Throughout much of the Cretaceous, as in the Jurassic, the whole of Britain and much of western Europe fell within the Boreal Realm which was characterized by the rarity or absence of a number of major invertebrate groups including many important ammonite families, larger Foraminiferida and rudistid bivalves (Casey and Rawson, 1973; Hallam, 1973). The composition of tne faunas of the Boreal Realm was probably not controlled simply by temperature and salinity, but more likely by prevailing patterns of environmental stability. The Boreal Realm was probably an area of high stress, with fluctuations of temperature, salinity and environmental energy while the Tethyan Realm had relatively stable environments. This is supported by the known palaeogeography of the Jurassic and early Cretaceous and the differences in faunal diversities and densities between the two realms. These realms persisted throughout much of the early Cretaceous, but from the Cenomanian onwards the differences declined.
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