Petrolacosaurus kansensis was discovered in a smoothed layer of shale in eastern Kansas. The discovery included a skull with two temporal fenestrae (part of the defining characteristics of diapsids), a large orbit and teeth. It also included over 60 caudal vertebrae, 7 cervical vertebrae, the pectoral girdle, radius, ulna, fibula, and articulated tibia and astragalus. This articulation tells us a lot about the morphology and development of the reptilian astragalus and ankle joint, which is why many papers have been written on this specific aspect of the fossil remains. Manual and pedal phalanges were also discovered, allowing researchers to know the arrangement of the digits; this is actually quite important evolutionarily as digit order and retention has changed over time (some animals have presented with certain digits lost and sometimes knowing which digits were retained or lost can inform evolutionary relationships). These characteristics have helped to inform the position on the reptilian and diapsid family tree that Petrolacosaurus currently maintains. Further finds of this animal and others will help to refine that positioning further of course, but at this moment we have a good idea of where the animal is phylogenetically and what its position means for the origins of specific regions and parts of the reptilian body plan, making this an extremely important animal.