Article: Homotaxy and biostratigraphical theory

The principal problem in biostratigraphical theory is a justification for using fossils to identify isochronous horizons. Biostratigraphers establish the sequential order of fossil events but there is no theoretical justification for equating constancy in stratigraphical position (homotaxy) with constancy in time of deposition. The problem is important as the way in which it is solved greatly influences the direction of biostratigraphical research. A partial solution is to use homotaxy as a weak test for diachroneity. Events that do not maintain invariant stratigraphical order are regarded as diachronous. While surviving events are not shown to be isochronous the amount of potential diachroneity throughout their individual distributions is inversely related to their stratigraphical spacing. The closer are homotaxial events in stratigraphical space the less they have wandered in time. Thus a major objective of biostratigraphical research should be to make tests of homotaxis more rigorous by raising the density of events.