The number and spool-like shape of the vertebrae, in addition to the poses that the reptiles died in, show that they were also very flexible creatures. The vertebral centra were amphicoelous (concave from the front and behind) and had large ventrolateral depressions. A similar depression can be seen on the massive neural arches, running from the prezygapophyses to the postzygapophysis. These depressions allowed the vertebral column to be lighter while still retaining a sturdy build. The pectoral girdle in "Petrolacosaurus" is light in comparison to the massive girdles found in taxa such as pelycosaurs. Petrolacosaurus specimens had a clavicle with an expanded ventral blade, yet shorter than that of C"aptorhinus" or pelycosaurs. Also, there is a large cleithrum with a broad, rounded end. The limbs of "Petrolacosaurus" were long and slender in comparison to other primitive diapsids such as "Araeoscelis". The radius and ulna in "Petrolacosaurus" are of relatively equal length. Unlike these two bones, the fibula is significantly shorter than the tibia, because the tibia has distal articulation with the astragalus. This length difference is further pronounced because the astragalus has a long-neck and buttressed, oblique platform providing an inflexible, concave articulation with the tibia. This articulation creates a virtually immobile joint between the two. "Petrolacosaurus" has a phalangeal formula for the manus of 2-3-4-5-3. For the pes, the formula is 2-3-4-5-4. The digits increase in length from digit I-digit V. Metatarsal IV is 3.5 times as long as metatarsal I. Many lines of post-Paleozoic reptiles have a reduction in digit IV in the manus and pes, indicating that "Petrolacosaurus" is more primitive than those reptiles. "Petrolacosaurus" is a member of the basal diapsid order Araeoscelidia, along with the aquatic, late Pennsylvanian "Spinoaequalis". The specific taxonomic placement of "Petrolacosaurus" is a highly debated topic. Because of the unusual osteological characters and how old the reptile is, scientists go back and forth between what position makes the most sense.