Diadectomorphs possess both amphibian and reptilian characteristics. Originally these animals were included under the order Cotylosauria, and were considered the most primitive and ancestral lineage of reptiles. More recently they have been reclassified as tetrapods, closely related to both reptiles and amphibians. The Diadectomorpha seem to have evolved during late Mississippian times, although they only became common during the Late Pennsylvanian and Early Permian periods.