Global palaeobiogeography of Upper Ordovician acritarchs

Peri-Gondwana and Baltica acritarch provinces are easily recognized during the Early-Middle Ordovician, illustrating provincialism of the global phytoplankton. However, acritarch assemblages become increasingly similar with a general cosmopolitanism towards the Late Ordovician, although acritarch assemblages from Laurentia and Baltica differ from those from peri-Gondwana. Acritarch assemblages from South America, Baltica, Iran, and Siberia, and new material from South China and Tarim, provide additional data to better understand the Late Ordovician palaeobiogeographical evolution of the phytoplankton. Sselected papers were used to build datasets for Cluster Analysis (CA), Nonmetric Multidimensional Scaling (NMDS) and Network Analysis (NA), to constrain acritarch palaeogeography. The global palaeobiogeographical distribution points to a clear acritarch provincialism during the Late Ordovician, particularly in the Katian, mainly controlled by palaeotemperature and palaeolatitude. A modified latitudinal diversity gradient shows the acritarch diversity peak moved from middle latitudes in the Early and Middle Ordovician to lower latitudes in Late Ordovician within the Southern Hemisphere. The peak of acritarch diversity correlates with strong palaeogeographical differentiation in the Katian. The proportion of cosmopolitan acritarch taxa is higher in the Hirnantian than in the Sandbian and Katian, which may be affected by the Late Ordovician Mass Extinction (LOME). Acritarch diversity in South America and North Africa increased in the Hirnantian and were less affected by the LOME, while those from the Middle East were affected moderately, and Baltica, Laurentia, Avalonia and South China were affected strongly. The different trends of the acritarch diversity changes in different palaeoplates could be linked to local environmental changes.