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This study describes the conodont palaeontology of Upper Ordovician sections in Avalonia and Baltica. 24 species from 17 genera are systematically described and are attributed to the North Atlantic Realm. Sections can be correlated using graptolites and conodonts. The taxa are typical of the accepted Aphelognathus to Periodon shallow to deep-water biofacies. From the late Caradoc in Avalonia and Baltica, the Amorphognathus and deeper-water biofacies persisted in shelf settings. The stability of this distribution through the Ashgill, a period when Avalonia and Baltica drifted towards sub-tropical latitudes, suggests ocean cooling associated with glaciation was the dominant control on biofacies.Microfacies analysis of the phosphatic Amorphognathus superbus Biozone limestones from the Nod Glas Formation of the Welsh Borders indicates the presence of the oxygen minimum zone. Biofacies distribution in this section reflects the subtle variations in temperature within this unique habitat. A hypothesis is presented for the evolution of Amorphognathus ordovicicus in which range expansion into slope settings enabled parapatric speciation. Amorphognathus ordovicicus evolved gradually from a deeper water ancestor by the loss of the lateral process and cusp adjacent denticles on the M element. The initial and subsequent transgressions of the Ashgill brought Amorphognathus ordovicicus, and its cool water niche, into shelf areas. Gradual evolution in deep-water is predicted by the Plus ça Change model. The crown enamel of Periodon, Protopanderodus and Drepanodus records seasonally entrained growth with periods of refractional growth followed by longer functional episodes. Periodon exhibits reduced growth and comparatively short growth duration. Drepanodus and Protopanderodus show continued growth. It is hypothesised that Periodon was nektobenthic and adapted to harsh but stable conditions in the deep-sea, an r-strategist. Drepanodus and Protopanderodus were nektonic and grew to a large size indicating that they were AT-strategists. Upper Ordovician North Atlantic Realm nektobenthic conodonts were characterised by a high diversity and abundance of small sized individuals compared with coeval shelf faunas, a situation analogous to the modem oceans.

The end-Ordovician mass extinction dramatically altered the course of conodont evolution. This extinction event is probably unique in that it can be strongly correlated with a glacial climatic control. This study has identified, through the application of high-resolution stratigraphy, events within the extinction and recovery intervals. Elements of the uppermost Ordovician Shelf-edge Biofacies were severely affected by the oceanic cooling and introduction of cold-water currents associated with the initiation of the glacial maximum. In contrast, elements of the Shelf Biofacies were more severely affected by the intense cooling, shallowing and overcrowding during the glacial maximum. A number of the Shelf-edge taxa that had survived the glacial maximum suffered extinction at the hands of increasing water temperatures, rising anoxia and/or the cessation of oceanic circulation during the post-glacial transgression. Recovery was initiated by the appearance of Crisis Progenitor Taxa within the glacial maximum in the Shelf Biofacies and during the post-glacial transgression in the Shelf- edge- Slope biofacies. The Shelf-edge Biofacies identified within the uppermost Ordovician is not recognised in the Lower Silurian. Two main biofacies occurred on the Shelf and Slope, which had directly evolved from their Upper Ordovician equivalents. The long-term recovery involved the evolution of Crisis Progenitor Taxa and Ecological Generalists within the Shelf and Slope Biofacies (autochthonous taxa). Punctuated equilibrium likely predominated in the Shelf Biofacies as a consequence of widely fluctuating physical conditions. In contrast, the more stable environments of the slope encouraged gradualistic evolution within the Slope Biofacies (Plus ça change Model). Transgressive episodes within the Llandovery, possibly linked to eccentricity cycles, caused the iterative appearance of Long-term Refugia Taxa (allochthonous taxa), sourced from a Pterospathodontid Biofacies. The transgressive episodes also drove elements of the Slope Biofacies onto the shelf. It has been observed that the mechanisms driving extinction, namely environmental disruption and temperature changes, were also responsible for fuelling the subsequent recovery.

No description available.