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...