COORDINATED BIOTIC AND ABIOTIC CHANGE DURING THE GREAT ORDOVICIAN BIODIVERSIFICATION EVENT: DARRIWILIAN ASSEMBLY OF EARLY PALEOZOIC BUILDING BLOCKS (Invited Presentation)
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 coincident 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 of the GOBE. We 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.