Fossils from the Hell Creek and Tullock Formations in northeastern Montana provide detailed documentation of terrestrial faunal and floral evolution during latest Cretaceous (Lancian) and early Paleocene (Puercan) time. Here the replacement of Lancian faunas by those of Puercan age, most obviously signaled by the extinction of dinosaurs, and the changes in pollen floras sometimes used to mark the Cretaceous-Tertiary boundary occurred during a period of reversed magnetic polarity. Paleontological correlations suggest that dinosaur extinction and the change in pollen floras took place in the Red Deer Valley area, Alberta, during the same period of reversed polarity. Furthermore, also on the basis of paleontological correlations, the extinction of dinosaurs in the San Juan Basin, New Mexico, appears to have occurred either during the same period of reversed polarity or, possibly, during the preceding period of normal polarity.

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