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