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Understanding transformations of the climate system in the geological past is essential for predicting and mitigating the effects of global climate change in the next future. The geological record provides a unique archive that documents long-term fluctuations of environmental variables, including seasonality. Seasonality appears to have played a crucial role in extreme climate transitions, highlighting the importance of constraining its variability in the past. Increased seasonality is often associated with colder conditions and the development of ice accumulations, making it a key parameter for understanding and forecasting climate change.Species of the brachiopod Gigantoproductus are giants within the Palaeozoic sedentary benthos, characterised by exceptional size and thick shells, reaching over 30 cm in width and more than 1 cm in shell thickness. These features make them unparalleled bioarchives for palaeoecological and palaeoclimatic reconstructions, enabling the investigation of long-term changes during key intervals of past climate change.In this study, specimens of Gigantoproductus semiglobosus from upper Visean (Mississippian, Carboniferous) successions of western Ireland (Aran Islands and the Burren) were subjected to detailed diagenetic screening and subsequently analysed using a sclerochemical approach (δ18O, δ13C). These analyses were used to reconstruct seasonal variability and to provide additional evidence for the timing of Mississippian phases of the Late Palaeozoic Ice Age (LPIA).Our results show that δ18O profiles from well-preserved shells record high seasonal variations (Δδ18O = 0.9 to 1.9 ‰ corresponding to a ΔT = 4 to 11 °C) for palaeoequatorial settings, as also observed in coeval species of Gigantoproductus from the UK (Angiolini et al., 2019). This seasonal variation is much higher than that recorded in comparable shallow water, low latitude environments both nowadays and in the distant past. The pronounced seasonality recorded by several species of Gigantoproductus from western Ireland and the UK at low palaeolatitudes supports the onset of a sustained Gondwanan glaciation in the late Visean. Also, the palaeogeographic distribution of the species of Gigantoproductus and the geochemical composition of their shells indicate that low-latitude Mississippian ocean waters did not experience a temperature decrease at the onset of the Gondwanan glaciation, but rather a marked increase in seasonal variability.Overall, this study highlights the importance of resolving long-term changes in seasonality, using fossil carbonate shells as palaeoclimatic archives during different intervals of climate change, in both the recent and distant past, to better understand and predict long-term transformations of the climate system. ReferencesAngiolini et al. (2019). The giants of the phylum Brachiopoda: a matter of diet? Palaeontology, Vol. 62, Part 6, pp. 889–917