Westlothiana lizzaie (Paton, Smithson & Clack 1999) lived ~338 mya, earlier than any other known reptile. Derived from a sister to Brouffia and Casineria.Westlothiana was a phylogenet dead end with its long torso, but was a sister to Paleothyris.
Distinct from the more primitive Brouffia, the skull of Westlothiana had a longer postorbital portion. The prefrontal extended nearly to the naris. The squamosal posterior rim was slightly convex. The teeth were smaller. The jawline was straight and the mandible was reduced to three major bones in lateral view. The postfrontal did not overlap the postorbital. The premaxilla did not descend.
Distinct from Casineria, the presacral vertebrae had increased in Weslothiana to 36. The cervical ribs were not robust. The dorsal ribs developed from the proximal presacrals. 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. The puboischium 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 1 had elongated to line up with metatarsals 2 and 3.
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). Here Westlothiana nests as one of the most primitive reptiles and the oldest of all.