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Brachiopods are among the most common elements of marine Paleozoic faunas. Many species are suitable for study of widely separated basins.
Costate specimens representing 11 species were obtained from Upper Ordovician (Richmondian) strata in the Ohio Valley, Illinois, Iowa, Minnesota, Missouri, Oklahoma, Tennessee, and Texas in order to examine geographic patterns in costation and to determine whether observed variations were uniform within each sedimentary province. Regardless of wide variations in costae between different species, most species showed some tendency toward geographic variation in costation. In the Ohio Valley, the variations generally followed a trend toward lower costation. Specimens of Lepidocyclus capax (Conrad), Austinella sooyellei (Miller), Plaesiomys subquadrata Hall, Zygospira kentuckiensis James, and Z. modesta (Hall) are typically less costate than related members in adjacent basins. These relat ons support earlier observations of provincialism in the type Richmond fauna. Exceptions to this general pattern were noted in Glyptorthis insculpta (Hall) and Rhynchotrema dentatum (Hall), both of which exhibited little geographic variation in their costation. Observations for the widespread species, Lepidocyclus capax, are somewhat contradictory. Representatives from the Ohio Valley generally are less costate than those observed in Tennessee, Iowa, and Minnesota, but are somewhat more costate than the "Fernvale" variant in Oklahoma. The latter may be an older form.
Preliminary results suggest that ecologic factors, operating within a sedimentary basin, did affect costation but not uniformly for all species. Studies are continuing to determine more precisely the degree of uniformity of costation patterns and whether such patterns are related to variations in lithology.
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