The Chappel limestone of Early Mississippian (late Kinderhook and probably partly early Osage) age crops out in the Llano region of Texas where it is as much as 45 feet thick. It contains three conodont faunal zones which, from youngest to oldest, are herein designated: (1) the Bactrognathus communis zone of probable early Osage age; (2) the Gnathodus punctatus zone of late Kinderhook (Chouteau) age—this zone ranges throughout the greater thickness of the Chappel; and (3) the Siphonodella cooperi zone, also of late Kinderhook (Chouteau) age. This zonation is based on an examination of the disjunct conodonts in more than 150 collections, as well as on a study of the frequency distribution of species in 68 collections. These 68 collections contain 22,946 identifiable specimens, some representing reworked material derived from Ordovician, Devonian, and lower-most Mississippian sources. Four generic names, Dollymae, Branmehla, Hindeodina, and Pandorinellina, are new; the last named replaces Pandorina Stauffer, 1940, which is preoccupied.
A new classification of disjunct conodonts is presented. This classification, utilitarian rather than biologic, is based on the fact that each individual conodont was built up by the accretion of lamellae about the apex of the pulp cavity. This cavity is used as the point of reference for all other parts of the conodont structure. Separation of the lamellae from one another, along one or more growth axes and in one or more directions, resulted in the development of fanglike structures without dentides (Distacodontidae); fanglike structures with denticles (Belodontidae); denticulated blades and bars (Coleodontidae, Prioniodinidae, and Prioniodontidae); platelike structures with platforms (Polygnathidae); and platelike structures with greatly expanded pulp cavities (Idiognathodontidae).
