Article: Shell structure of Terebratulid brachiopods
Ultrastructural studies of living and fossil brachiopods belonging to the Terebratulidae show that the shell is invariably penetrated by unbranched puncta and generally consists of a three-fold mineral succession: a primary layer composed of acicular and granular crystallites, a secondary layer of orthodoxly stacked fibres, and a prismatic tertiary layer. The constituents of the last layer are not 'prisms' in the crystallographic sense but discrete units with interlocking boundaries arranged normal to the surface of accretion. Each prism arises from a fibre first by lateral spreading of the terminal face and then by vertical accretion so that growth banding no longer indicates an oblique increase in length as in fibres. Sections of decalcified mantle show that the epithelium secreting the tertiary layer is like that underlying the secondary shell except that it does not exude proteinous sheets between prisms. Amalgamation of prisms may, therefore, be inhibited by either sheets of water-soluble organic compounds or crystallographic incompatibility between adjacent prisms as indicated by non-alignment of cleavage. The tertiary layer was not developed in Lobothyris, the oldest terebratulid studied, so that the layer was possibly first acquired geronto-morphically. But it is also lacking in the shell of other genera like Rhombothyris and Terebratula and this may reflect repeated neotenous suppression. Despite the uniform texture of the tertiary layer, expansion of muscle bases across overlying epithelium causes different types of myotests to develop in Liothyrella and Gryphus as in other living articulates. The characteristic well-ordered pits in Liothyrella, in particular, suggest that the disposition and strength of muscle fibres contribute to the shaping of myotest topography.