The standard stages of the Cretaceous are summarized above. As has been mentioned, the ammonite faunas of western Europe were restricted, and, for much of the interval, correlation with areas outside this region is difficult, particularly during the early Cretaceous. An added difficulty in the late Cretaceous is the absence of rarity of ammonites in chalk facies, due either to their ecological exclusion, or to pre-burial dissolution of their skeletons. In consequence, the standard Upper Cretaceous zones are based on various groups, including echinoids, crinoids, belemnites, bivalves and others. In non-marine facies, subdivision and correlation is based primarily on ostracodes and plant microfossils.
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