| Summary: | The Pliocene Epoch represents a significant period in the palaeozoogeography of North Atlantic, Arctic and northern Pacific podocopid ostracods. A climate crash during Marine Isotope Stage (MIS) G6 (c. 2.74 Ma) resulted in the extinction of early-mid Pliocene thermophilic ostracods and the subsequent emergence of oligothermal assemblages during the late Pliocene. During the numerous cold stages of the late Pliocene-early Pleistocene Arctic ostracod species were able to disperse southwards into the mid-latitudes of the North Atlantic. The Pre-Ludhamian Stage of the Red Crag Formation, Suffolk, England spans one such episode (c. 2.7–2.41 Ma) and thus offers new insight into the vagility of shallow marine ostracods as they responded to glacio-eustatic and isostatic changes. Five podocopid ostracods are described from the Sizewell Member (Pre-Ludhamian Stage) of the Red Crag Formation and new information regarding their phylogeny and palaeozoogeography is presented. Four of the species are new to science, Microcytherura gelida sp. nov., Thaerocythere liebaui sp. nov., Thaerocythere russus sp. nov. and Pectocythere hollowayae sp. nov., while the fifth, Neomonoceratina tsurugasakensis (Tabuki), is reexamined and reassigned. The comparative morphology of Pectocythere Hanai and Kotoracythere Ishizaki is reviewed and a morphological continuum recognized between phenotypic end-members. Pectocythere hollowayae sp. nov. forms part of a distinctive morphological group comprising Arctic and northern Pacific species: P. ishizakii Irizuki and Yamada, Pectocythere? sp. cf. P.? dentarticulata (Smith) sensu Swain and Gilby, Kotoracythere sp. Irizuki, P. janae Brouwers and P. parkerae Swain and Gilby. Although lacking a distinctive caudal process, Paijenborchella tsurugasakensis Tabuki is reassigned to Neomonoceratina sensu Zhao and Whatley. Neomonoceratina tsurugasakensis displays some minor polymorphism over an enormous geo-temporal range that extends from the early Miocene of the northwestern Pacific to the late Pliocene-early Pleistocene of the southern North Sea Basin. Despite its lack of pelagic larvae N. tsurugasakensis was capable of transoceanic dispersal, evidenced by its occurrence in the early Pliocene (c. 4.90–4.81 Ma), Oosterhout Formation, Western Netherlands Basin, alongside other pan-Arctic and North Pacific ostracod taxa. Mechanisms of dispersal remain unclear but could involve passive transport on Fucus brown algae, aided by the Transpolar Drift. Further agents of dispersion could include fish and birds. The viability of water birds as vectors of invertebrate dispersal is given further credence by the discovery of Phoebastria anglica (Lydekker), a fossil albatross, in the Pliocene crags of East Anglia.
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