2 BIOEROSION ICHNOFACIES PRODUCED BY EARLY AND LATE BURIAL ASSOCIATEDWITH SEA-LEVEL CHANGE

In bioerosion, as in trace fossils as a whole, deeply emplaced structu res have greater survival value than shallow structures. That is to sa y, tiering (the relative depth to which rasping, etching and boring or ganisms penetrate their substrate) is of paramount importance for the preservation potential of individual trace fossils. An Entobia ichnofa cies is established for trace fossil assemblages dominated by deep tie r borings and arising from long-term bioerosion, such as occurs on sed iment-free submarine cliffs or hardgrounds. A Gnathichnus ichnofacies comprises assemblages containing all tiers, including superficial scul ptures produced by radulation that have very little preservation poten tial. Such assemblages occur in short-term bioerosion situations as on shell surfaces and hardgrounds buried early by sedimentation.