Article: The composition and palaeogeographical significance of the Ordovician ostracode faunas of southern Britain, Baltoscandia, and Ibero-Armorica
A review of more than 250 genera has established the taxonomic composition, patterns of geographical and stratigraphical distribution, and faunal links for the Ordovician ostracodes of the British Isles, Ibero-Armorica, and Baltoscandia. Compositional and diversity changes of the faunas can be correlated with tectonic and ecological controls.Four orders (Beyrichiocopa, Platycopa, Podocopa, and Leperditiocopa), comprising over fifty families and more than 800 species are represented. In the three regions ostracodes show high diversity at all taxonomic levels, especially in Baltoscandia. Palaeocopes and binodicopes represent some 85% of the total number of Ordovician genera in the three regions. Palaeocope dominance increases from Ibero-Armorica to the British Isles and then to Baltoscandia. The same dominant families of palaeocopes and binodicopes (tetradellids, ctenonotellids, bolliids, and circulinids) occur in the three domains. Palaeocope and binodicope diversity increases during the Arenig-Llanvirn and maximum diversity at all taxonomic levels is reached in all three regions during the Llandeilo-early Caradoc interval. The later Ordovician is marked by a general decline of palaeocopes.Faunal links most clearly occur between Baltoscandia and the British Isles, with twenty-seven genera and some uppermost Ordovician species in common. There is a uniform increase in faunal similarity between Britain and Baltoscandia throughout the Ordovician, whereas British/Ibero-Armorican and Ibero-Armorican/ Baltoscandian generic contacts show an irreversible decline (for the Ordovician) after Llanvirn/Llandeilo times.Faunal composition and diversity vary with environment: binodicope-rich faunas are typically associated with clastic and unstable environments (e.g. Ibero-Armorica); more diversified, palaeocope-rich faunas are typically associated with more stable conditions and carbonate sedimentation (e.g. Baltoscandia). The biological effects of sea-level changes are attested by, for example, a diversity increase during the Llandeilo-early Caradoc (transgression) and a diversity decrease during the later Caradoc-Ashgill (regression).The evolving pattern of ostracode links support: 1, the northwards movement of the southern part of the British Isles (microcontinent Avalonia) towards Baltica and the consequent closing of Tornquist's Sea by the Caradoc/Ashgill; 2, the development of the Rheic Ocean between Gondwana (including Ibero-Armorica) and Avalonia by the mid-late Ordovician.