Ostracoda (crustacea) of the Great Estuarine Group (Bathonian, Middle Jurassic), Inner Hebrides, Scotland.

The Ostracoda of the Great Estuarine Group, Inner Hebrides, Scotland are monographed. These are referred to Order Podocopida, Suborder Podocopina, superfamilies Cytheracea, Cypridacea and Darwinulacea, seven families, five subfamilies, 17 genera (three new) and 41 species (25 new, 11 in open nomenclature). The stratigraphic occurrence of the ostracod species is determined for the Islands of Skye, Eigg and Muck. The major lithostratigraphic subdivisions of the Great Estuarine Group are reflected in the ostracod fauna. The ostracod species are largely endemic to the Hebrides; there are generic level affinities with other British sequences. Ostracod genera are used to interpret the palaeosalinity of deposition of the Duntulm and Kilmaluag formations. In some cases the palaeosalinity of deposition of individual beds is corroborated using C & O stable isotope analyses. Three salinity controlled ostracod assemblages are recognised and are interpreted to have migrated within the Kilmaluag lagoon due to increased freshwater or brackish-marine water input. By comparison with the associated molluscs, conchostracans and algae, four salinity events are detected within the type section of the Kilmaluag Formation. The Kilmaluag Formation was deposited within shallow, low energy freshwater lagoons with a tenuous link to a water body of brackish-marine salinity producing a salinity gradient. Salinity is the primary control upon faunal and floral occurrences within the Lealt Shale Formation. Relative salinity tolerances are estimated for 26 ostracod species from the formation. Based upon ostracod and mollusc data this formation has rapidly and frequently fluctuating palaeosalinity values. O isotope analyses of the bivalve Praemytilus strathairdensis show a positive correlation with the palaeosalinity fluctuations; the percentage abundances of the alga Botryococcus show a negative correlation. Variations in the adductor muscle-scar rosette of Darwinula muscula are due to the mostly ontogenetic subdivision of scars. Different fossil and Recent species of Darwinula are shown to have different rosette patterns.