Figure 12.1 Brachiopod morphologies: (a) internal features of a lingulate, (b) exterior of a burrowing lingulate, (c) internal terminology of a craniform calciate, (d) internal features of a terebratulide, (e) external terminology of a typical articulate, (f) internal terminology of both valves of a terebratulide, and (g) main types of brachiopod lophophore.
change their shape and life mode during ontogeny from being attached to the seabed to lying untethered in the mud.
Ultramorphology: brachiopod shell_
The brachiopod shell is a multilayered complex of both organic and inorganic material that has proved of fundamental importance in classification. The shells of most rhynchonelliformean brachiopods consist of three layers (Fig. 12.6). The outer layer (peri-ostracum) is organic, and underneath are the mineralized primary and secondary layers. These layers are sequentially secreted by cells within the generative zone of the mantle, forming first a gelatinous sheath followed by the organic periostracum, and then the granular calcite of the primary layer. The subsequent secondary layer is thicker and composed of calcite fibers, and in some brachiopods a third prismatic layer is secreted. There are a number of variations of this basic template.
The linguliformeans, for example, have phos-phatic material as part of their shell fabric. The shells of rhynchonelliformean brachio-pods are composed of low-magnesian calcite; these shells may have fibrous, laminar or cross-bladed laminar shell fabrics in their secondary layers. The mineral fabrics themselves, when investigated at the nanoscale, may be of particular ecological importance. Those with calcite seminacre, rather like mother-of-pearl, can cement directly to the seafloor whereas those with fibrous shells can not (Pérez-Huerta et al. 2007).
Many shells are perforated by small holes or punctae, in life holding finger-like extensions of the mantle or ceca. Their function is uncertain but they increased the amount of the brachiopod's soft tissue. Some strophom-enates have pseudopunctae, with fine inclined calcite rods or taleolae embedded in the shell fabric.
The relatively stable brachiopod shell substance can tell much about the secretion of the shell but also about environmental conditions
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