Two distinct regimes of extinction dynamic are present in the major marine zooplankton group, the graptolites, during the Ordovician and Silurian periods (486−418 Ma). In conditions of "background" extinction, which dominated in the Ordovician, taxonomic evolutionary rates were relatively low and the probability of extinction was highest among newly evolved species ("background extinction mode"). A sharp change in extinction regime in the Late Ordovician marked the onset of repeated severe spikes in the extinction rate curve; evolutionary turnover increased greatly in the Silurian, and the extinction mode changed to include extinction that was independent of species age ("high-extinction mode"). This change coincides with a change in global climate, from greenhouse to icehouse conditions. During the most extreme episode of extinction, the Late Ordovician Mass Extinction, old species were selectively removed ("mass extinction mode"). Our analysis indicates that selective regimes in the Paleozoic ocean plankton switched rapidly (generally in <0.5 My) from one mode to another in response to environmental change, even when restoration of the full ecosystem was much slower (several million years). The patterns observed are not a simple consequence of geographic range effects or of taxonomic changes from Ordovician to Silurian. Our results suggest that the dominant primary controls on extinction throughout the lifespan of this clade were abiotic (environmental), probably mediated by the microphytoplankton.T he importance of the marine plankton in both the carbon cycle and in the food web that supports the diversity of marine life is undisputed. However, the evolutionary dynamics of planktic species and the factors controlling their diversity and evolutionary turnover are still poorly known (1, 2). This is particularly so for the Paleozoic, where problems of preservation and sampling bias, and poor time resolution, have precluded detailed analysis. How does the marine plankton respond to environmental perturbations arising from climate change over geological time? How does background extinction differ from episodic and mass extinction in the pelagic realm? Is the risk of plankton species extinction dependent on the amount of time since the species originated (3)? These questions have important implications for macroevolutionary process, stability of marine ecosystems, and modern biodiversity conservation (3-5). Here, using a new global data set of unparalleled temporal resolution, we attempt to answer these questions.The graptoloid clade (order Graptoloidea) constituted the main component of the early Paleozoic macrozooplankton from the beginning of the Ordovician to the Early Devonian (6). Graptoloids were colonial filter-feeding protochordates, generally ranging from a few millimeteres up to ∼200 mm in maximum dimension, which lived suspended in the water column in a range of depth zones. They have been used extensively for correlation and zonation (7-10), and the stratigraphic distributions of species are well docum...