IMPLICATIONS OF BORING AND ENCRUSTATION OF LATE CRETACEOUS RUDISTS

During the Santonian and Campanian large radiolitid rudists flourished in the warm shallow waters of the southern Western Interior Seaway. Sea levels were falling and the Seaway was beginning its retreat. It is suggested that these substantial organisms were part of a reef-like community that developed around small centers of submarine volcanic activity.

These unusual bivalves, with commissural diameters of as much as 25 cm, were tubular or conical in shape with the second valve opercular in form. Vertical dimension could reach 35 cm; it is thought that they partially buried themselves in sediment for support during life. Although the inner shell layer was aragonitic, the broader outer cellular structure was calcite. The cell structures were filled at some point with later secondary calcite increasing the chance of fossilization.

Analysis of this fossil record as evidenced in the collections of the Texas Natural Science Center provides some interesting patterns of boring and encrustation. Juvenile oysters, serpulid worms and several types of predatory features are present. Initial results imply that much of this bioerosion occurred post-mortem.