Profound changes in the biodiversity and biocomplexity of marine life occurred during the early to mid-Ordovician through an interval of some 25 Myr. The planet’s seascapes were changed forever with massive hikes in biodiversity, the installation of a benthos dominated by suspension feeders together with the introduction of many new megaguilds; increased bioturbation, biotic tiering above and below the sediment–water interface, the more widespread prevalence of hard substrates, in a calcite sea, and the development of new reef formers helped drive these changes. The strong and varied regional and taxonomic components of the global signal emphasize the extrinsic controls on the radiation. A model involving the cascading development of g (interprovincial), h (inter-community) and a (intra-community) diversity highlights the interplay between ecology, environment, geography and adaptive strategy during the event. Further deconstruction of the event into individual taxonomic and regional components together with the acquisition of more environmental and isotopic data are required to elucidate the causes of this marked and multifaceted biodiversification.