Phreatophasma aenigmaticum is a mysterious tetrapod from theearliest middle Permian of Russia, represented by a single femur. At varioustimes since its original description it has been considered a therapsidsynapsid, a pelycosaurian-grade synapsid from the family Caseidae, and mostrecently a seymouriamorph amphibian. Using up-to-date knowledge of thepostcranial morphology and evolution of early synapsids, the specimen isre-evaluated and subjected to cladistic analysis. Seymouriamorph andtherapsid affinities are rejected, and a caseid affinity is supported basedon the deep intertrochanteric fossa; the widely spaced distal condyles; theshort, robust femoral shaft; and the lack of a longitudinal ridge enclosingthe posterior margin of the intertrochanteric fossa. When included in twocladistic matrices, the first a global analysis of basal synapsids and thesecond devoted to caseids, Phreatophasma is found to occupy a basalposition within caseids, retaining plesiomorphic characters such as the lackof compression of the anterior condyle of the femur and the almost identicaldistal extent of the two condyles. The recognition of Phreatophasmaas a basal caseid has great implications for the evolution and biogeographyof this family. This is only the second example of a caseid from thepalaeo-temperate region of Russia, and it is not closely related to the first(Ennatosaurus tecton), implying at least two distinct dispersalevents from the palaeoequatorial to temperate latitudes. It also impliesthat a number of plesiomorphic characteristics of caseids, including smallbody size and a relatively long femur, were retained as late as the middlePermian, a time when caseids were otherwise represented by large herbivoroustaxa.