Paleoecology of epizoans and borings on some Upper Cretaceous chalk oysters from the Gulf Coast

Abstract

Analysis was made of a hard substrate fauna found on right valve interiors and exteriors of the epifaunal reclining oyster Pycnodonte mutabilis from the Maastrichtian (Navarroan) Saratoga Formation (southwestern Arkansas). Comparison of boring and encrustation patterns on both sides of valves indicates that a major portion of colonization on valve exteriors occurred while host oysters were alive. Paleoautecologic information derived from such valve exterior patterns includes evidence of rheotropic orientation by encrusting juvenile P. mutabilis and preferential location of Trypanites sp. borings in superficial shell grooves. Valve exteriors supported a hard substrate paleocommunity which had the following non-interactive progressive colonization sequence: Trypanites sp. and P. mutabilis juveniles; Entobia sp., serpulid worm tubes and Bullopora sp.; and cheilostome bryozoans. This sequence could have been caused by low seasonality and ranked success of colonizing encrusters and borers. Colonization of valve interiors generally differed from exteriors only in that many interiors were 1st colonized by the clionid sponge that created Entobia sp., which had already occupied the exterior, and which quickly bored through the valve to occupy the interior upon the host''s death.