The shell fabric of Palaeozoic brachiopods : patterns and trends

The varied microstructures of brachiopod biominerals represent a robust archive to understand the evolution and adaptations of marine calcifiers in time. Therefore, a detailed study of the shell microstructure of Cambrian to Devonian brachiopods from Iran is here presented. The shell of 38 brachiopod species, representatives of 22 families and nine orders, has been analysed using scanning electron microscope (SEM), and a database has been built, including macro- and micro-morphological features used to characterize the two- or three-layered brachiopod shells. Two main microstructural variants of the secondary layer have been analysed: fibrous and laminar fabrics. The fibrous layer has a fabric comparable to that of recent brachiopods, whereas the laminar fabric is more complex in its structural organization and has no recent analogue. In cross-section, the laminae are thinner than the fibres, and much less variable in size. There is evidence that taxa with laminar microstructure have diverged from the Billingsellida and then followed a trend implying a decrease in thickness of the laminae. Our linear discriminant analysis (LDA) shows that shell fabric and shell thickness are powerful predictors of shell shapes, which in turn approximate the brachiopod lifestyles and ecological strategies. Taxa with a fibrous fabric are mostly biconvex, whereas the groups with a laminar secondary layer are associated to a variety of shell shapes and lifestyles. Even if the relations between shell fabric and shell thickness remain enigmatic, as well as the metabolic cost they imply, shell fabrics, and the possible structural and mechanical advantages conferred, could have played a role in the evolutionary success of the Strophomenata during the Palaeozoic.