New deep-water brachiopod resilient assemblage from the South-Iberian Palaeomargin (Western Tethys) and its significance for the brachiopod adaptive strategies around the Early Toarcian Mass Extinction Event

Abstract

The Pliensbachian-Toarcian transition was a period of changes in long-term environmental conditions leading up to the Early Toarcian Mass Extinction Event (ETMEE), which resulted in a noticeable extinction and turnover in the marine biota. The westernmost Tethyan basins, especially the peri-Iberian platforms, provide an exceptional brachiopod record to better understand the adaptive strategies and the severe ecological effects of these faunas within the marine ecosystems. This event marks a critical interval in the evolutionary history of the Phylum Brachiopoda as two orders, the Athyridida and Spiriferinida, became extinct. Evolutionary patterns displayed by several taxa from these groups and some rhynchonellids typifying deep-water habitats are analyzed across this biotic crisis spanning several Mediterranean and NW-European basins. New work performed in La Cerradura section, a deep pelagic trough from the South-Iberian palaeomargin, reveals two new taxa (Koninckodonta sumuntanensis and Atychorhynchia falsiorigo) herein described. This newly documented fauna supports pre-extinction dwarfing and resilience in deep refugia linked to the ETMEE, and an episode of speciation which is interpreted in terms of a pre-extinction radiation. In the ETMEE repopulation phase an opportunistic strategy occurs typified by Soaresirhynchia bouchardi, and a case of homoplasy involving post-extinction pioneers (Elvis taxon) is detected. Similar adaptive strategies occurred associated with other mass extinctions such as the Permian/Triassic and the Cretaceous/Paleocene events, supporting a possible standard pattern in the response of the brachiopod fauna to such biotic crises and shedding light on the ecological effects of the mass extinction events.