ABSTRACT

Rudists are a major component of Cretaceous carbonate strata. Studies of rudists from these strata have demonstrated that epizoans are sparse but macroborings are relatively common. Previous studies have not been made of macroborings and epizoans on rudists from nearshore, elastic, active-margin environments.

Beds of the Rosario Formation lower sandstone containing the late Campanian rudist Coralliochama orcutti crop out along the northern coast of Punta Banda on the Pacific coast of Baja California, Mexico. Serial sections of rudist skeletons from this area show that borings include Entobia and varying forms of Trypanites, while epizoans include serpulid worms, the bivalve Acila and juvenile Coralliochama. Macroborings occur on 30% of examined specimens, primarily on the upper portion of the attached valve and on the free valve, while epizoans are found on 10% of examined specimens, typically from the middle third to the commissure of the attached valve and on the free valve. Only a small portion of the skeleton may be covered by epizoans, and some boring and encrusting appears to have occurred during life. Inhibition by rudists of epizonas, a concept proposed by Kauffman and Sohl, probably was not as effective in Coralliochama as, in rudists from Antillean reefs. In addition, Coralliochama appears to be more heavily bored than Antillean reef rudists.

These results aid in understanding the broad environmental range of Late Cretaceous rudists and the adaptive response to this range. Rudists of clastic environments may have been more susceptible to boring and encrustation than those from reef associations.

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