Extreme faunal endemism, super-island faunas, and the Late Ordovician mass extinction

The majority of studies on mass extinction events (MEEs) have focused on the various extrinsic or palaeoenvironmental killing mechanisms, most associated with habitat destruction and some of which remain hotly contested. The ‘mass extinction by insularization and kill’ (MEIK) model of this study is proposed as a significant intrinsic cause of the Late Ordovician MEE based on palaeoecological and palaeobiogeographical analyses of Ordovician brachiopod faunas from Laurentia and its adjacent tectonic plates. During the Late Ordovician pulses of a first-order sea-level rise, the MEIK model reveals that biodiversity hotspots shifted from open ocean to epicontinental seas associated with a drastic increase in faunal endemism. Continental-sized ‘island faunas’, exemplified by the Laurentian intracratonic-sea brachiopods, were characterized by high abundance but relatively low diversity, but more critically the eventual loss of their inter-plate dispersal ability. The onset of mass extinction of these highly endemic ‘super-island’ faunas, specialized in living in shallow intracratonic seas for ∼8 million years during the Katian, occurred at the start of the Hirnantian glaciation due to draining of epicontinental seas and loss of their specialized habitats. The data presented here suggest that during a major sea-level rise, global biodiversity epicentres migrated from the ocean to epicontinental seas, becoming genetically isolated, rendering marine shelly benthos intrinsically susceptible to rapid environmental change and subsequent mass extinction.