Biogeography of Ordovician sponges

Sponges have an unrealized potential importance in biogeographic analysis. Biogeographic patterns determined from our analysis of all published data o n distribution of Ordovician genera indicate Early Ordovician sponge faunas have relatively low diversity and are completely dominated by demosponges. Early Ordovician (Ibexian) faunas are characterized by the widespread co-o ccurrence of Archaeoscyphia and the problematic Calathium. This association is commonly found in biohermal structures. Middle Ordovician faunas show a n increase in diversity, and two broad associations are differentiated: App alachian faunas (including Southern China and the Argentine Precordillera) and Great Basin faunas. Late Ordovician faunas show important changes in diversity and provincialis m. Hexactinellid and calcareous sponges became important and new demosponge families appeared. Four Mohawkian-Cincinnatian associations are recognized here, including: 1) Midcontinent faunas; 2) Baltic faunas; 3) New South Wa les faunas; and 4) Western North American (California and Alaska) faunas. H owever, two separate biogeographic associations are differentiated based on faunal differences. These are a Pacific association (western North America n and New South Wales) and an Atlantic association (Midcontinent Laurentia and Baltica). Distribution of sponge genera and migration patterns are utilized to consid er paleogeographic dispositions of the different continental plates, climat ic features, and oceanic currents. Such an analysis points to close paleoge ographic affinities between the Argentine Precordillera and Laurentian Appa lachian faunas. However, significant endemicity and the occurrence of extra -laurentian genera suggest a relative isolation of the Precordillera terran e during the Late Ibexian-Whiterockian. The study also shows a faunal migra tion from the Appalachian region to South China during the Middle Ordovicia n and the migration of faunas from Baltica to Laurentia in the Late Ordovic ian. The occurrence of Laurentian migrants in New South Wales during the La te Ordovician could be related to inferred oceanic current circulation betw een these two areas, although other paleogeographic features may be involve d.