During much of the Triassic, Britain was largely an emergent landmass with little marine deposition. The main phase of marine deposition only occurred at the end of the Triassic, within the Rhaetian Mercia Mudstone Group, Blue Anchor Formation and the succeeding Penarth Group. A brief marine incursion, probably from the north, also occurred in the middle Triassic. Few ostracod assemblages are known from this period. The main Triassic outcrop and key sites in the British Isles are illustrated in Figure 1.
Globally, Triassic ostracod assemblages are generally of low diversity following the end-Permian extinction and the British record is no exception. It is the late Triassic eustatic sea-level rise that opens new biotopes on the transgressed landmasses and mixes marginal marine faunas with new pioneer species.