Craspeditidae (Ammonoidea) of the Russian Platform at the Jurassic–Cretaceous Boundary. II. Genus Hectoroceras Spath

Resumo

The short geochronological span and wide geographic distribution of the genus Hectoroceras in the Boreal Paleogeographic Superrealm make it exceptionally important for correlation of the Volgian–Ryazanian boundary beds. The Ogarkovo Section on the Unzha River (Kostroma Region), which is essential for this correlation, is described. The revised genus Hectoroceras includes the type species H. kochi Spath, H. larwoodi Casey, H. tolijense (Nikitin), and its microconch H. pseudokochi (Mesezhnikov), the latter two previously assigned to the genus Shulginites Casey, 1973 which is here considered as a junior subjective synonym of Hectoroceras. The FAD of Hectoroceras kochi is an interregional marker for Panboreal correlation, and the coincidence of this event with the invasion of ammonites of Tethyan origin into the Central Russian ecotone improves the prospects for a Boreal – Tethyan correlation of the Jurassic–Cretaceous boundary interval.