A new specimen from the Upper Pennsylvanian of Garnett, Kansas, is referable to the edaphosaur Ianthasaurus hardestii. It is the second articulated skeleton known of this species, and possesses previously undescribed midline elements of the skull roof. This specimen features a new autapomorphy, an elongate cross-barred dorsal process on the axial neural spine. The presence of ventral webbing and multiple tubercles on several basal lateral protuberances of the presacral neural spines in this skeleton may represent a sexual dimorphism of Ianthasaurus. Two synapomorphies of sphenacodonts and edaphosaurs, the presence of a lateral lappet on the frontal and exclusion of the reduced quadratojugal from the ventral margin of the temporal bar, are confirmed in this specimen. The presence of these traits in Ianthasaurus supports the hypothesis that edaphosaurs and sphenacodonts form a clade more derived than other pelycosaur groups. New information provided by this specimen indicates that edaphosaurs can be recognized only by the morphology of their distinctive presacral neural spines. Small, problematical edaphosaur species assigned to the genus Edaphosaurus may be reinterpreted as insectivores, closely related to the Garnett edaphosaur and distinguishable from the large, bulky herbivore Edaphosaurus.