Understanding the Great Ordovician Biodiversification Event ( GOBE ) : Influences of paleogeography , paleoclimate , or paleoecology ?

The Great Ordovician Biodiversification Event” (GOBE) was arguably the most important and sustained increase of marine biodiversity in Earth’s history. During a short time span of 25 Ma, an “explosion” of diversity at the order, family, genus, and species level occurred. The combined effects of several geological and biological processes helped generate the GOBE. The peak of the GOBE correlates with unique paleogeography, featuring the greatest continental dispersal of the Paleozoic. Rapid sea-floor spreading during this time coincided with warm climates, high sea levels, and the largest tropical shelf area of the Phanerozoic. In addition, important ecological evolutionary changes took place, with the “explosion” of both zooplankton and suspension feeding organisms, possibly based on increased phytoplankton availability and high nutrient input to the oceans driven by intense volcanic activity. Extraterrestrial causes, in the form of asteroid impacts, have also been invoked to explain this remarkable event. INTRODUCTION Although the five major mass extinctions (in particular, the Permian-Triassic and the Cretaceous-Tertiary events) have been extensively documented, until recently, the major biodiversifications and radiations of life on Earth have attracted much less attention. The so-called “Cambrian explosion” is in many ways much better known than the Ordovician and Mesozoic-Cenozoic radiations of marine invertebrates. Although the Cambrian explosion resulted in a range of new and spectacular animal body plans, mostly known from famous FossilLagerstätten, such as the Burgess Shale (Canada), Chengjiang (China), and Sirius Passet (Greenland), the Ordovician radiation is dramatic in different ways (Droser and Finnegan, 2003) and is evident in the “normal” shelly fossil record. The term “The Great Ordovician Biodiversification Event” (GOBE) has been introduced to designate what is arguably the most important increase of biodiversity of marine life during Earth’s history (Webby et al., 2004). While the “Cambrian explosion” involved the origins of skeletalization and a range of new body plans, the Ordovician biodiversification generated few new higher taxa but witnessed a staggering increase in disparity and biodiversity (e.g., Harper, 2006). Barnes et al. (1995) reviewed the global bio-events during the Ordovician, and two international research projects have since targeted the Ordovician biodiversification. International Geoscience Programme (IGCP) Project 410, “The Great Ordovician Biodiversification Event” (1997–2002), resulted in a compilation of biodiversity curves for all fossil groups of the Ordovician biota (Webby et al., 2004). In this compilation, the dramatic increase of diversity of all groups at the specific and/or the generic level became obvious and confirmed the patterns based on previous diversity counts (e.g., Sepkoski, 1981). IGCP 503 started in 2004 under the banner of “Ordovician Palaeogeography and Palaeoclimate” and has focused on the causes and the geological context of the Ordovician biodiversification, including radical changes in the marine trophic chains. Possible triggers of the GOBE may include the near-unique paleogeography, the distinctive paleoclimate, the highest sea levels of the Paleozoic (if not the entire Phanerozoic), enhanced nutrient supply as a result of pronounced volcanic activity, and major ecological changes. In addition to these Earth-bound physical and biological drivers of biodiversity change, Schmitz et al. (2008) linked the onset of the major phase of the Ordovician biodiversification with the largest documented asteroid breakup event during the past few billion years. It seems likely that the GOBE was linked to a variety of coincident and interconnected factors. Here we review recent studies, ask “What generated the GOBE?” and indicate the perspectives for future research in this exciting and rapidly advancing field. GSA Today, v. 19, no. 4/5, doi: 10.1130/GSATG37A.1 E-mails: thomas.servais@univ-lille1.fr, dharper@snm.ku.dk, junli@nigpas.ac.cn, axel.munnecke@pal.uni-erlangen.de, alan.owen@ges.gla.ac.uk,