Turkish Journal of Earth Sciences




The rudist bivalve Thyrastylon is redescribed based on extensive material collected from Jamaica. The new material shows that the left valve contains slits, and that this is the diagnostic feature of the genus. The genus is redefined as follows: a radiolitid rudist with no ligamental infold, stretched cellular microstructure and compact layers in the outer layer of the right valve; the radial bands are partially infolded into the shell layer of the right valve; the left valve has oscules developed above the radial bands and has concentric, slit-like pores that penetrate the shell and open out onto the inner part of the commissure close to the body chamber. Two species are recognized, the type species Thyrastylon adhaerens (Whitfield) from the mid to late Maastrichtian of Jamaica and Guatemala and T. chubbi Alencáster from the lower to mid Maastrichtian of Mexico (Chiapas) and Jamaica. The slits allow a reassessment of the feeding strategy of radiolitid rudists. Water entered around the entire length of the commissure, was carried across the apertural surface and expelled through the slits; the gills acting both to create the current and filter suspended detritus from seawater. The oscules acted to carry pseudofaeces out of the mantle cavity. The increase in thickness of the outer layer of the right valve in radiolitids is seen as an adaptation to increase the area of the commissure and increase filter feeding efficiency; it was not for the harboring of symbiotic algae.

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