Middle and Upper Pennsylvanian strata in the Colorado River Valley and Llano area south of Brownwood, Texas, have not been extensively studied, and little is known regarding their precise correlation. Conodont faunas have been recovered from many units, particularly Smithwick Shale (Atokan), lower Strawn Group (Atokan-Desmoinesian), upper Strawn Group (Desmoinesian-Missourian), and lower Canyon Group (Missourian). Conodonts permit more refined correlation of this important sequence with Mid-Continent and Appalachian series than has previously been possible. In addition, age relationships of stratigraphic units places constraints on developmental models for the Fort Worth basin and adjacent Concho platform.
The Smithwick conodont fauna is indicative of a middle to late Atokan age. Diagnostic species are Neognathodus atokaensis, N. bothrops, Neogondolella clarki, N. n. sp. A, and Idiognathoides spp. Approximately the lower one-half of the lower Strawn Group is latest Atokan based on occurrences of Idiognathoides spp. The appearance of Gondolella laevis and advanced neognathodids distinguish the early Desmoinesian portion of the lower Strawn Group. Neognathodus roundyi, N. dilatus, terminal neognathodids, Gondolella bella, and G.magna are typical conodonts of middle to late Desmoinesian strata of upper Strawn Group. Missourian conodont faunas are relatively depauperate, and seemingly have less value for precise correlation than older Desmoinesian and Atokan faunas.