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Studies of conodonts from the North American Lower Ordovician have concentrated on taxonomy and biostratigraphy. These objectives will continue to have high priority in the near future; however, sequences of conodonts now known in the Lower Ordovician of central and western United States allow preliminary assessment of the geographic distributions of the faunas. By earliest Ordovician time, conodonts had differentiated into a population that inhabited the shallow seas of the craton and another population that was adapted to the deeper conditions of the marginal basins. The conodonts of younger Ibexian rocks were segregated into biofacies whose regional distributions suggest concentric belts around the central craton. Present knowledge of occurrences of these conodonts permits recognition of associations of genera and species that are characteristic of deposits that accumulated on (1) intertidal and shallow subtidal carbonate banks and flats, (2) shelf areas in the open ocean, and slope-rise regions in marginal basins. The possibility of further paleobiogeographic subdivision of shelf faunas is suggested by the evidence at hand, but greater geographic control will be needed to verify it. The factors that controlled the distribution of these conodonts along a probable bathymetric gradient are not known.

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