Morphology, Microstructure and Stratigraphy of Some Late Cretaceous Radiolitid Rudists from Jamaica

Abstract

Radiolitid rudists from the Upper Maastrichtian of Jamaica includegenera Bournonia, Biradiolites and Thyrastylon. Three species ofBournonia are recognised, B. cancellata WHITFIELD, B. barretti TRECHMANN and B. thiadensi VERMUNT. B. cancellata evolvesinto B. barretti by the acquisition of costae on the anterior side. Thespecies of Bournonia are characterised by a distinctive cardinal apparatus, a thick inner shell layer, and an outer shell layer composed of cellular microstructure. Two species of Biradiolites are recognised– B. rudissimus TRECHMANN has an outer layer composed of cellular microstructure with occasional bands of compact microstructure, particularly in the radial bands, and B. jamaicensis TRECHMANNhas a very well organised outer layer with an inner band of cellular microstructure and an outer band of compact microstructure. Biradiolitesand Thyrastylon have a similar dentition, a thin inner layer andsimilar wall structures in the outer layer. Thyrastylon is distinguished from Biradiolites by the infolding of the radial bands in RV and bythe presence of oscules in LV. The evolutionary change from Bo. cancellata to Bo. barretti, and the first appearance of Bi. jamaicensis are probably useful biostratigraphic markers of the Upper Maastrichtian.