Upper Ordovician ostracods from the Cautley district, northern England: Baltic and Laurentian affinities

The Cautley Mudstone Formation and Cystoid Limestone Member of the Ashgill Formation (Windermere Supergroup; Ashgill Series), from the Cautley distric t of northern England, has yielded an ostracod fauna of more than 30 specie s. Many of these have short ranges, permitting recognition of stratigraphic ally successive Pusgillian-lower Cautleyan, middle upper Cautleyan, and Raw theyan ostracod faunas. Several species are also known from the upper Ordov ician of North America (Anticosti Island), Scotland (Girvan district) and t he Baltic region (Estonia, glacial erratic boulders of northern Germany), p roviding evidence to correlate upper Ordovician successions in these areas. The ostracods include abundant podocopes, at some horizons accounting for more than 80% of the fauna. Binodicopes are also common, but palaeocopes ar e rare. Assemblages are typical of a clastic dominated open marine shelf se tting. Diversity at most horizons is low (c. 3-5 species), but reaches a pe ak of between 13-14 species in middle Cautleyan horizons. Lower diversity a t Pusgillian and Rawtheyan horizons coincides with the encroachment of deep er marine-shelf facies which were probably hostile to Ordovician benthonic ostracods. Some of the ostracods (particularly Aechmina) have distributions suggesting tolerance of a range of mid- to deep shelf benthonic palaeoenvi ronments, but none were pelagic. During Ashgill times the Cautley district (part of palaeocontinental Avalonia) was replete with ostracod genera and s pecies which also occur in the Baltic region (palaeocontinental Baltica, mo re than 90% generic similarity) and to a lesser, but nonetheless significan t extent in North America and Scotland (parts of palaeocontinental Laurenti a). Such trans-Tornquist Sea and Iapetus Ocean distributional patterns add to previous ostracod data that support models which show palaeogeographical proximity of Avalonia and Baltica, and Avalonia and Laurentia, by Ashgill times. The widely cited observation, that trans-Iapetus ostracod faunas rem ained strictly provincial until the mid-or late Silurian, cannot be sustain ed.