Pterotocrinus is an echinoderm of the class Crinoidea that is restricted to the Chesterian Series (Chestnut and Ettensohn, 1988; Sutton, 1934). The most identifiable and best preserved remains of Pterotocrinus are the wing plates (Welsh, 1978). These wing plates are specialized tegmen plates that give Pterotocrinus its name (from pteron, Greek for "wing") (Welsh, 1978). This study questions whether or not wing plates can be used for biostratigraphy throughout the Chesterian Series, as well as what the functions of the wing plates were. New morphologies of Pterotocrinus found within the Menard Limestone during this study bring into question the biostratigraphical usefulness of many of the morphologies of Pterotocrinus wing plates. Certain species of Pterotocrinus appear to remain valuable index fossils within the Menard Limestone and the Kinkaid Formation. Four new morphologies were collected within the Menard Limestone during this study. These new finds draw questions about our knowledge of this genus. This study also attempted to explain the function of the wing plate, and how it may have changed over time. Pterotocrinus wing plates evolved rapidly during the Chesterian Series developing vastly different morphologies from the time the Menard Limestone was deposited to the time when the Kinkaid Formation was deposited. This study suggests a functional shift over time, with the wing plates of the Menard Limestone acting as rudders to orient the calyx to either assist in feeding or reduce stress on the calyx, and the wing plates of the Kinkaid Formation acting as an antipredatory defense mechanism. This study presents new conclusions and new questions regarding the wing plates of Pterotocrinus.