The specimen was recovered from an outcrop of the "Bull Run" flora, University of California Museum of Paleontology location P562, which preserves lacustrine sedimentation associated with extensive intermittent volcanism between 43 to 38 million years ago.

Radiometric dating of volcanic ash layers above and below the flora reported in 1966 indicated an age of approximately 41 million years ago.

The Bull Run, Copper Basin, and Elko floras were formed during the same time frame as a result of the volcanic activity.

[1] The specimen was studied by paleobotanists Jack A. Wolfe of the United States Geological Survey, Denver office and Toshimasa Tanai of Hokkaido University, with Wolfe and Tanai publishing their 1987 type description for A. taurocursum in the Journal of the Faculty of Science, Hokkaido University.

[1] A. eonegundo is one of six Acer species to be described by Wolfe and Tanai in 1987 from the "Bull Run" flora, with the others being A. axelrodi, A. cadaver, A. elkoanum, A. eomediunum, and A. taurocursum.