For environmental interpretation, biogenic sedimentary structures have the obvious advantage that they are absolutely autochthonous (rare exceptions can be easily recognized) and that they reflect a direct behavioral response to environmental conditions. On the other hand, the palichnological record is largely biased by preservational factors. The majority of the biogenic traces that we observe in present-day environments, particularly the exichnial ones--those on the upper surface of a bed (Martinsson, 1970; Chapter 1, Table 2) -- may have an almost zero fossilization potential. We, therefore, consider the study of modern traces as a prerequisite to understanding the origin of individual trace fossils, but our paleoenvironmental classification is based mainly on comparisons between fossil ichnocoenoses; i.e., on the other side of the “f ossilizati barrier”.