characters, a humerus found in a Rhaetic-type bonebed in unequivocally Rhaetian sediments (marine Exter Formation) in a clay pit at Bonenburg (eastern Westphalia, Germany) was identified as pertaining to the temnospondyl cf. Cyclotosaurus sp. The humeral midshaft histology also supports temnospondyl affinities and serves to exclude plesiosaurs and ichthyosaurs from consideration. This find is the geologically youngest record of a non-brachyopoid temnospondyl, indicating that cyclotosaurids survived well into the Rhaetian, likely falling victim to the end-Triassic extinction.