The temnospondyl Wantzosaurus elongatus Lehman, 1961, from the Lower Triassic of Madagascar, is redescribed on the basis of a new specimen, which is the most complete trematosaur ever found. Detailed osteological observations and comparisons provide new data on the anatomy, ontogeny, palaeobiology and palaeoecology of this peculiar marine 'amphibian'. The morphology of this aquatically readapted taxon is compared to that of marine 'reptiles': Wantzosaurus might have been able to swim by undulation. A phylogenetic analysis of the trematosaurs is performed for the first time and suggests that Wantzosaurus is a derived taxon within the clade Trematosauridae. The family is defined on the basis of derived character states and is shown to be monophyletic.