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During the middle Albian and continuing through the late Cenomanian, changes in the spore and pollen record of the Middle Atlantic Coastal Plain suggest cooling climatic trends. The decrease in temperature is indicated by a reduction in palynomorphs associated with humid tropical conditions and an increase in gymnosperm pollen. These trends are paralleled by a gradual but continuous evolution of angiosperm types from tricolpate to triporate pollen. The latter events may be associated with cooling and increasing seasonality that would favor selective pressures for the evolution from early insect-pollinated angiosperms to well-developed wind-pollinated types by late Cenomanian. This climatically driven evolutionary trend reaches its acme during the Coniacian-Santonian clima ic maximum.
Climatic cooling during the middle Cretaceous, as suggested by oxygen isotope studies, is believed to be related to increased sea-floor spreading. Such plate movements resulted in the fractionation of the circumglobal tropical Tethyan seaway as well as an increasing rate of northward and counterclockwise movement of the Middle Atlantic coastal margin during middle to Late Cretaceous times.
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