ИСТИНА

The shell secretion of recent brachiopods remains poorly known, especially if compared with the studied in detail biomineralization processes in recent mollusks. The lingulids are best known of all brachiopods in microstructural sense as they possess the unique shell composed of alternating biomineralized and organic layers that remained structurally stable throughout the Phanerozoic. Three parts are distinguished in the lingulide shell: a periostracum and primary and secondary layers. The mineralized part of the linguloid shell consists of up to 10 nm in diameter nanogranules of apatite, specifically the fluorapatite francolite, aggregated into up to several microns in size spheroids or variously long rods. Though the superfamily Linguloidea appeared in the Early Cambrian, only two genera, Lingula Bruguière, 1797 and Glottidia Dall, 1870, survived up to recently. Studies on the linguloid primary layer provided quite controversial data on its presence and structure. Watabe and Pan (1984) described 40–50 μm thick primary layer composed of aggregates of spherulites for Glottidia pyramidata (Stimpson, 1860). However, Iwata (1981) did not find any mineralized primary layer in Lingula unguis (L.) (this species was considered as a synonym of Lingula anatina Lamarck, 1801 by Emig (1982). Williams et al. (1994) reported a 40 μm thick organic primary layer in L. anatina composed mainly of glycoaminoglycans (GAGs). We studied ten specimens of L. anatina from Vietnam and the Philippines. The Vietnam specimens were preserved in a 4% formaldehyde solution in filtered sea water and the Philippine specimens were dried without any chemical treatment. All specimens were studied with SEM first with periostracum and then after bleaching with collagenase/proteinase mixture. The shell substance was studied externally and along the fractures. In many places on the shell outer surface after bleaching preserved a well-ordered sheets of about 0.5 μm high and 0.2 mm thick cylindroids directed perpendicularly to the valve surface. Such dolioform crystals were previously unknown for linguliform brachiopods. The sheets look as a thin light film in SEM images in contrast with the underlying compact lamina and cover the whole shell surface including the protegulum and brephic shell. On the brephic and adult shells, the cylindroids are neatly packed and their outer and inner ends are obtuse while on the protegulum they are more random and fusiform and somewhat resemble the acicular primary layer of rhynchonelliform brachiopods with carbonate shell. The external surface of the sheets of cylindroids bear reflections of the radial striation of periostracum and thus this is the first shell layer underlying the periostracum, i.e. the primary layer. As it preserved after the bleaching, it is mineralized. Thus three types of the primary layer are known for recent linguloids: 40–50 μm thick primary layer composed of aggregates of spherulites in G.pyramidata (Watabe and Pan 1984), about 40 μm thick primary layer composed of GAGs in L. anatina from Japan (Williams et al. 1994) and about 1 μm thick layer of cylindroids in L. anatina from Vietnam and the Philippines. One of the possible explanations for strongly differing primary layers in one species is that in fact they are different taxa but additional data on the molecular phylogenetics of these brachiopods are needed in order to resolves this problem.