Recurrent Patterns and Processes: The Significance of Ichnology in Evolutionary Paleoecology

Abstract

Evaluation of the trace-fossil record of several major evolutionary events allows detecting recurrent patterns that may help to enhance our understanding of the underlying dynamics of animal-substrate interactions through geologic time. In order to detect these recurrent motifs, organism-substrate interactions during evolutionary radiations, benthic fauna response to mass extinctions, patterns of ecospace colonization, and environmental shifts through time are evaluated. Comparison of the ichnologic record during the Cambrian Explosion, the Great Ordovician Biodiversification Event and the Mesozoic Marine Revolution as well as of events in the continental realm indicates that evolutionary radiations are invariably associated with an ichnodiversity increase. Analysis of ichnodiversity changes through geologic time supports Sepkoski’s three-phase kinetic model, which was originally based on analysis of marine body fossils. In contrast, increases in ichnodisparity are not linked to evolutionary radiations per se, but to the colonization of empty ecospace, as illustrated by the Cambrian Explosion for softground colonization, the Great Ordovician Biodiversification Event for colonization of hardgrounds, and the colonization of paleosols by the end of the Mesozoic. Ichnologic evaluation of mass extinctions suggest common responses from the benthos, typically a reduction in ichnodiversity, decrease in degree of bioturbation, reduction of bioturbation depth, size reduction, dominance of simple trace fossils in post-extinction strata, preferential survival of ichnotaxa produced by deposit feeders, and selective extinction in shallow-marine ecosystems. The severity of the mass extinction event essentially controls how pronounced were these effects, with the end-Permian mass extinction being the only one displaying evidence of the collapse of the mixed layer. The recurrent presence of certain trace-fossil assemblages in rocks of different ages and formed under a wide variety of environmental settings reveals a limited repertoire of behavioral strategies that allow benthic faunas to colonize empty or underutilized ecospace. Finally, the ichnologic record provides further empirical support to the onshore–offshore model across a broad range of scales, from that of individual ichnotaxa to the one displaying initial colonization of habitats.