FREQUENCY DISTRIBUTION OF CONODONTS AT THE MAXIMUM FLOODING SURFACE OF THE LOST BRANCH CYCLOTHEM (LATEST DESMOINESIAN STAGE), CENTRAL AND WESTERN UNITED STATES

The Nuyaka Creek Shale and its equivalent sea-level maximum deposits of the Lost Branch Cyclothem (latest Desmoinesian Stage) extend from Texas to Iowa, and correlate with time-equivalent lithofacies in the Illinois and Paradox basins of the central and western United States. The Lost Branch is not considered a major cyclothem because it has not been identified in the Eastern Interior (Appalachian) Basin. Nevertheless, its biostratigraphic significance is recognized for containing the last occurrences of numerous distinctive Desmoinesian genera just below the Missourian Stage (for example, the conodont Neognathodus, the fusulinid Fusulina sensu stricto, and the brachiopod Mesolobus).

The standard characterization of black shale conodont faunas implies a consistent geographic distribution of genus-level taxa. However, observation of numerous collections from across the midcontinent suggested that Swadelina is proportionally more abundant in (modern) northern outcrops, while Idiognathodus is proportionally more abundant in southern outcrops. The relative abundance of Neognathodus seemed widely variable and random. To test these qualitative observations, a north-south and an east-west transect of samples were taken from the maximum-flooding surface within Lost Branch outcrops. Quantitative frequency analyses were performed at both the genus- and species-levels. Ontogenetic groupings were also quantitatively analyzed. The database consisted of Adetognathus lautus, Diplognathodus sp., Ellisonia sp., Gondolella bella, G. ovata, G. cf. G. denuda, Hindeodus minutus, Idiognathodus expansus, I. swadei, I. heckeli, I. n. sp. A., Idioprioniodus conjunctus, Neognathodus dilatus, N. expansus, N. n. sp. A., N. n. sp. B., Swadelina neoshoensis, S. nodocarinata, S. recta, and juveniles assigned to “idiognathodids” and “neognathodids”.

Our results do not support a proportional continuity of conodont genera over vast geographical areas, nor do our results support an inverse relationship that links frequencies between any genus-level taxa. At the species level, however, some cases of inverse frequencies were documented. Furthermore, our ontogenetic analyses suggest that conodont faunas recovered from core shale maximum flooding surfaces are time-averaged and do not represent a kill event.