First record of plicidentine in Synapsida and patterns of tooth root shape change in Early Permian sphenacodontians.

Recent histological studies have revealed a diversity of dental features in Permo-Carboniferous tetrapods. Here, we report on the occurrence of plicidentine (infolded dentine around the base of the tooth root) in Sphenacodontia, the first such documentation in Synapsida, the clade that includes mammals. Five taxa were examined histologically, Ianthodon schultzei, Sphenacodon ferocior, Dimetrodon limbatus, Dimetrodon grandis, and Secodontosaurus obtusidens. The tooth roots of Ianthodon possess multiple folds, which is generally viewed as the primitive condition for amniotes. Sphenacodon and D. limbatus have distinctive "four-leaf clover"-shaped roots in cross section, whereas Secodontosaurus has an elongate square shape with only subtle folding. The most derived and largest taxon examined in this study, D. grandis, has rounded roots in cross section and therefore no plicidentine. This pattern of a loss of plicidentine in sphenacodontids supports previous functional hypotheses of plicidentine, where teeth with shallow roots require folds to increase the area of attachment to the tooth-bearing element, whereas teeth with long roots do not. This pattern may also reflect differences in diet between co-occurring sphenacodontids as well as changes in feeding niche through time, specifically in the apex predator Dimetrodon.