Brachiopod giants from the Mississippian (Asbian) of western Ireland: Fossil bioarchives of seasonality and symbiosis and far-field harbingers of climate change

Brachiopod species of Gigantoproductus have long fascinated researchers, not only because of their exceptional size and thick shell, but also as unparalleled bioarchives for palaeoecological and palaeoclimatic information. In this paper, we describe faunas containing Gigantoproductus semiglobosus from upper Visean (upper Asbian) successions in two regions of western Ireland, and report geochemical analyses that improve understanding of the palaeobiology of these brachiopods. The two regions are the Burren, where the Aillwee Member (Burren Formation) comprises thick-bedded cyclic bioclastic packstone to grainstone, interpreted as the deposits in predominantly shallow-water (subtidal) marine environments with episodic subaerial exposure, and the Aran Islands, where the Slievenaglasha Formation comprises cyclic crinoidal limestones with chert, deposited in slightly deeper water conditions. Shallowing upward fourth-order cycles in both regions have previously been interpreted as being under a glacioeustatic control. Reconstructed δ¹³C_org and δ¹⁵N_org of soft tissues of G. semiglobosus are respectively −29.0 to −30.1 ‰ (VPDB) and − 1.4 ‰ and + 6.1 ‰ (Air) and serve as proxies for identifying photosymbiotic relationships and a mixotroph lifestyle for this species. Well-preserved δ¹⁸O_carb profiles record high seasonal variations (Δδ¹⁸O = 0.9 to 1.9 ‰ corresponding to a ΔT = 4 to 11 °C) for palaeoequatorial settings as a far-field proxy of the onset of sustained Gondwanan glaciation in the late Visean and provide evidence of warm tropics during the glaciation. The δ¹³C_carb profiles are mostly controlled by local influences and changes in productivity. Our geochemical analyses of growth patterns, seasonal variation, diet and endosymbiosis in G. semiglobosus, sheds new light on the paradox of these unusual brachiopods, and provides a greater understanding of their massive size.