Abstract: This paper describes the microanatomical and histological structure of different-sized femora and hypocentra of the archegosauroid amphibian Platyoposaurus stuckenbergi from the Middle Permian (Urzhum horizon, Estemmenosuchus uralensis zone) of Eastern Europe. Age variability is discussed in a comparative context, along with the ecology of the species. P. stuckenbergi femora are characterized by the complete absence of calcified cartilage and the presence of up to 15 lines of arrested growth (LAGs); hypocentra have an osteoporotic structure, lacking both calcified cartilage and LAGs. Microanatomical and morphological features indicate the predominantly aquatic lifestyle of Platyoposaurus as an active predator in river and lake habitats.