Quercus hiholensis (Fagaceae)

 
This study documents the presence of white oaks in the middle Miocene of central Washington state, based on the presence of characteristic morphological and developmental features of the acorns. These acorns have a highly reduced perianth, and are thus thought to demonstrate the presence of annual fruit habit by the middle Miocene. They also contain insect damage patterns, galls and fungi that are the subject of a later study.
Borgardt, S. J., and K. B. Pigg. 1999. Petrified acorns from the Miocene of North America and their significance to the evolution of Quercus L. (Fagaceae). Amer. J. Bot. 86:307-325. http://www.amjbot.org

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