Shell microstructures of latest Permian Rugosochonetidae (Brachiopoda): evidence from SEM‐ and CT‐scanned shell materials.

Microscopic perforations in brachiopod shells, such as endopunctae, pseudopunctae, extropunctae, aditicules and epipunctae, have various biological functions. Based on excellently preserved material, aditicules are found to be widely developed in the Fusichonetes and Neochonetes, and setae‐bearing aditicules are shown to exist in rugosochonetid brachiopods for the first time. SEM and Micro‐CT images show that aditicules obliquely insert into the primary layer and are arranged in a single column along the crest of a costellae. The setae are supposed to play a role in triggering the closure of shells by sensing dangers, preventing large particles from entering the inner cavity and helping to increase the water pressure by forming a functional siphon. The SEM and Micro‐CT images also show that papillae are solid cones on the inner surface of the shell, and the density of papillae varies over different regions of the shell surface, and with environment. The variation in density of the papillae is most likely related to diverse functions in the 'filtering system'. In the marginal area, the small and densely spaced papillae can prevent intermediate and fine particles from entering the mantle cavity, whereas the large and sparse papillae in the mid‐valve can trap fine particles. In this study, these two microstructures are interpreted as a sediment 'filtering system' in Fusichonetes and Neochonetes, which is corroborated by the predominant occurrence of these genera in shallow‐water clastic shelves and deep‐water siliceous basins across the Permian–Triassic boundary interval in South China. [ABSTRACT FROM AUTHOR]