Westlothiana lizzaie (Paton, Smithson & Clack 1999) lived ~338 mya, earlier than any other known reptile.
Westlothiana nested at the base of the Microsauria, a clade which heretofore has been considered to nest just outside of the Reptilia. Some scientists consider Westlothiana one of the anamniote sisters of "true reptiles", citing the lack of fusion in the ankle. However a suite of other traits indicates this was likely a reversal, a product of the lengthening of the torso (think limb reduction in snakes) that was retained inmicrosaurs, such as Anthracodromeus and Tuditanus.
Compared to the more primitive Casineria and Gephyrostegus, Westlothiana was longer with shorter limbs and shorter girdles.
In the skull, the prefrontal extended nearly to the naris. The quadratojugal was deeper posteriorly. The squamosal posterior rim was striaght to slightly convex. No tabular horns appeared at the rear of the skull. The intertemporal bone was fused to the parietal, a different pattern than that seen in Cephalerpeton among the Lepidosauromorpha. The premaxillary teeth were all about the same length. The mandible was reduced to three major bones in lateral view.
The presacral vertebrae had increased to 36. The tail was deep with elongated chevrons.
The entire forelimb was smaller and much more slender. The manual digits were slightly longer than the antebrachium (forearm).
The ilium was reduced and the rotation anteriorly had just begun. The puboiscium was longer and shallower. The astragalus elements (tibiale and intermedium) were no longer co-ossified but distal tarsal 4 was also reduced to two elements, distal tarsal 4 and its associated centrale. Metatarsal I had lengthened to line up with metatarsals II and III.