Acer clarnoense is represented by a series of fossil specimens which were recovered from late Eocene, Priabonian stage,[1] outcrops of the lower John Day Formation which outcrops at the Teater Road locality, also known as the Sheep Rock Creek flora. This site, Oregon Museum of Science and Industry site number 256, was originally included by Wolfe and Tanai in the older Clarno Formation, which underlies the John Day formation. This placement was based on floral similarities between the plant species at the Teater Road site and dated sites of the Clarno formation. The placement of the site in the Clarno Formation was accepted until 2007 when white ash deposits at the base of the Teater road outcrop were dated using Argon–argon dating analyses. A series of twenty-six sanidine crystals taken from the white ash tuff were dated with an average age of 36 million years ago which is younger than the 39.2 million years ago ignimbrite at the base of the John Day Formation.[1]
The species was described from a group of type specimens, the holotype leaf specimen UCMP 9010A, B, and three paratype leaves, specimens UCMP 9011, UCMP 9012, and UCMP 9013. The fourth paratype, UCMP 9014 is a single complete samara specimen. All of the type specimens are currently preserved in the paleobotanical collections housed at the University of California Museum of Paleontology, in Berkeley, California.[2] The specimens were studied by paleobotanists Jack A. Wolfe of the United States Geological Survey, Denver office and Toshimasa Tanai of Hokkaido University. Wolfe and Tanai published their 1987 type description for A. clarnoense in the Journal of the Faculty of Science, Hokkaido University.[2] The etymology of the chosen specific name clarnoense is in recognition of the Clarno Formation to which the type locality was thought to belong when the species was described.[2]
