A sequence of essentially worldwide, major diastrophic episodes, which had a considerable influence on biological events, occurred during or near mid-Cretaceous time. Mountain ranges were built and a concurrent 95-105-million-year-old episode of granitization is confirmed by accumulated radiogenic data. There is also evidence of widespread withdrawal of seas at or near the end of the Early Cretaceous. Late Early Cretaceous and initial Late Cretaceous times were characterized by diastrophic events, which resulted in the deposition of clastic facies and in the formation of unconformities of different magnitude, the magnitude depending on proximity of the areas involved to mobile belts. Moreover, the Cenomanian or basal Late Cretaceous sediments widely overlap Early Cretaceo s beds, or the rocks of older systems, in many geographic locations. A major biological "crisis" also occurred during an interval which may be considered to include Albian-Cenomanian time. The paleontologic record indicates that the resultant biological changes are as significant as those which distinguish most other systemic boundaries. Thus the physical and biological evidence generally suggests that, according to classical concepts, the more than 70-million-year-long Cretaceous time interval should be regarded as comprising two periods.

Significant biological changes (i.e., those involving the higher taxonomic categories) tend to occur at or near system boundaries. Such worldwide organic "crises" probably are caused by major diastrophic episodes which alter environments faster than highly specialized groups in the lower evolutionary categories can adapt to them. The primitive representatives of highly evolved organisms then invade and occupy the vacated ecological niches. The importance of mass extinctions resulting in conspicuous paleontological breaks between periods or eras probably has been overemphasized, compared with the importance of newly introduced and rapidly expanding types of plants and animals.

The mid-Cretaceous paleontological break is expressed in several ways. For example, among the higher taxa which were present but greatly restricted during the Early Cretaceous, and which are marked by population explosion in the Late Cretaceous, are the planktonic Foraminifera, pelagic crinoids, and angiosperms. Groups which were present in some abundance in the Early Cretaceous, but which expanded in the Late Cretaceous, are teleostei fish, bryozoans, and pulmonate gastropods. Significant declines occurred, however, in the abundance of ammonites, stromatoporoids, and sponges. Both extinctions and introductions occurred, for example, among the heterodont pelecypods, scleractinian coelenterates, opisthobranch gastropods, ostracods, and red algae. Examples of introductions during the La e Cretaceous include the ceratopsian dinosaurs, and the octopi and baculitids among the cephalopods.

The accumulating physical and biological evidence for a major mid-Cretaceous stratigraphic break, detailed in this paper, is more noteworthy than has commonly been appreciated in recent years.

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