Conodonts of the Desmoinesian (middle Pennsylvanian) Lost Branch Formation, Oklahoma and Kansas
Rosscoe, Steven J.
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The Lost Branch Formation of Oklahoma and Kansas was deposited during the last Desmoinesian (Middle Pennsylvanian) transgressive-regressive cycle of the Midcontinent Sea. The regression of the Midcontinent Sea during Lost Branch deposition records the final occurrences of the conodont genera Neognathodus and Swadelina as well as numerous ammonoids, the Desmoinesian brachiopod Mesolobus, the fusulinid Beedina, and some sponges and palynomorphs. The Lost Branch Formation begins at the top of the Dawson Coal in a sandy shale deposit of the Upper Holdenville Shale. The transgression of the Midcontinent Sea is recorded in the Upper Holdenville Shale, the Homer School Limestone, and the Nuyaka Creek Black Shale Bed. The regression of the Midcontinent Sea is recorded in the “offshore” Upper Holdenville Shale, the Glenpool Limestone, and the upper sandy shale deposit of the Upper Holdenville Shale. The lower outside shale is characterized by a conodont fauna of Idiognathodus species A, Adetognathodus lautus, and Idioprioniodus. As the transgression progressed there was further diversification of the fauna. The Homer School Limestone has the first appearance of Neognathodus dilatus dilatus, N. expansus expansus, and Hindeodus minutus. Specimens of I. expansus, Swadelina nodocarinata, N. dilatus bifurcatus, N. roundyi, N. expansus subspecies B, Gondolella magna, G. denuda, grooved morphotypes of I. expansus, and rounded and nodose morphotypes of Sw. nodocarinata are found in the overlying Nuyaka Creek Shale. The Nuyaka Creek Shale is the only interval where G. magna, G. denuda, and the rounded and nodose morphotypes of Sw. nodocarinata are recovered. The regression of the Midcontinent Sea is reflected in the conodont fauna by a loss of diversity. In the offshore shale of the Upper Holdenville Shale the conodonts Swadelina nodocarinata and Hindeodus minutus are absent. The Glenpool Limestone lacks the grooved morphotype of Idiognathodus expansus, Neognathodus dilatus dilatus, N. dilatus bifurcatus, N. roundyi, N. expansus expansus, N. expansus subspecies B, and Idioprioniodus. The only species found in the upper outside shale are I. expansus, I. species A, and Adetognathodus lautus. A paleoecological model for the Lost Branch Formation is presented that shows species tolerant of turbid waters living in the surface waters of the sea, and species preferring clear waters appearing only in the deeper offshore intervals. Gondolella and Swadelina dominate in clear-water assemblage, while Idiognathodus dominates in turbid waters. The well-preserved conodont fauna and easy clay-shale processing of the Lost Branch Formation provided an opportunity to study conodont Pa element function as it relates to microwear and element morphology. The original texture of blade denticles is fibrous, while the bar of the blade is smooth and featureless. The original texture of the platform forms a polygonal texture on the oral surface and a smooth texture on the aboral surface. Abrasive microwear removes original texture from the elements. Reshaping microwear takes element features, like denticles and ridges, and reshapes them into asymmetric surfaces indicative of their functional significance. By comparing the morphology of an element with the areas of intense microwear it was determined that the Pa element of Idiognathodus expansus served a food-processing and a food-transport function. The blade of the element pushed large food particles onto the ventral platform for crushing. The blade may also have sliced larger food particles. The ventral platform served a crushing and bruising function. The dorsal platform of the element pushed food back to the ventral platform for further processing.