Sclerosaurus armatus (Meyer 1859) Middle Triassic ~50 cm in length, was originally considered a procolophonid, then a pareiasaurid, then back and forth again and again, with a complete account in Sues and Reisz (2008) who considered it a procolophonid. After Procolophon, Sues and Reisz (2008) considered Tichvinskia, Hypsognathus, Leptopleuron and Scoloparia sister taxa to Sclerosaurus. These all nest with Diadectes in the large reptile tree, not pareiasaurs.
Here, based on data from Sues and Reisz (2008), Sclerosaurus nests between pareiasaurs and basal softshell turtles like Odontochelys andTrionyx. It is a sister to Arganaceras, but was smaller with larger supratemporal horns. Hard shell turtles have a different ancestry starting with Elginia and Meiolania.
Overall smaller than other pareiasaurs, like Arganaceras, Sclerosaurus had a wide, flat body, like the horned lizard, Phrynosoma. The backbone remained quite flexible, as shown by the fossil (below). Only a sparse sprinking of dermal bones lined the dorsal vertebrae.