Article: Rhaetian (Late Triassic) Monotis (Bivalvia: Pectinoida) from the eastern Northern Calcareous Alps (Austria) and the end-Norian crisis in pelagic faunas
Christopher A. McRoberts, Leopold Krystyn and Adriel Shea
Species of marine bivalves of the pectinoid genus Monotis provide useful biochronologic indices for the Late Triassic (middle Norian–earliest Rhaetian). We report the succession of Monotis at Hernstein in Lower Austria where typical late Norian Monotis salinaria (Schlotheim) are overlain by strata with apparently the youngest Monotis known of demonstrable Rhaetian age: Monotis hoernesi Kittl and Monotis rhaetica sp. nov., a species closely related to M. rudis Gemmellaro. A Rhaetian (Sevatian 2) age is confirmed by the co-occurrence of Monotis with the platform conodont Misikella posthernsteini and close association with the ammonoid Paracochloceras. A review of late Norian monotid species indicates that a profound extinction event occurred in the pelagic realm at or close to the Norian/Rhaetian boundary where c. 15 monotids (Monotis s.l. and Maorimonotis) became extinct. The surviving Monotis are dwarfed when compared to their Norian predecessors and may represent an ecological/phylogenetic response to the crisis.