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Age and Environment: A Survey of North American Tertiary Floras in Relation to Paleoecology

E. S. Barghoorn
Journal of Paleontology
Vol. 25, No. 6 (Nov., 1951), pp. 736-744
Stable URL: http://www.jstor.org/stable/1299814
Page Count: 9
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Age and Environment: A Survey of North American Tertiary Floras in Relation to Paleoecology
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Abstract

Seven Upper Cretaceous and 25 Tertiary floras have been analyzed with respect to their generic composition. The genera described from each flora have been divided into three clearly defined categories: 1) native genera, now living in the geographic region of the fossil flora, 2) exotic genera, no longer represented in the geographic region of the fossil flora, and 3) genera which are botanically unidentified, extinct or designated as form genera. Statistical representation of each of the three categories in each flora has been determined and quantitative change in the three categories related to geologic age. It is found that there is a close correlation between increasing geologic age and the percentage of "native" to "exotic" genera. The "native" genera in fossil floras, regardless of geographic location, show a gradual decrease between the Upper Cretaceous and the late Eocene and an almost exponential increase between early Oligocene and late Pliocene. The "exotic" element reaches its peak in the late Oligocene and rapidly declines to the late Pliocene. Changing percentages of the three generic components have been interpreted as representing major migrations of plant populations in response to profound climatic changes during the Cenozoic. Relationship between geologic age and the generic composition of Cenozoic floras is proposed as a means of validating the age assignment of continental deposits bearing plant fossils.

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