Barghoornia oblongifolia was described from a single type specimen, a leaf, the holotype being USNM 32695 A&B, in the paleobotanical collections of Smithsonians National Museum of Natural History.

Working from this specimen, collected in the Republic, Washington, area in the early 1980s, the fossil was studied by Jack A. Wolfe, then of the University of California and Northwest School artist[1][2] and Wesley Wehr, affiliate paleobotany curator of the Burke Museum.

[3] They published their 1987 type description for the genus and species in a United States Geological Survey monograph on the North Eastern Washington dicot fossils.

[3] Based on the asymmetry of the leaf, secondary venation, and probability of the genus being pinnately compound Wolfe and Wehr suggested Barghoornia to be affiliated with the "Rosidae" clade.

[3] Barghoornia oblongifolia fossils have been recovered from a single location in the Okanagan highlands, an outcrop of the early Eocene, Ypresian[4] Klondike Mountain Formation in Republic.

However further study has shown the lake system was surrounded by a warm temperate ecosystem that likely had a mesic upper microthermal to lower mesothermal climate, in which winter temperatures rarely dropped low enough for snow, and which were seasonably equitable.