Marine Borers: Trace Fossils and Geological Significance

The purpose of this chapter is to briefly review what we know about borers as trace makers, as bioeroders, and paleoecological and sedimen- totogical indicators. Most of the discussion is documented and illustrated with previously published work, and our intent simply is to draw attention to basic principles and to the literature without excessive detail; however, some recently published and unpublished data also are presented.

Modern or fossil borings made by extant or ancient marine borers have many uses for the reconstruction of ancient environments. Because borers represent taxa that had specialized morphological characteristics for example, they are useful for biological and paleobiological studies of both the borers and the substrates that they bore. Such structures also document the evolution of borers the boring habit, and thus, in some situations borings are geologic age indicators.

Most borers discussed herein are marine forms, although freshwater and non-aquatic borers exist. Most are best known from intertidal and shal- low-water marine habitats, although some examples of deep-marine borers are documented (see below).

Much of the species richness of modern rocky coasts, and also especially of tropical reef tracts, is due to borers and to the addition of other endolithic residents as nestlers or cavity-dwellers in abandoned borings. Most of these species, are not readily apparent and must be sought by purposefully breaking open the substrate. Many of the species furthermore, are small-bodied and as yet poorly understood. Whereas most trace fossils appear best expressed in siliciclastic rocks, borings are both abundant and