Electron microscopic studies of growth margins of articulate brachiopods

Summary The brachiopod shell is secreted by the mantle epithelium lining the internal surfaces of its two valves. Growth lines, seen on their external surfaces, have been interpreted in terms of mantle regression and transgression from the valve margins. This scanning electron microscope study of the shell microstructure in recent brachiopods confirms these views and shows the skeletal evidence upon which such interpretations can be made. Electron micrographs reveal that from a growth line a plane dips posteriorly into the shell substance along which normal skeletal secretion was interrupted. Commonly a mosaic of secondary fibres, similar to that seen on the inside surface of the valve, is preserved upon this regression plane, most of the inside surface of which is covered by primary shell, usually extending posteriorly well into the secondary shell layer. The regression plane marks the area from which the mantle withdrew and the area over which shell secretion was interrupted. During mantle transgression primary shell was deposited over much of this surface, before the redevelopment of secondary fibres, so that the old internal surface of the valve was preserved as a false mosaic within the shell. In this way it is possible to recognise the extent of mantle regression and to note the position of the primary — secondary shell secreting junction of the mantle at the time when shell secretion was resumed.