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Ammonoids and nautiloids from the earliest Spathian Paris Biota and other early Spathian localities in southeastern Idaho, USA.

Abstract : Intensive sampling of three earliest Spathian sites represented by the Lower Shale unit and coeval bedswithin the Bear Lake vicinity and neighboring areas, southeastern Idaho, yielded several new ammonoidand nautiloid assemblages. These new occurrences overall indicate that the lower boundary of theTirolites beds, classically used as a regional marker for the base of the early Spathian, and therefore theregional Smithian/Spathian boundary, must be shifted downward into the Lower Shale unit and coevalbeds. Regarding ammonoids, one new genus (Caribouceras) and two new species (Caribouceras slugenseand 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 reportedoccurrence of the early Spathian Columbites fauna in coeval beds of the Middle Shale unit. As acomplement to ammonoids, changes observed in nautiloid dominance are also shown to facilitatecorrelation with high-latitude basins such as Siberia during this short time interval, and they alsohighlight the major successive environmental fluctuations that took place during the late Smithian–earlySpathian transition.
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Arnaud Brayard, James F. Jenks, Kevin G. Bylund, Gilles Escarguel, Emmanuel Fara, et al.. Ammonoids and nautiloids from the earliest Spathian Paris Biota and other early Spathian localities in southeastern Idaho, USA.. Geobios, Elsevier, 2019, 54, pp.13-36. ⟨10.1016/j.geobios.2019.04.007⟩. ⟨hal-02168391⟩

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