There is considerable debate regarding the paleogeography of the Western Interior Seaway (WIS) near the close of the Cretaceous. To investigate this issue, the paleogeographic implications of recent advancements in the biostratigraphy of the lower to upper Maastrichtian, transitional-marine strata in the Raton Basin are explored. In southern Colorado, the western shoreline of the WIS should be shifted about 100 km farther west than current projections for the end of the early Maastrichtian. Strong marine connections among the WIS, the Gulf of Mexico, and the North Atlantic appear to have persisted at least until the end of the early Maastrichtian. A marine connection between the WIS and the Gulf of Mexico is projected to have lasted through the end of the Cretaceous. Closure of the WIS to the Arctic Ocean is projected to have occurred earlier and farther north than other models, which close the WIS to the Arctic Ocean through the formation of the Dakota Isthmus in the latest Maastrichtian. Closure of the WIS in Canada during the early late-Maastrichtian (Hoploscaphites birkelundae ammonite biozone) appears to have permitted the dispersal of land plants, such as “Cissitespanduratus and Credneria protophylloides, among landmasses previously isolated by epeiric seaways covering much of North America and western Greenland during the Late Cretaceous and is consistent with preexisting biostratigraphic constraints on the paleogeography of the WIS.

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