The temnospondyl amphibian Erpetosaurus radiatus is known only from the Upper Freeport Coal, Allegheny Group, Middle Pennsylvanian of Linton, Ohio, USA. Restudy of its morphology demonstrates that it is a dvinosaurian and the sister taxon of the dvinosauroid (sensu Yates and Warren) clade comprising the temnospondyl taxa Isodectes, Acroplous, Dvinosauridae and Tupilakosauridae. Erpetosaurus exhibits a mosaic of primitive and derived characters. It shares synapomorphies with Isodectes and Acroplous, such as rows of square dermal plates bordering the ventromedial edge of the mandible, modified dermal scales posterior to the interclavicle and extreme elongation of the ilium. However unlike them, it retains a tympanic embayment and lacks the winged basal plate of the paraphenoid found in Isodectes and Acroplous. Unique features within the temnospondyls include a fang + pit pair and a single fang on each premaxilla and a slot on the anterior dentary to receive a premaxillary fang; pits on the premaxilla and the anterior maxilla for the reception of two dentary fangs; and an elongate, tube-like posterior extension of the parasphenoid. The immediate relationship of Erpetosaurus to a clade of temnospondyls that lack tympanic notches documents one of the nodes at which the tympanic notch was lost in phylogeny, presumably as a specialization associated with an aquatic existence.