Newly identified material belonging to the holotype specimen of Ianthodon schultzeisubstantially increases our knowledge of this poorly known basalsphenacodont synapsid from the fossil site in Garnett, Kansas (Missourian, LatePennsylvanian). The original description, based on a partial dermal skullroof, is augmented with information on the palate and braincase, togetherwith data on the mandible and a few postcranial elements. The known skeletalmorphology resembles that of Haptodus garnettensis, another synapsid taxon known from thislocality, but with fewer marginal, distinctly recurved teeth and smallerteeth on the transverse flange of the pterygoid. Although recognizing thatthe holotype and only known specimen represents a juvenile individual,Ianthodon appears to reflect a more basal sphenacodontian condition than H. garnettensis. Arestricted phylogenetic analysis based on previous work and newly scoredcharacters for Ianthodon, Cutleria and Pantelosaurus supports this hypothesis. The Garnett localityappears to preserve an assemblage of synapsids (Haptodus, Ianthasaurus, Ianthodon) that are close to the baseof the large clade that includes Edaphosauridae and Sphenacodontia,suggesting that an initial diversification of this clade occurred well within theCarboniferous Period.