Microfloral changes across the Ordovician-Silurian boundary: Evidence from Canada, Britain and Estonia.

Acritarchs have been recovered from late Ordovician-early Silurian strata in Anticosti Island, the type Ashgill and Llandovery areas, and from subsurface material in Estonia. Samples have yielded 54 genera (3 new: Acutesphaera, Asperasphaera and Magnasphaera), 229 species (36 new) and 100 species described in open nomenclature. The acritarchs from Anticosti Island and Estonia are diverse and well preserved, allowing for the establishment of a detailed biostratigraphy. The type Ashgill and Llandovery areas have low to moderate diversity and less well preserved assemblages due to a greater degree of thermal maturation. The International Stratotype for the Ordovician-Silurian boundary at Dob's Linn, Scotland, yields poorly preserved acritarchs and was excluded from further study. Large numbers of acritarch taxa were casualties of the end-Ordovician extinction event, especially thick/double walled acritarchs. Morphologically complex forms also temporarily disappeared from the carbonate environments of Anticosti Island and Estonia, as did species of Veryhachium. The composition of floras from the carbonate rocks of Anticosti Island and Estonia show a greater similarity than with the clastic rocks of the Llandovery area, thus suggesting facies control on acritarch distribution. Lowermost Llandovery assemblages are dominated by geographically widespread and long ranging taxa. Recovery of morphologically complex forms, the reappearance of Veryhachium and the introduction of new thick/double walled genera occurred by the middle Aeronian. Recovery appears to have occurred earlier in the clastic environments of the type Llandovery area than the carbonate environments of Anticosti Island and Estonia. Current models for the end-Ordovician extinction do not adequately explain the selective extinction and temporary disappearance of certain groups of acritarchs. The main phase of extinction, during the mid Hirnantian, coincided with the glacial maximum, and it is likely that associated changes in oceanic circulation and stratification, and nutrient supply combined to affect acritarch productivity.