The body is elongated, with about 36 to 40 vertebrae between the skull and the hip. No known specimen of "Westlothiana" preserves a complete tail, so the length of that region is unknown. The vertebrae of the body are multi-part bones, primarily formed by a large pleurocentrum bone at the rear of each segment, with a smaller intercentrum at the front of each segment. Vertebrae which possess this format are defined as "gastrocentrous". The pleurocentra are spool-shaped cylinders, pierced by a large canal for the spinal cord and fused to low, triangular neural spines. The intercentra are crescent-shaped, and despite being smaller than the pleurocentra, they are relatively large compared to those of most amniotes and advanced reptiliomorphs. Overall the vertebrae of "Westlothiana" most closely resemble those of the early sauropsid "Captorhinus," although the short neural spines are more similar to lepospondyl vertebrae. However, the vertebrae also possess a unique feature not shared by any other reptiliomorphs. This feature is the presence of several large keels which run along the underside of each pleurocentrum. Although keels also run along the underside of the pleurocentra in synapsids, these keels are much closer to the midline in members of that group compared to in "Westlothiana". The ribs of "Westlothiana" connect to the intercentra, and are present in every vertebra between the skull and the hip.