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.