CHEMICO-STRUCTURAL DEGRADATION OF CARBONIFEROUS LINGULID SHELLS

Shells of Lingula squamiformis from argillaceous sediments at three ho rizons within the Dinantian Series, exposed at three localities in Sco tland, have been studied to determine chemico-structural changes resul ting from fossilization. Biomineral structures are essentially the sam e as those of living Lingula anatina with apatitic granules aggregatin g into spherules, up to 60 nm in size, and larger spheroidal bodies as well as rods and rarer lath-like plates. These aggregates and the ori ginal organic constituents were secreted as stratiform successions in two distinct layers, as in Recent L. anatina. The outer, lithified par t of the primary layer bears microstructural moulds of a totally degra ded periostracum and was probably composed mainly of acidic glycosamin oglycans (GAGs); the inner part evidently contained a higher proportio n of spherular apatite within the GAGs than in living species. The sec ondary layer consists of variably complete rhythmic sets of compact, r od and plate (virgose), and membranous laminae. Compact laminae are no rmally cleaved along degraded walls of GAGs whereas the original GAGs- filled spaces of virgose laminae are partly occupied by recrystallized apatitic sheets with kaolinite. The phosphatized membranous laminae p robably contained more spherular apatite in life than present day Ling ula. The shell is canaliculate with chambers and galleries well develo ped in the virgose laminae. There is a decrease in concentration of am ino acids from the posterior to the anterior of the valves of living L ingula anatina and Glottidia pyramidata resulting from the proteinaceo us coat of the apatitic spherules. A similar distribution of hydroxypr oline occurs, indicative of collagen in the body platform of living li ngulids. Nearly all organic constituents have been degraded in the Car boniferous valves but threads, about 50 nm thick, occasionally travers e spaces in virgose laminae and even form a network coated with spheru lar apatite, which resembles webs of collagens or actin found in livin g lingulids. Acidic and aliphatic amino acids were extracted from L. s quamiformis valves from Calderwood and Kinghorn whereas the narrower r ange of amino acids from the heavily pyritized valves from Ardross con firmed differential degradation of organic material during the fossili zation of penecontemporaneous samples. The fossilization of complete s hells of L. squamiformis is not due exclusively to catastrophic burial as has been deemed necessary to preserve Recent Lingula intact. The l iving shells of Carboniferous species were more apatitic than those of Recent Lingula, especially in the anteriomedian sectors of the second ary layers.