Historically, sandy paleochannel deposits of the 2,000 m- thick Ferris (Maastrichtian–Danian: 66–62 Ma) and overlying 3,000 m-thick Hanna Formation (Danian–Thanetian: 62–55 Ma) in southern Wyoming's Hanna and Carbon basins have been interpreted as fluvial channel fills. New selachian paleofaunas from both formations are concentrated in stratigraphic intervals that also exhibit an abundance of mechanical tidal indicators and low-diversity marine ichnofossil assemblages (indicating brackish-water). Their presence in these two continental formations supports the reinterpretation of some sandy paleochannel deposits and shaly intervals as brackish-water estuaries, tidally influenced distributary channels, and interdistributary bays. Cederstroemia and Cretorectolobus are reported for the first time from Maastrichtian deposits (they previously have been documented only from Campanian and older strata). A new species of dasyatid is preserved in the middle Ferris Formation (earliest Paleocene: 64 Ma) and awaits fuller description. Prior interpretations of the Hanna Basin area as being far removed from the influences of the Western Interior Sea (WIS) during the latest Cretaceous and Paleocene must now be re-evaluated. New paleogeographic reconstructions of the western shoreline of the WIS, based in part on the stratigraphic and geographic distribution of selachian teeth in the Hanna Basin area, suggest that a complete marine regression and continental draining at the end of the Cretaceous did not occur. When combined with sedimentological and ichnological data, selachian tooth assemblages are useful tools for interpreting depositional environments and for basin-scale research.

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