Coordinated biotic and abiotic change during the Great Ordovician Biodiversification Event: Darriwilian assembly of early Paleozoic building blocks

The Ordovician Period records an extraordinary biodiversity increase known as the Great Ordovician Biodiversification Event (GOBE), which coincided with a series of environmental changes to the Earth System, notably a cooling global ocean, increased oxygenation, and increased nutrient supply from volcanism and continental weathering. The co-evolution of Earth and its biota during this interval has been studied in various contexts on multiple paleocontinents. Emerging patterns depend on the lens of investigation. Here we summarize the current state of understanding by reviewing and synthesizing the fossil and sedimentary records. Recent paleontological studies, mainly focused on rhynchonelliform (articulated) brachiopods, bryozoa, cephalopods, trilobites, graptolites, echinoderms, and reef organisms, have documented details of diversification, body size increase, development of ecosystem complexity, and intensification of inter-continental dispersal from the late Cambrian through Late Ordovician. Biomass increased markedly between the Early and Middle Ordovician. Furthermore, diversification rates increase statistically during the mid-Darriwilian Age both globally and regionally. Coincident with these biotic changes, geochemical proxies record significant changes to Earth's physical system. Oceanic temperatures decreased, and atmospheric oxygen levels increased to near modern levels in concert with the Middle Ordovician diversification of shelly fauna. Anoxic pulses ceased and evidence for deep ocean ventilation prevails in Middle Ordovician strata. Furthermore, a major Middle Late Ordovician change in oceanic strontium isotopic composition indicates increased weathering of juvenile volcanic rocks and delivery of nutrients to marine settings. This multi-proxy dataset records near-simultaneous changes in fossil-rich shallow marine environments during exactly the interval of greatest diversification. By integrating biotic and geochemical datasets, a clear picture of the co-evolution of Earth and its biota emerges indicating that the Darriwilian was the critical interval facilitating increased capacity of ecosystems. We, therefore, recommend restricting the term GOBE to indicate this short interval of rapid diversification and ecosystem change, and using Ordovician Radiation when referencing the sum of diversifications that occurred throughout the Ordovician Period.