Collagenous chitinophosphatic shell of the brachiopod Lingula

Abstract

The structure and composition of the shell of living Lingula have been studied in detail to provide a measure for evaluating the evolution of the lingulid shell since early Palaeozoic times. Four constituents are identifiable at ultrastructural level. Apatite, specifically the fluorapatite francolite, occurs as coated granules, up to lOnm in diameter, aggregated into spheroidal masses up to several microns in size, cylindroids up to 600 nm long and rare plates. Glycosaminoglycans (GAGs) are present throughout the shell as an elastic isotropic gel. Chitin is normally associated with a protein and is best seen under the scanning electron microscope after digestion of its associate by proteinases. As strands about 30 nm thick, chitin can form mats or be part of granular rods aligned as anastomosing ridges. Fibrillar collagens, with a periodicity of about 45 nm, occur mainly as sporadically developed mats throughout the body platform succession and as the core of the dorsal median septum. Elsewhere they appear sparingly as vertical and horizontal strands. At least ten proteins have been identified in the shell but it is not yet known which are covalently attached to GAGs, associated with the chitin or fabricated into membranes. The entire integument consists of: a periostracum, about 4 pm thick, made up of closely packed fibrous tubes with axial strands presumably of chitin; a primary layer, about 40 pm thick, composed mainly of GAGs; and a mineralized secondary layer of variable thickness. The secondary layer is a succession of isotopic and isochronic laminae with thicknesses in microns compared with areas frequently of several square millimetres. Compact laminae, composed of closely packed spheroidal aggregates of apatitic granules are succeeded: by botryoidal or walled laminae, in which apatitic aggregates form botryoidal masses or vertical walls in a GAGs matrix; or by rod and plate laminae with apatitic rods accreting into anastomosing ridges disposed transversely on the body platform and radially in peripheral regions. The preponderance of the botryoid and rod laminae in the body platform and of anastomosing ridges in the lateral areas of a valve respectively explains the complex patterns of c-axis orientation of apatite obtained by X-ray diffraction. Membranous laminae, consisting of chitinoproteinaceous membranes (and sporadic collagenous mats) in GAGs, occur throughout the succession; while stratified laminae, characterized by gentle inclined, alternating organic and apatitic units, each just over a micron thick, are especially well developed (with compact laminae) at the junction with the primary layer.