A boring is any biogenic structure that involves erosion of an already consolidated substrate by an organism; the process of forming a boring by an organism is bioerosion. Although most geologists may think of borings as formed exclusively in rocks, other substrates suitable for borings include wood, shells, bones, or eggs. Under the definition given here, toothmarks also could be considered as borings.
Tracemakers of borings are quite diverse, ranging from algae to tyrannosaurs. A partial listing of bioeroders includes some types of fungi, foraminiferans, poriferans, cnidarians, nematodes, phoronids, bryozoans, brachiopods, sipunculids, polychaetes, bivalves, gastropods, cephalopods, amphineurans, arthropods, echinoids, and fish. Probably the best-known bioeroders are bivalves, of which certain species can bore into rocks and wood. In continental environments, wood has been a preferred substrate for some insect species, such as termites, ants, and bees, as well as woodpeckers. Claw marks left by modern bears on trees are borings, although I would also call them tracks because they represent an impression made by a foot. Toothmarks, which I will restrict in usage here to mean marks made by vertebrates with teeth, have been attributed to fossil organisms such as sharks, mosasaurs, or theropods. However, some invertebrates, such as predatory octopi or gastropods, can leave toothlike marks in bivalve shells through their beak and radula, respectively.
A continuum of biogenic structures in sedimentary rocks can range from softground to firmground to hardground traces; this continuum represents a change in relative water content in the original sediment to a state of cementation. Borings made in marine-related environments often represent a hiatus in deposition that formed a hardground or, in the case of a wood accumulation, a woodground. Hardgrounds are typically represented by the Trypanites ichnofacies, whereas woodgrounds are represented by the Teredolites ichnofacies. The Scoyenia ichnofacies includes borings made in continental settings, although subdivision of this ichnofacies would be very helpful for better representation of the diversity of borings reflecting environmentally sensitive factors.
Further Reading on Borings:
Bromley, R. G. 1992. Bioerosion: Eating rocks for fun and profit. In Maples, C. G., and West, R. R. (eds.), Trace Fossils. Short Courses in Paleontology No. 5, Paleontological Society, Knoxville, Tennessee, p. 121-129.
Warme, J. E. 1975. Borings as trace fossils, and the processes of marine bioerosion. In Frey, R. W. (ed.), The Study of Trace Fossils. New York, Springer-Verlag, p. 181-227.
Dinosaur Toothmarks as Trace Fossils - link to section in the Dinosaur Trace Fossil page.