SILICEOUS TABLETS IN THE LARVAL SHELLS OF APATITIC DISCINID BRACHIOPODS

The marine bivalved Brachiopoda are abundant throughout the geological record and have apatitic (CaPO4-rich) or calcitic (CaCO3-rich) shells . Vesicles covering the larval valves of living apatitic-shelled disci nids contain tablets of silica. The tablets are cemented into close-pa cked mosaics by spherular apatite in glycosaminoglycans. They are usua lly lost as vesicles degrade but leave imprints on the underlying apat itic shell. Similar imprints ornament larval surfaces of some of the e arliest Paleozoic apatitic-shelled brachiopods and may also be indicat ors of siliceous biomineralization.