THE ORDOVICIAN BIOEROSION REVOLUTION

It has become increasingly clear that the evolutionary radiation in the Ordovician included an extraordinary rise in bioeroders on carbonate hard substrates. Only one macroboring ichnogenus is known from the Cambrian, the simple, cylindrical Trypanites, and it is relatively rare and virtually confined to the Lower Cambrian. In contrast there are at least six macroboring ichnogenera in the Ordovician, and they are often important components of hard substrate communities. Trypanites reappears in abundance, and by the Late Ordovician the organisms producing it are recycling significant amounts of calcium carbonate. Palaeosabella is a similar macroboring with an expanded base; it shows a paleoecological distribution similar to that of Trypanites. Gastrochaenolites, a large clavate boring, is now known from Lower Ordovician hardgrounds in Sweden (Ekdale & Bromley, 2001, Lethaia 34: 1); its maker is unknown. Petroxestes is the facultative boring of mytilacean bivalves, and it is now known from the Middle and Upper Ordovician of North America. Ropalonaria represents ctenostome bryozoan etched borings especially common on calcitic shells of the Upper Ordovician. Cicatricula is a ramifying shallow boring found on Middle Ordovician hardground surfaces in Iowa; it appears to have been produced by some sort of boring sponge. These styles of bioerosion which appeared in the Ordovician remained dominant for the remainder of the Paleozoic. The only significant Paleozoic additions were acrothoracican barnacle borings in the Devonian, apparently bivalve-produced Gastrochaenolites in the Pennsylvanian, and the phoronid boring Talpina and the recurved worm boring Caulostrepsis, also in the Pennsylvanian. The boring communities do not significantly change after the Ordovician until the Mesozoic Marine Revolution brings a dramatic diversification in bivalve and sponge borings.