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

Abstract

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.

The early Eocene flora of Horsefly, British Columbia, Canada and its phytogeographic significance

“…(Penhallow 1908) and illustrated as a "burr" from Republic by Pigg and Wehr (2002). Disseminules of this type have been designated as Pteroheterochrosperma horseflyensis (Smith et al 2023). Another interesting disseminule is characterized by a hard central body with arched longitudinal ridges and a punctate surface (Textfig.…”

“…These include a small seed (Text-fig. 10f) recently described as Pteroheterochrosperma horseflyensis (Smith et al 2023) that is commonly found at Republic (Pigg and Wehr 2001) and a flower with a distinctive papillate surface that may be assignable to Apiaceae (Textfig. 10g, k).…”

“…(Manchester and McClain 2001) and Lagokarpos lacustris D.M. McMurran et MancHester (McMurran and Manchester 2010), pistillate and staminate reproductive structures of Palaeocarpinus P.r.crane (Douglas and Stockey 1996, Pigg et al 2003, Correa-Narvaez and Manchester 2021 and tiny seeds now designated as Pteroheterochrosperma horseflyensis Mack.a.sM., GreenwaLt et MancHester (Smith et al 2023). However, a comprehensive treatment of the Horsefly flora has been lacking.The purpose of this article is to provide an updated report on the floristic composition of the Horsefly plant fossil assemblage, based on an increased collection of megafossils, and to consider its phytogeographic implications.…”