LATE ORDOVICIAN EUXINIA DRIVES EXTINCTION OF THE DEEP-WATER BRACHIOPOD FOLIOMENA FAUNA

The concept of the deep-water brachiopod Foliomena fauna was established in the 1970s, almost 40 years ago. Following its initial description from the red mudstones of the Katian Jerrestad Mudstones in southern Sweden, the fauna has been recorded from the margins of most major Ordovician palaeocontinents, including the tropics, its stratigraphical range expanded from the lower Sandbian to upper Katian and its environmental range documented from the mid-shelf to the upper slope. This distinctive assemblage is dominated by small, thin-shelled taxa characterized by core elements of Christiania, Dedzetina, Foliomena, Leptestiina, Nubialba and Cyclospira. New data from the Katian of Bornholm, the North of England and Poland supplement a database for over 40 localities and 70 taxa worldwide that has been analysed multivariately. The initial derivation of the fauna from shallower-water during the early Sandbian, its maximum expansion environmentally and geographically during the early-mid Katian and its extinction in the late Katian is confirmed. It is unlikely that cool water, areal habitat restriction or ventilation of the oceans fatally affected the fauna. Rather, rising euxinia, invading the upper slopes and more distal parts of the shelf may have removed the deeper-water part of the benthos just prior to or during the earliest phase of the short-lived, Late Ordovician glaciations. Adjacent, shallower-water associations such as the Dicoelosia community were more adaptable and avoided the ravages of extinction. Recovery in the deep sea was slow, with initially the late Llandovery marginal Clorinda assemblages deriving elements from the Dicoelosia community.