The Albian to Maestrichtian plant sequence in northern Alaska is well documented stratigraphically and regionally, by megafossil assemblages from more than 250 localities through about 23,000 ft of strata (about 7,000 m), in 11 homoclinally-dipping sections, and from 7 study areas across more than 350 mi (about 600 km) of the North Slope. Common interbedding of marine and nonmarine units provides data for (1) inferring continuous coastal-plain environments for the Cretaceous vegetation, and (2) referring the floral records to European stages through marine fossil correlations.

About 380 species have been recognized, but this total will undoubtedly increase as the detailed morphologic analyses continue. There are examples of species that appear to be widely ranging through time; but upon closer examination they are recognized as complexes of similar species, commonly occurring in stratigraphic sequence. Only a few of these complexes have been examined in detail, and such studies are currently in progress.

The sequential plant records are divisible into 7 biostratigraphic zones based on proportional representation of general taxonomic categories: I (oldest) = pteridophyte-ginkgophyte-cycadophyte assemblages; II = pteridophyte-ginkgophyte-cycadophyte-coniferophyte assemblages; III = pteridophyte-ginkgophyte-cycadophyte-coniferophyte-dicotyledon assemblages; IV = coniferophyte-dicotyledon assemblages; V-VII = predominantly dicotyledon assemblages with coniferophytes subdominant.

Certain taxonomic and morphologic data obtained at numerous levels are interpreted to indicate rapidly changing and widely fluctuating climates over the Cretaceous coastal plain during the Albian to Maestrichtian interval: (1) records of cycadophyte representation and of entire-margined dicots as warmer climate indicators; and (2) records of coniferophyte representation and of maximum size of Ginkgo leaves as cooler climate indicators. When plotted on stratigraphic charts, the cycadophyte-dicot peaks correspond to each other as well as to the stratigraphic positions of coniferophyte-Ginkgo troughs (warmer climate trends). The coniferophyte-Ginkgo peaks correspond to each other and to the cycadophyte-dicot troughs (cooler climate trends). On the basis of such data it is suggested that ater Mesozoic climates may have fluctuated as widely and as rapidly as those of later Cenozoic time; that is, at intervals of about 100,000-250,000 years. Warming trends appear to have peaked in the subtropical range with no evidence to indicate climates as warm as tropical. Cooling trends appear to have "bottomed-out" in the cool-temperate range with no evidence to indicate climates as cool as boreal.

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