It is uncertain whether the ancestors of Westlothiana were ancestral to amniotes (making them the outgroup to amniotes) or were ancestral to the unrelated microsaurs.
Westlothiana possessed characters which were similar to reptiles. It had evolved thicker vertebrae in which the centrum fused to the neural arch. Its limb bones were slender, its eyes were large, and it reduced the bones in the temporal region. It lacked a phlange on palate found in amniotes and did not posses an astragalus and calcaneus in its ankle.