Ecostratigraphic criteria for evaluating the magnitude, character and duration of bioevents

Abstract

In order to have the capability for recognizing as many of the extinction and adaptive radiations in the fossil record as possible we should take advantage of the ecostratigraphic approach in our work. This means that we will carefully collect, stratum by stratum, data about the stratigraphic ranges of the individual taxa within individual community groups, biofacies narrowly construed, as opposed to the all too customary habit of lumping taxa from varied community groups together indiscriminately. Following this procedure enables one to far more easily recognize as well, those brief intervals when portions of the ecosystem were restructured, which is important owing to the fact that such restructuring commonly coincides with extinction and adaptive radiation events. It must be recognized that major changes in supra-specific abundance are fully as useful in pin pointing extinction and adaptive radiation events as are mere taxonomic compilations. The ecostratigraphic approach also emphasizes the fact that so-called "known" stratigraphic ranges are commonly far less than "true" ranges except for the small number of abundant genera and their species. Awareness of this last relationship makes it clear that there is no such thing as a "Background Extinction Rate" within any one community group, i.e., biofacies, because the species to species name changes within the genera of each community group are merely evidence of phyletic evolution, not the termination of a lineage. Emphasis is placed on the importance of separating out the major ecosystem components, such as the level bottom from the reef complex when trying to recognize event horizons, i.e., compilations that lump taxa from such ecosystem components together tend to blur the actual nature of the units being mixed together, giving rise to an artifactual background extinction (and adaptive radiation) rate. We now need to far more carefully sample beds above and below suspected event horizons, community group by community group, in order to discover whether or not the taxa involved in radiations and extinctions undergo a sigmoidal change in abundance or not. All of this requires that we carefully evaluate our data against a sound knowledge of classical biostratigraphy, based on the evolutionarily useful data developed during the past century and more.