LINGULID SHELL MEDIATION IN CLAY FORMATION

Organophosphatic shells of the brachiopod Lingula squamiformis, collec ted from Scottish Lower Carboniferous shales and mudstones of intertid al to sublittoral provenance, have been studied to ascertain chemico-s tructural changes resulting from fossilization. Enough original shell has been preserved at ultrastructural and molecular levels to confirm that Carboniferous and Recent integuments are homologous with stratifo rm successions of apatitic to organic laminae forming rhythmic sets. O ne of the main organic constituents, the acidic, hydrophilic gel glyco saminoglycans (GAGs), is the dominant component towards the tops of rh ythms. During fossilization of the Carboniferous shells, GAGs degraded incrementally without disturbing apatitic ultrastructures, and the sp aces so created became partly filled with sheets of recrystallized apa tite with some kaolinite or with books and plates of kaolinite. The ka olinite in the shells contrasts with the illite of the entombing sedim ents and suggests that degrading acidic GAGs mediated in clay formatio n in situ. The sediments also contain framboidal pyrite, which is virt ually absent from the shells themselves but is usually even more abund ant, with a greater range of trace metals, in the sedimentary fills of complete shells. This imbalance suggests mediation by another gel, th e glycocalyx, secreted by the inner epithelium of the brachiopod mantl e. The glycocalyx would have lined the shell interior and could have s erved as a sorption film for dissolved metals precipitated as compound s on decomposition of body tissue.