MICROFOSSILS/Ostracoda 461 Figure A Silurian palaeocopid ostracod (Beyrichicopina) A B: female, lateral and anterior views (note large anteroventral brood pouches); C: male Scale bar mm position in myodocopan evolution, but their affinities are controversial and there is some doubt that they are true ostracods The exclusively marine bairdioideans, one of the oldest extant podocopid lineages, are known from the mid-Ordovician onwards; they show a considerable variety of both deep- and shallow-water taxa by the Devonian, although post-Palaeozoic forms are most diverse in shallow carbonate environments The podocopid cytheroidean family Bythocytheridae originated in the Ordovician, possibly sharing a common ancestor with the Bairdioidea, and underwent major radiations in the Devonian and again in the Jurassic–Cretaceous; while many cytheroidean families include at least some taxa that have adapted to brackish waters, bythocytherids never seem to have achieved this and are found only in fully marine salinities Distinctive Palaeozoic marine assemblages from the Devonian onwards have been related to shallow benthonic (high energy; dominated by thick-shelled ornamented podocopans), deep benthonic (low energy; thin-shelled spinose podocopans), and pelagic environments (myodocopans) The low-energy benthonic assemblage may represent a ‘palaeopsychrospheric’ fauna inhabiting deep cold waters below the thermocline in a stratified ocean, but this interpretation remains controversial The pioneer colonization of pelagic marine environments by myodocopan ostracods took place in mid-Silurian (Figure 7), after an ecological shift from a nektobenthonic mode of life, probably in response to bottom water oxygen deficiency and the opportunities offered by the plankton-rich, well-oxygenated upper waters Another myodocopan group, the halocypridoidean halocyprids, may have undergone a similar shift, but they have an extremely poor fossil record and evidence for the existence of pelagic halocypridans since the Devonian, as shown on Figure 7, is limited and controversial; in the Silurian – Carboniferous they are represented by entomozooideans, a group of questionable affinity A Silurian–Devonian palaeocopid binodocopine family, the aechminids, with a large, hollow dorsolateral spine on each valve, are also thought to have been pelagic, since it is difficult to imagine such a carapace morphology as functionally suitable for a benthonic organism; since the end of the Palaeozoic, however, there have been no pelagic podocopans The first undoubted non-marine ostracods, the podocopid superfamilies Carbonitoidea and Darwinuloidea, entered freshwater environments in the Early Carboniferous and radiated to high diversity in the Late Carboniferous–Permian; at the end of the Permian the former became extinct and the latter reduced in diversity (Figure 7) Post-Palaeozoic nonmarine faunas are dominated by Cypridoidea and limnocytherid Cytheroidea; both lineages may have Late Palaeozoic origins but their early history is controversial, as is the possibility of a Carboniferous brackish-freshwater radiation of some platycopids, which today are an exclusively marine group Ostracods, which may represent the first limnocytherids, proliferated in Late Carboniferous and Permian lakes and coal swamps It is notable that darwinuloideans, which in Palaeozoic assemblages show distinctive sexual dimorphism (the females having expanded posterior brood chambers), have apparently been exclusively parthenogenetic since the Early Mesozoic Almost all of the Palaeocopida, so characteristic of Palaeozoic marine faunas, became extinct at the end of the Permian; only the Puncioidea survived and are represented today by a single living species PostPalaeozoic marine faunas are dominated by cytheroidean Podocopida Major radiations of cytheroidean families took place in the Mesozoic, for example the Cytheruridae (Triassic onwards), the Progonocytheridae and Schulerideidae (mid-Jurassic–Early Cretaceous), the Trachyleberididae and Brachycytheridae (Late Cretaceous onwards), and the Cytherettidae, Hemicytheridae, Loxoconchidae, Leptocytheridae, and Xestoleberididae (Tertiary) The origins of the diverse modern deep-sea (psychrospheric) benthonic ostracod fauna (predominantly cytheroideans) were in Mesozoic faunas which evolved in a thermospheric ocean and were forced to adapt to cooling conditions in the Tertiary Metacopine platycopids radiated in the Triassic and Early Jurassic and then became extinct Platycopina were often the dominant group (in terms of abundance rather than diversity) in the chalk seas of the Late Cretaceous; their highest diversity today is in sub-tropical or tropical shallow carbonate environments In contrast to podocopans, the fossil record of myodocopans is poor, since in many the valves are weakly or not at all mineralized The nektobenthonic cladocopine halocyprids are usually better calcified, however; they originated in the Palaeozoic and became relatively diverse in the Mesozoic, achieving their greatest diversity in the deep (bathyal) waters of