Late Ordovician (Hirnantian) shelly fossils from New Zealand and their significance

Abstract A poorly preserved fauna of brachiopods, trilobites, corals, molluscs, bryozoans, echinoderms, and ostracods is described from Wangapeka Valley, northwest South Island. It can be dated to the latest Ordovician Hirnantian Stage, equivalent to Bolindian Bo4‐Bo5, by immediately underlying graptolite faunas and also by some of its contained shelly fossils. It is one of only two shelly faunas of Middle or Late Ordovician age identified in New Zealand. The trilobite Mucronaspis and the brachiopods Eostropheodonta, Plectothyrella, Cliftonia, and Leptaena all occurring together identify the fauna as part of the very widespread Hirnantia Fauna, although Hirnantia itself and other key elements are absent. The development of this fauna in relatively outer shelf settings such as that at Wangapeka Valley was made possible because the contemporary latest Ordovician global glaciation oxygenated the sea water to far deeper levels than was normal, and also because of the associated glacio‐eustatic fall in sea level.