Evolutionary patterns and palaeobiogeography of Pliensbachian and Toarcian (Early Jurassic) Radiolaria

摘要: Recent studies on the global distribution of Pliensbachian and Toarcian polycystine radiolarians allowed us to examine faunal turnovers biogeography through this critical time interval around a major ecologic biotic crisis. The analysis is based 167 species belonging 69 genera. Significant variations in ratio between number originating extinct have been recognized. During early Early FADs greatly exceeded LADs maximum diversity was reached late Pliensbachian. trend then reversed with exceeding throughout Late (extinction interval). Recovery started Middle Toarcian, when again surpassed LADs. Three differing evolutionary patterns are observed amongst radiolarian genera studied interval. largest group diversified rapidly experienced higher extinction rates Toarcian; second exhibited no changes; third mainly spongy spumellarians successful during overall fairly good agreement that other marine faunas (ammonites also benthos), but shows an inverse correlation trends phytoplankton. Correlation concomitant environmental changes indicates radiation/extinction were not consistently linked temperature fluctuations or sea-level changes. It evident decrease well before negative δ13C peak Oceanic Anoxic Event (OAE). corresponds duration short-term anomaly strontium-isotope record, including rapid 87Sr/86Sr values as increase Toarcian. This coincidence supports hypothesis predominance extinctions over originations caused by series climate related intensified magmatic activity. Some distinct biogeographic differences observed. Generic most strongly displayed presence absence particular genus abundance while range from greater variability having completely different separate palaeolatitudinal realms. Two groups distinguished: those common abundant Tethys (low latitudes) rare absent mid high latitudes, latitudes Tethys.