Anatomical and developmental study of petrified Quercus (Fagaceae) fruits from the Middle Miocene, Yakima Canyon, Washington, USA

The first reported petrified acorns to show internal anatomical structure a re here described from Middle Miocene (similar to 15.6 million years old) c hert of the Columbia River Basalt Group in Yakima Canyon, Washington. Querc us hiholensis Borgardt ct Pigg sp. nov, is described from anatomical and mo rphological fruit features, as well as a little recognized anatomical featu re, the umbilical complex. Acorns, each comprising a nut and its cupule, ar e up to 15.3 mm long and 18.8 mm wide with helically arranged, imbricate, t uberculate cupule scales. They show basal aborted ovules, short styles, bro ad stigmas, and lack grooves in their cotyledons. These characters and the developmental pattern seen in these fossil acorns demonstrate that Q. hihol ensis conforms to genus Quercus (Fagaceae), subgenus Quercus, section Querc us (the white oaks). The correspondence of Q. hiholensis to the modern sect ion Quercus reveals that the derived floral and fruit characters that disti nguish section Quercus within the genus had evolved by the Middle Miocene.