The conceptual and methodological tools of ichnology

In order to be able to turn our attention to the potential of trace fossils to unravel information on evolutionary events, the conceptual and methodological tools of ichnology need to be briefly reviewed. Ichnology is the multidisciplinary science that focuses on the study of traces produced by organisms (both animals and plants) on or within a substrate, including all issues related to bioturbation, bioerosion, and biodeposition. Preservational aspects need to be assessed in any study involving trace fossils and a number of classification schemes are available. Because trace fossils are primarily evidence of animal behavior, evaluation of the ethologic significance of trace fossils is central to any application of ichnology, and the ethologic classification of trace fossils is an important component of the conceptual framework of ichnology. Ichnologists tend to frame their work within two different research traditions: the ichnofacies model and the ichnofabric approach. Both research strategies may be employed in conjunction, resulting in a more holistic view of the trace-fossil record. The strength of the ichnofacies model relies on its archetypal nature, whereas the strength of the ichnofabric approach resides in the evaluation of the taphonomic controls that filter the biogenic signal through the fossilization barrier. The concepts of ichnodiversity and ichnodisparity are useful to evaluate changes in types of animal-substrate interactions through geologic time. The three components of global diversity (alpha, beta and gamma) commonly used in paleobiology for body fossils have been adapted for ichnology through the definition of alpha, beta and gamma ichnodiversity. The concept of ichnodisparity is based on the definition of several categories of architectural designs. Whereas ichnodiversity refers to ichnotaxonomic richness, ichnodisparity provides a measure of the variability of trace-fossil morphologic plans. A model is proposed to analyze benthic ecospace occupation on the basis of tiering, motility, feeding mode, and ways in which animals modify and rework sediments. The conceptual and methodological tools briefly summarized in this chapter allow exploration of the potential for trace fossils to reconstruct macroevolutionary events, such as radiations, mass extinctions, and ecosystem colonization and construction.