Ostracods are a diverse group of arthropod crustaceans with a geological record from the Ordovician. Their radiation from marine to non-marine environments is a key step in the evolution of the group. The nature and chronology of this transition is examined, and proxies for non-marine environments determined. The Mississippian of the Midland Valley of Scotland contains a wide range of marine to non-marine ostracods, macrofauna and sediments that make it an ideal study area. This study documents the evidence for early Mississippian freshwater ostracods, 20 million years older than previously recorded. \ud Twentyfive ostracod species from the orders Myodocopida, Palaeocopida and Podocopida are described, four of which are new species. Macrofossils are used to interpret the environmental tolerance of the ostracods. Important brackish to freshwater macrofauna are the bivalves Carbonicola, Curvirimula and Naiadites, the vermiform microconchid “Spirorbis”, conchostracans and fish. Eurytopic ostracods are species of Cavellina, palaeocopes and Shemonaella siveteri n. sp. Key brackish to freshwater ostracods are species of Geisina arcuata, and Paraparchites circularis n. sp. Freshwater ostracods are species of Carbonita. Non-marine sediments contain three new types of algal palynomorph, Botryococcus sp., and arthropod fragments. The algal palynomorphs are interpreted as freshwater. Brackish to freshwater habitats identified include estuaries, lakes, temporary pools and swamps. \ud A protocol for the examination of diagenetic alteration of ostracods and macrofossils is proposed, which is essential prior to any isotope analysis. Diagenetically altered ostracods may be mistaken as pristine specimens, without a study of the carapace ultrastructure. This is assessed by comparing Carboniferous and Recent specimens. The Carboniferous ostracods have undergone seven diagenetic stages: 1. neomorphic calcite; 2. dissolution and pitting; 3. euhedral pyrite; 4. ferroan calcite; 5. ferroan dolomite; 6. iron oxide; 7. sphalerite and barite. The carbon and oxygen stable isotope data from the ostracods reflect these stages of diagenesis
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