Schematic stratigraphic chart of Permian rocks discussed in this paper. The tetrapod-bearing parts of the San Angelo and Chickasha formations are highlighted. Although no overall vertical scale is implied, the relative thicknesses of the El Reno formations are adapted from Fay (1962). Abbreviation: FS: Flowerpot Shale Formation. The age of the Dog Creek Shale shown here assumes a mid-Wordian age of the Illawarra reversal; see text for details.