Macro-borings on late cretaceous oysters of Egypt

摘要

The present study investigated traces of bioerosion in parautochthonous associations of oysters from the Upper Cretaceous of Egypt. These traces occurred in great quantity in the exterior surfaces of left valves and more abundantly than occurred in the interior surfaces, as well as in the surfaces of right valves. The most common bioerosion traces were formed pre- and/or post-mortem. The recognised borings appeared as systems of chambers and tunnels that were attributed to the ichnogenera Entobia (traces of clionaid sponges), Maeandropolydora (traces of polychaetes) and Gastrochaenolites (mostly traces of bivalves). Of these, the ichnogenus Entobia represents the most destructive component. The distribution and frequency of the observed borings varied greatly in the shells from one species to another, and sometimes within the same species. Patterns of borings (i.e., size, shape, distribution and frequency) indicated that the studied oyster species were lived in nearshore, shallow, relatively low-energy marine environments with low sedimentation rate and high level of productivity, i.e., optimal conditions for several bioeroders and, hence, pronounced bioerosion. These environmental aspects prevailed during and after the life of these oyster assemblages before final burial in the Late Cretaceous.