Osteology and relationships of the temnospondyl genus Sclerocephalus

The temnospondyl Sclerocephalus from the Permo‐Carboniferous of Germany is one of the most completely preserved and most abundant Palaeozoic tetrapods. Here, we review the complete osteology of the genus, based on a range of fully grown specimens housed in public collections. Among the four valid species, Sclerocephalus haeuseri and Sclerocephalus nobilis reached an adult size of well beyond 1 m in length, and had robust postcranial skeletons. In the skull, the exoccipital and sphenethmoid bones were ossified, completing the well‐known ossification sequence in S. haeuseri. Large adults had an elongate trunk and a laterally compressed tail, and some individuals also retained lateral line sulci: features that taken together suggest an aquatic life. The coracoid, pubis, carpals, tarsals, and the bony tail are fully ossified in the largest specimens. The genus Sclerocephalus forms a weakly supported clade nesting firmly at the base of the Stereospondylomorpha, and the close resemblance between Sclerocephalus, Onchiodon, and Eryops is found to be partially based on shared derived states, but is mostly based on symplesiomorphies. Cladistic analysis of 54 characters and 18 taxa finds more support for the Eryopoidea hypothesis (Eryopidae + Stereospondylomorpha) than for the Euskelia hypothesis (Eryopidae + Zatracheidae + Dissorophoidea). This indicates that the large temnospondyls of the Permian and Mesozoic probably formed a natural group, and that the terrestrial adaptations of Eryops and the dissorophoids probably evolved by convergence.