THE GENUS KRITHE (OSTRACODA) FROM THE CAMPANIAN AND MAASTRICHTIAN (UPPER CRETACEOUS) OF THE NORTHERN US GULF COASTAL-PLAIN

The ostracode genus Krithe is one of the most common genera in the Upp er Cretaceous (late Santonian to Maastrichtian) deposits of the northe rn Gulf Coastal Plain of North America. Although it is never abundant, the genus occurs in sediments that were deposited under a wide range of palaeoenvironments, including nearshore sandy marls to offshore, ne arly pure, chalk. The taxonomy of this taxon has been problematical, a nd what is herein considered to be a single species, K. cushmani, has been referred to in the literature under five different names. Two mor photypes were observed: relatively large individuals with 'mushroom'-s haped vestibules collected from chalk, and smaller individuals with po cket-shaped vestibules collected from nearshore deposits. Species of K rithe have been hypothesized to be useful in estimating dissolved oxyg en concentration in ancient ocean floors, based on details of their mo rphology. Whereas the relationship between size and environment corrob orates with previous predictions (larger individuals live in deeper wa ter), the morphology of the vestibules contradicts predictions (the la rger vestibules occur in the nearshore deposits and the smaller, more constricted vestibules occur in the chalk). A causal relationship betw een environment and morphology is discussed.