Daghlian, C. P. and W. L. Crepet. 1983. Oak catkins, leaves and fruits from the Oligocene Catahoula Formation and their evolutionary significance. American Journal of Botany 70 (5): 639-649.

Abstract: Remains of staminate inflorescences, leaves, and fruits of Quercus from Oligocene Catahoula Formation show striking similarity to modern subgenera Erythrobalanus (catkins and leaves) and Lepidobalanus (fruits). The appearance of modern subgenera in the Oligocene, only a short period of time after the first occurrences of Quercus in the fossil record suggest a period of rapid evolution resulting in the modernization of Quercus. It is suggested that this period of relatively rapid evolution was in response to global climatic changes initiated at the Eocene-Oligocene boundary that may have been exaggerated by an Oligocene global lowstand of sea level. The climatic deterioration induced by these events may have allowed Paleocene Quercus to enter new adaptive zones through migration and hybridization culminating in the final major modernization of the oaks.

FOSSIL PHOTOS

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Quercus
Actual width in ( ) unless otherwise noted.

Quercus catahoulaensis
leaf
(length= 9.1 cm)
Veniation
(image= 1.4 mm)
Quercus huntsvillensis fruit
(1.6 cm)
Quercus oligocenensis
catkin
(length= 1.9 cm)

Florets with anthers
(1.5 mm)
Pollen grain
(17 µm)
Pollen grain cut in half
(14 µm)