The Great Ordovician Biodiversification Event

The Global Ordovician Biodiversification Event (GOBE) was undoubtedly one of the most significant evolutionary events in the history of the marine biosphere. A continuous increase in ichnodiversity occurs through the Ordovician in both shallow- and deep-marine environments. The earlier view that early Paleozoic deep-marine ichnofaunas are of low alpha diversity has been challenged by discoveries of moderately diverse associations. Interestingly, however, the increase in global ichnodiversity through the Ordovician is not paralleled by an increase in ichnodisparity of bioturbation structures. In fact, whereas global ichnodiversity in the Ordovician almost doubled Cambrian levels, Ordovician ichnodisparity of bioturbation structures is roughly similar to that resulting from the Cambrian explosion. Macroboring organisms also display significant evolutionary innovation and diversification in shallow-water hardgrounds and other carbonate substrates, resulting in the Ordovician Bioerosion Revolution. Along with this macroboring ichnodiversity and ichnodisparity increase is a significant rise in the rate of bioerosion in carbonate substrates. Ichnofaunal changes in lower-shoreface and offshore siliciclastic deposits through the Ordovician reveal faunal turnovers resulting from the evolutionary radiation. Lower Ordovician deposits tend to be dominated by abundant trilobite-produced trace fossils. Middle to Upper Ordovician shallow-marine ichnofaunas tend to show more varied behavioral patterns and trilobite trace fossils are rarely the dominant components. During the early Paleozoic, the tiering structure of ichnofaunas became more complex, as a result of both the addition of deeper tiers and of a wider variety of biogenic structures in previously occupied tiers. Infaunalization by deposit feeders in offshore siliciclastic environments was most likely diachronous, with the establishment of a mid-tier infauna first in Laurentia and Baltica, and only subsequently in Gondwana.