Latitudinal gradient of taxonomic richness of ammonites in the Kimmeridgian-Volgian in the Northern Hemisphere

The latitudinal gradient of taxonomic richness (LGTR) of Kimmeridgian and Volgian ammonite genera of the Northern Hemisphere is evaluated, and the LGTR evaluation methods and factors influencing LGTR are discussed. In the Kimmeridgian-Volgian the LGTR values for ammonites were largely influenced by the paleogeography of the Middle Russian Sea, which was directly connected with the Neotethys and, to a lesser extent, by the exchange through the connection between the Arctic and Pacific oceans. The Middle Russian Sea is considered to have been a major source of immigrant taxa to the Arctic basins. The highest latitude ammonite faunas were influenced by the Pacific faunas, and due to the permanent presence of oceanic phylloceratids and lytoceratids, ammonite taxonomic richness was relatively high compared to some mid-latitudinal sites. The gradual decrease in taxonomic richness that occurred from the end of the Kimmeridgian to the end of the Volgian in all Subboreal basins probably resulted from the eventual isolation from the Neotethys. Subboreal ammonite associations from the ecotone between the two superrealms (Panboreal and Tethys-Panthalassa) were affected by short-term climatic oscillations, which led to rapid changes in the ammonite assemblages.