Two of the greatest evolutionary events in the history of life on Earth occurred during Early Paleozoic time. The first was the Cambrian explosion of skeletonized marine animals about 540 million years ago. The second was the "Great Ordovician Biodiversification Event," the focus of this book. During the 46-million-year Ordovician Period (489-443 m.y.), a remarkable array of adaptive radiations of "Paleozoic- and Modern-type" biotas appeared in marine habitats, the first animals (arthropods) walked on land, and the first nonvascular bryophyte-like plants colonized the land. This is the first book devoted specifically to establishing the global patterns of differentiation of Ordovician biotas through time and space. It provides extensive genus- and species-level diversity data for the many Ordovician fossil groups and presents an evaluation of how each group diversified, with assessments of patterns of diversity change, and rates of origination and extinction.