Record of microendoliths in different facies of the Upper Ordovician in the Cincinnati Arch region USA: The early history of light-related microendolithic zonation

This study provides the first documentation of microendolithic borings made by cyanobacteria, algae, and fungi in invertebrate skeletons from the Late Ordovician of eastern North America. Samples from the upper Mohawkian to Cincinnatian Series (Katian Stage, Caradoc to lower Ashgill of traditional usage) limestones and calcareous shales of Ohio, Indiana, and Kentucky were selected for study, because of the excellent preservation of fossil shell material; the substrates mostly are brachiopods and corals. Strata of the Mohawkian Lexington Limestone and Cincinnatian Kope Formation, Maysville, and Richmond Groups have been recently studied in terms of facies, sequence stratigraphy, and paleoenvironments, which facilitates a reliable comparison of the distribution of microborings and inferred environmental settings, especially relative paleobathymetry. aper reports the first occurrence of 10 ichnospecies plus various Conchocelis traces in Late Ordovician (Cincinnatian) marine shells. These traces have been previously observed in shells from younger geological periods. Their modern counterparts are produced by species of cyanobacteria, green algae, red algae and heterotrophic microendoliths. Considering the bathymetric index ichnocoenoses of microendoliths defined for the later Phanerozoic, it appears that only one or two light-related bathymetric zones can be recognized in a few samples from the Upper Ordovician (Cincinnatian) of Ohio and Kentucky (but not in all deposits where they might have been expected): the Shallow Euphotic Zone III and — with less certainty — the Deep Euphotic Zone. On the contrary, our samples from the underlying Lexington Formation contain only borings of “worms” (Macrotubular Form) and some forms with unknown producers, although they were collected in facies realms comparable to the younger samples. the present state of knowledge, the best explanation for the appearance of these 10 ichnospecies and the first bathymetric index ichnocoenoses within the Late Ordovician is that they represent the early evolution of microendolithic ichnospecies and early emplacement of ichnocoenoses.