Taxonomy, biostratigraphy and biofacies of an Upper Ordovician (Katian) conodont fauna from the Casaio Formation, Northwest Spain

Abstract

regions are characterised by siliciclastic deposition under mostly shallow, cold-water regimes. These vast regions, currently represented by central southern Europe and North Africa, experienced a major environmental change during the Late Ordovician with the abrupt appearance of calcareous deposits of highly variable thickness (e.g. Villas et al. 2002, Boucot et al. 2003). The limestone deposition was associated either with a sharp climatic global warming named as Boda Event after the Boda Limestone of Sweden (Fortey & Cocks 2005), or with a general cooling (Cherns & Wheeley 2007). Currently, the Late Ordovician (Katian–Hirnantian) through earliest Silurian (Rhuddanian) time interval is regarded as a period of variable climate and sea level conditions, with at least two separate pulses of glacial advance and one of retreat during a late Katian global warm interval (Melchin et al. 2013, Ghienne et al. 2014, Kröger et al. 2017). The latter cooling triggered widespread marine anoxia by reorganisation of the thermohaline circulation, which resulted in the second pulse of the Hirnantian mass extinction, the first of the ‘Big Five’ Phanerozoic mass extinctions (Bartlett et al. 2018). The pioneer study by Fuganti & Serpagli (1968) on the Katian fauna of the Urbana Limestone of the Central Iberian Cordillera started the Ordovician conodont stud ies in the Iberian Peninsula (Fig. 1). Since then, several conodont studies have focused on the Upper Ordovician limestones present in the different tectonometamorphic domains of Spain (summarised by Sarmiento et al. 2011). Particularly, the common record of conodonts of the Amorphognathus ordovicicus Zone (Ka3–4 time slices of Bergström et al. 2009) has allowed correlation of the the Urbana Limestone with the Cystoid Limestone in the Eastern Iberian Cordillera, the ʻPelmatozoan Lime­ stoneʼ in the Ossa­Morena Zone, the ʻunidad calcárea superiorʼ in the Cantabrian Zone, the Estana Formation in the Pyrenees, and the Ferradosa Formation from the Portuguese Central Iberian Zone (e.g. Hafenrichter 1979; Sarmiento 1990, 1993, 2002; Sarmiento et al. 2001; del Moral 2003, 2007; del Moral & Sarmiento 2008).