SYSTEMATIC PATTERNS AND ECOLOGICAL PROCESSES IN THE BIOEROSION OF PALEOZOIC MARINE HARD SUBSTRATES

Biological erosion of rocks, shells and hardgrounds (bioerosion) was a prominent ecological and sedimentological process in the Paleozoic. There are over two dozen ichnogenera of macroborings which appear in the Paleozoic, along with several microboring ichnotaxa which have not been surveyed as systematically. The macroborings of the Cambrian through the Lower Ordovician are simple cylindrical tubes (Trypanites) and ovoid punctures in shells (Oichnus). A significant diversification takes place in the Middle to Upper Ordovician when roughly half the Paleozoic macroborings appear. This has been termed the Ordovician Bioerosion Revolution because of the flourishing of new ichnotaxa showing diverse behavior by endobionts, and because of the significant amount of hard substrate consumed in the excavation processes. Another radiation of macroborings occurs in the Devonian, especially among sponge and barnacle endobionts. By the end of the Paleozoic, macroborings are known from “worms” of various kinds, bivalves, sponges, bryozoans, barnacles and brachiopods. Most of these macroborings will appear again in the Middle Mesozoic, but with very different relative abundances. Tubular worm-like borings are most common in the Paleozoic, for example, whereas Jurassic and later borer assemblages are dominated by bivalves and sponges. Our perceptions of Paleozoic bioerosion have changed considerably in the last three decades. At first Paleozoic borings were little noted, even by ichnologists. Recognition of their behavioral diversity slowly grew as the ichnosystematics were sorted out. We now see that Paleozoic borers were important contributors to the breakdown of marine carbonate substrates, with boring intensities often approaching those of the Middle Mesozoic and later. These endobionts were also significant members of Paleozoic hard substrate communities. The ecological interactions between endobionts and epibionts, for example, were rich and complex. As far back as the Ordovician borers changed hard surfaces from smooth and uniform to rough and microcavernous, producing additional microhabitats and thereby increasing niche space for encrusters and nestlers.