The description of the atlas–axis complex of Secodontosaurus (an Early Permian synapsid) indicates that sphenacodontids share with primitive therapsids the derived feature of a tall atlantal pleurocentrum that extends to the ventral edge of the vertebral column. Although similar, the postcranial skeletons of Dimetrodon and Secodontosaurus can be differentiated by the distinct shape and size of the latter's axial neural spine and posterior zygapophysis.

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