Borings as Trace Fossils, and the Processes of Marine Bioerosion

Marine borers are nearly ubiquitous in the modern seabed. Their distinctive excavations provide abundant potential trace fossils, and their general erosional activities (bioerosion) are important factors in marine sedimentation and benthic ecology. Species of excavators include protozoans, plants, and animals. Those best studied are boring fungi, algae, sponges, sipunculids, polychaetes, gastropods, bivalves, and echinoids; however, several other groups also contain species of borers. Many excavations in the fossil record are distinctive enough to be identified and named as trace fossils. Rock borings mark ancient shorelines, hardgrounds, and unconformities. Shell borings are useful for paleoecologic and taphonomic reconstructions. Most borers penetrate for their protection, but in the process they sculpt and significantly bioerode exposed substrates. This process is especially evident on coral reefs and limestone outcrops, because these substrates are very susceptible to biochemical attack. The mechanisms of penetration are understood for very few borers. Certain borers use solely mechanical processes, whereas others supplement or replace these with chemical means. Although bioerosion is an important natural process, little is known of the rates at which it proceeds.