In broad terms, the faunas of the Cretaceous were very similar to those of the Jurassic. In marine environments, there were new families of ammonites, including heteromorphs (where the coiling of the whorls depart from the regular planospiral), and two important groups of bivalves: the rudists, which were important rock builders in tropical regions, and the inoceramids, which were among the commonest bivalves in many environments. On land, although many Jurassic plants continued to flourish, the development and radiation of the angiosperms (the flowering plants) was important during the later part of the period.
During the Cretaceous, the area of Britain and western Europe lay between 30° and 45°N (Smith and Briden, 1977). As in the Jurassic, the now shrinking Tethys Ocean lay to the south (Fig. o) but to the west, lay the Atlantic. In the southern Atlantic initial rifting led first to the development of deep salt basins which were destroyed by marine transgression during the Albian (see table below). Some time between 150 to 80 million years ago (Triassic
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