Diadectes sideropelicus (Cope 1878a, b) Early Permian ~270 mya, 1.5-3 m in length, is widely considered a bulky pre-reptile, but here Diadectes nests in the middle of the Lepidosauromorpha. Derived from a sister to Orobates, Diadectes phylogenetically was a sister to Procolophon, a relationship that was never previously considered. Apparently Diadectes represents the last of its lineage.
Distinct from Orobates, the skull of Diadectes had rounder profile and more strongly descending cheek flanges. The posterior skull was strongly embayed. The orbit and naris were relatively smaller. The teeth were shorter.
The presacral column was reduced to 20 vertebrae. The neural spines were tallest near the pelvis. The ribs were widest anteriorly, as in Limnoscelis. The tail was shorter.
The legs were shorter and the feet were proportionately smaller.