Fig. 1. Interpolation map of acritarch genus diversity calculated per 5° latitudinal bands.

Diversity was calculated with the standard range-through approach excluding single-interval taxa [RTexS; see (48)] for each series of the Cambrian and each stage of the Ordovician. Note the slight southward drift of peak diversity through the Cambrian, the colonization of intertropical latitudes during the Early Ordovician, and the subsequent duplication of the diversity peak. The null diversity observed above ~30°N is a consequence of the particular paleogeography of the early Paleozoic, with a Northern Hemisphere mainly constituted by the Panthalassa Ocean. Data are lacking for this large area, because the vast majority of its oceanic crust has long been recycled in subduction zones.