The Late Paleozoic Ice Age (LPIA, 360 - 255 Ma) was a climatic event with glacial and interglacial periods (Upper Devonian to late Permian), interpreted as a dynamic glaciation. Several continental glacier growths and melting events have been interpreted from Carboniferous to Permian, by combining sedimentological proxies and geochemical data, especially the stable isotope records (δ13C and δ18O) from conodonts and brachiopod shells. Most geochemical data come from high paleo-latitudes, whereas data from low paleo-latitudes are relatively scarce. Geochemical signatures of brachiopod shells are commonly used due to their great abundance in geological record and its relative resistance to diagenesis. Among the brachiopods, gigantoproductids have the largest and thickest shells known, which are extremely useful for the sampling in these kinds of studies. Besides, its expansion coincided with the onset of the Late Paleozoic Ice Age, being very abundant in the Paleotethys at low latitudes. All these make the gigantoproductids key for reconstructing the paleoclimate at low paleo-latitudes, using isotopic and Mg/Ca data. Unfortunately, processes like diagenesis or ‘vital effects’ may vary the original seawater geochemistry recorded by the shells. Its characterization is crucial to obtain nonbiased geochemical data suitable for paleo climatological studies. Therfore, it is required to study the shell morphology and microstructure in order to understand all the intrashell geochemical and crystallographic variations produced during biocrystallization, but also during diagenesis. Moreover, some features like shell spiral and microstructure could help to add more information to shell descriptions and the growth dynamics understanding. In this study, more than 700 samples have been collected from Sierra Morena (Spain), Montagne Noire (France) and the Azrou-Khenifra basin (Morocco), identifying up to six gigantoproductid genera: Globosoproductus, Semiplanus, Kansuella?, Latiproductus, Gigantoproductus, and Datangia with Viséan and Serpukhovian ages. Microstructural features from 290 thin sections have been studied and six crystal morphologies have been distinguished into the tertiary layer of ventral valve: subhorizontal, imbricated, crenulated, acicular, short and long columnar morphologies. Furthermore, 60 productid specimens, including gigantoproductids and non-gigantoproductid shells, have been selected to investigate the shell size and curvature using 2D and 3D scans. Results from this study emphasize that shell size, curvature, and crystal morphology are taxa related. Finally, a remarkable microestructural change in the

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