A semi-quantitative analysis of the distribution of conodonts in the maximum flooding deposits of the Lost Branch Cyclothem, "Latest Desmoinesian" midcontinent and Western North America




Canchola, Mary Katherine

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Conodonts are common index fossils ranging from the Late Cambrian to the Late Triassic, and represent the phosphatic "teeth," "jaws," and "remains" of a soft-bodied animal that is analogous in outward appearance to an eel (Sweet, 1988). Most conodonts are comprised of 16-17 element types. They are concentrated in sedimentary deposits called cyclothems that represent rapid sea-level changes. The Nuyaka Creek Shale (Latest Desmoinesian) and its equivalents in the Lost Branch Cyclothem (Latest Desmoinesian) extend from the state of Texas to the state of Iowa, and correlate to time equivalent lithofacies in the Illinois, Paradox, and Great Basins of the Eastern and Western United States. For many decades it has been assumed that populations of conodont genera and species are evenly-distributed geographically across cyclothems; however, we are discovering that this may not be the case. The distribution of conodonts is dependent upon ecological and ontogenetic factors; these two factors affect the distribution and frequency of distribution of the individual species within the Lost Branch Cyclothem. Using quantitative analysis and paleoecological methods, a qualitative observation that conodont genera and species frequencies vary systematically from Texas to Iowa and Nevada to Utah was tested. The list of conodont fauna included Idiognathodus expansus, I. swadei, I. heckeli, I. n. sp. A., Swadelina neoshoensis, S. nodocarinata, S. recta, Neognathodus dilatus, N. expansus, N. n. sp. A., N. n. sp. B., Adetognathus lautus, Gondolella bella, G. denuda, G. ovata, Idioprioniodus conjunctus, Hindeodus minutus, Diplognathodus sp., Ellisonia sp., and juveniles ("idiognathodids" and "neognathodids"). Analysis of the recoveries disproved that conodont proportions of adults, subadults, and juveniles are equal, and instead suggests that unequal quantities represent a time-averaged population in which conodont elements accumulated over time. Analysis has also disproven that conodont taxa vary systematically across a geographic region; however, taxa are evenly-distributed in the study tract and systematic variation occurs on a species-level.


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conodonts, cyclothems, Desmoinesian, Lost Branch, Pennsylvanian