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Geological Society, London, Special Publications; 2002; v. 194; p. 85-98;
DOI: 10.1144/GSL.SP.2002.194.01.07
© 2002 Geological Society of London

Euconodont diversity changes in a cooling and closing Iapetus Ocean

H. A. Armstrong1 & A. W. Owen2

1 Department of Geological Sciences, University of Durham, Durham DH1 3LE, UK H.A.Armstrong{at}durham.ac.uk
2 Division of Earth Sciences, University of Glasgow, Gregory Building, Glasgow G12 8QQ, UK

Constrained seriation of euconodont generic presence-absence matrices for four time slices between the late Llanvirn and late Llandovery provides a qualitative method for defining shelf and oceanic biofacies, recosntructing biofacies architecture and analysing biodiversity within a regional context.

We propose many North Atlantic Province taxa had a pelagic mode of life and ranged widely across the Iapetus Ocean. Oceanic biofacies are considered to reflect water mass structure. Changes in vertical distribution of one such biofacies (including Amorphognathus and Spinodus) suggest adaptation to cold, nutrient-rich, oxygen-poor upwelling water. Biofacies distributions suggest that upwelling occurred along the Avalonian margin throughout the Ashgill, but was only initiated along the Laurentian margin immediately prior to the Hirnantian glacial maximum.

Clade diversities and trajectories differ between biofacies and latitudes, reflecting different causal mechanisms. In Laurentia, diversity fell in the early Ashgill, coincident with the onset of ocean cooling. Diversity declined in Avalonia when the microcontinent drifted into tropical latitudes. The stability of euconodont biofacies architecture during the Late Ordovician indicates that global cooling and plate reorganization had a low palaeoecological impact despite decreases in alpha and beta diversity.





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