The appendicular skeleton of the Lower Permian temnospondyl Eryops megacephalus Cope, 1877, described and figured in detail, is similar to that of most temnospondyls, except that it is highly ossified. It displays terrestrial adaptations, including a reduced dermal pectoral girdle and comparatively large limbs, characterized by well-developed processes for muscle attachment. While many features that were previously unknown or uncommon among temnospondyls were identified, no apomorphies of the appendicular skeleton particular to Eryops were found. Some characteristics of the endochondral postcranial skeleton found in well-ossified temnospondyls, such as Eryops, are absent in less well ossified temnospondyls due to immaturity or paedomorphism. The effects of heterochronic processes on the morphology of the postcranial skeleton of temnospondyls and the implications for cladistics are discussed; the appendicular skeleton of Eryops is considered hypermorphic. Within the Temnospondyli, the Eryops appendicular skeleton is most similar to that of the Dissorophoidea, and most dissimilar to both the most plesiomorphic temnospondyls and the secondarily aquatic Mesozoic stereospondyls. The appendicular skeletons of well-ossified Late Permian and Mesozoic temnospondyls are not as robust as that of Eryops.
Surprisingly, Eryops, in common with other well-ossified temnospondyls, shares many derived features of the appendicular skeleton with seymouriamorphs and diadectomorphs. The presence of these previously unrecognized synapomorphies (relative to stem tetrapods and embolomeres) provides evidence for an alternative hypothesis of relationships of early tetrapods, suggesting that the Temnospondyli and seymouriamorphs plus diadectomorphs are sister taxa.