Mikael Calner

Following the Cambrian Explosion and the appearance in the fossil record of most animal phyla associated
with a range of new body plans, the Ordovician and Silurian periods witnessed three subsequent major biotic
events: the Great Ordovician Biodiversification Event, the end-Ordovician extinction (the first animal
extinction and second largest of the five mass extinctions of the Phanerozoic), and the Early Silurian postextinction
recovery. There are currently no simple explanations for these three major events. Combined
extrinsic (geological) and intrinsic (biological) factors probably drove the biodiversifications and radiations,
and the appearance and disappearance of marine habitats have to be analysed in the frame of changing
palaeogeography, palaeoclimate and sea-water chemistry. The present paper reviews the relationships of the
three biotic events to chemical and physical processes occurring in the ocean and atmosphere during the
Ordovician and Silurian, including sea-level changes, geochemical proxies (δ13C, δ18O, 87Sr/86Sr) of the ocean
waters, and the evolution of the atmosphere (oxygen and carbon dioxide content).