Climate and Evolution in Western North America During Middle Pliocene Time

Axelrod, D.

Fossil floras from California, the Great Basin and the High Plains suggest a mild and warm semiarid climate characterized the western United States during Middle Pliocene time. This climate seems sufficiently distinctive to provide a basis for inter-provincial correlation of dis-similar types of vegetation. Warm climate also seems indicated for the mid-Pliocene of Europe and may form a means for wide correlation if substantiated by further investigation. Middle Pliocene climate is inferred to have had an important role in evolution for it apparently initiated grassland and sub-desert environments of subcontinental extent. Genetically variable populations of herbaceous and semi-woody plants probably spread into these areas from more restricted semiarid habitats already existing on the borders of forest, woodland and scrub vegetation where they had been evolving slowly during earlier parts of the Tertiary. Evolution in the newly-expanded Middle Pliocene environments seems to have been rapid. The continued development of more localized environments over the subcontinental dry regions during post-Middle Pliocene time presumably had a selective role in differentiating various taxonomic categories. Middle Pliocene and succeeding climates are considered to have been a major factor in the final breakup and segregation of major continental Tertiary Floras into restricted areas, and in their evolution into modern plant communities. Certain fundamental patterns in present-day plant distribution, termed latitudinal, radial and meridional, seem explainable on the basis of this late Cenozoic floristic evolution.

Climate and Evolution in Western North America During Middle Pliocene Time

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