Diverse fruits and seeds of the mid-Eocene Kishenehn Formation, northwestern Montana, USA, and their implications for biogeography

Lacustrine shales of the Kishenehn Formation of northwestern Montana provide an important window to the local mid-Eocene (Lutetian) ecosystem including evidence from insects, molluscs, vertebrates and plants. However, little has been published on the macrofossils flora, which includes abundant compressed fruits and seeds as well as foliage. Here we provide a preliminary survey, with particular attention to reproductive remains from the Middle Fork Region. Identified families include Equisetaceae, Cupressaceae, Pinaceae, Betulaceae, Brassicaceae, Cercidiphyllaceae, Eucommiaceae, Juglandaceae, Oleaceae, Platanaceae, Rutaceae, Salicaceae, Sapindaceae, Simaroubaceae and Ulmaceae. With at least 107 entities, this is among the most diverse lacustrine megafossil floras in North America. This flora shares elements with the early to mid-Eocene Green River Parachute Creek Member flora of Colorado and Utah, the Thunder Mountain flora of Idaho and the Okanogan Highland floras of British Columbia and Republic, Washington, as well as some with the late Eocene Ruby flora of Montana. We estimate the mean annual temperature to have been between 8.91 and 12.10 °C and mean annual precipitation to have been between 945 and 1,204 mm using the Bioclimatic Analysis/Mutual Climate Range Technique. This summary of floral elements complements the faunal record of the Kishenehn Formation and fills a gap in prior knowledge of the paleofloristic distributions.