Intensive sampling of three earliest Spathian sites represented by the Lower Shale unit and coeval beds within the Bear Lake vicinity and neighboring areas, southeastern Idaho, yielded several new ammonoid and nautiloid assemblages. These new occurrences overall indicate that the lower boundary of the Tirolites beds, classically used as a regional marker for the base of the early Spathian, and therefore the regional Smithian/Spathian boundary, must be shifted downward into the Lower Shale unit and coeval beds. Regarding ammonoids, one new genus (Caribouceras) and two new species (Caribouceras slugense and Albanites americanus) are described. In addition, the regional temporal distribution of Bajarunia, Tirolites, Columbites, and Coscaites is refined, based on a fourth sampled site containing a newly reported occurrence of the early Spathian Columbites fauna in coeval beds of the Middle Shale unit. As a complement to ammonoids, changes observed in nautiloid dominance are also shown to facilitate correlation with high-latitude basins such as Siberia during this short time interval, and they also highlight the major successive environmental fluctuations that took place during the late Smithian–early Spathian transition.