GEOGRAPHIC DIFFERENTIATION OF BOREAL AMMONOIDS IN THE LADINIAN AGE (<i>Middle Triassic</i>)

Studying the spatial distribution of specific groups of marine invertebrates in the geological past and analyzing their geographic differentiation over time are crucial for understanding both their evolutionary patterns and the history of marine basin development. This study, based on modern paleontological and biostratigraphic data of the Boreal Triassic, refines the taxonomic composition and distribution of Ladinian ammonoids across various regions of the Boreal Realm. Zonal correlation on ammonoids of Ladinian deposits has been carried out for Northeast Asia, British Columbia, the Canadian Arctic Archipelago, northern Greenland, Svalbard, and Franz Josef Land, providing a chronological basis for comparative analysis of coeval ammonoid faunas. The qualitative and quantitative comparative analysis of ammonoid assemblages for different phases of the Ladinian Age has established that Northeast Asia consistently belonged to the Siberian Province of the Boreal Realm. The separation of the Canadian Province of the Boreal Realm occurred at the end of the constantis Phase due to the penetration of trachyceratids (genus Protrachyceras) into the paleobasins of the Canadian Arctic Archipelago. Subsequently, starting from the maclearni Phase, the area of the Canadian province expanded due to the migration of Tethyan forms into the paleobasins of Svalbard. The migrations of trachyceratids, gymnitids, and lobitids into boreal paleobasins from the Tethys Ocean, as well as the dispersal of Boreal tsvetkovitids and nathorstitids into ecotonal and Tethyan paleowaters of British Columbia, were influenced not only by circumpolar currents but likely, by the lifestyle of ammonoids and their adaptation to a broader or narrower range of marine basin depths.