Pteroheterochrosperma

Pteroheterochrosperma horseflyensis was first reported from the Horsefly Shales in the Horsefly River valley of central eastern British Columbia. Further Eocene Okanagan Highlands occurrences have been found in the Tranquille Formation at the McAbee Fossil Beds near Cache Creek, British Columbia and the Klondike Mountain Formation around Republic, Washington. The only known occurrence outside of the Eocene Okanagan Highlands was reported in 2023. A single fossil was recovered from the Kishenehn Formation's "Dakin Site" on the Middle Fork Flathead River in Flathead County, Montana. This occurrence is younger then that of the others, being confidently placed around 44 Myr old and within the Lutetian of the Middle Eocene. == History and classification==

The oldest mentions of these fruits come from the detailed description of Lawrence Lambe's expeditions through the greater Central British Columbian region from Horsefly southeast to Princeton. Within the fossil material from Horsefly were "a few" fossils of minute fruits that Penhallow had not seen before. While giving them a brief description and providing an illustration of one, it was not named as a new species. Instead, based on the finding of several small elm leaves, Penhallow placed both leaf and fruit fossils into the European species Ulmus minuta. While Smith et al. (2023) did not place the genus to any angiosperm group, Smith did posit a possible relationship in their PhD dissertation. In the discussion of Pteroheterochrosperma, Smith suggests that a possible modern relation to be the brassicaceous genus Boechera. == Description ==

Pteroheterochrosperma horseflyensis disseminules are comprised of very small seeds surrounded by a wing-like membrane. The specimen described from Horsefly by Penhallow (1908) had an ovate fruit of in height and in width, while the Kishenehn specimen possesses an obovate outline and is described as having a minutely wrinkled surface texturing. Its smaller than the Horsefly specimen with proportions of by only . Both specimens have an orbicular to elliptical wing-like membrane enclosing the seeds body. Again, the Kishenehn specimen is smaller than the Horsefly specimen at versus . The wing membranes have numerous veins radiating out from the seed body towards the membrane edges, with the majority reaching the edges and terminating. Some veins do not reach the edges, while some others are forked into two parallel veins. The Kishenehn fossil has an estimated 27 veins and a few are noted to fork into three veins. == Paleoenvironment ==

, Albertine Rift, Democratic Republic of the Congo The Republic, McAbee and Horsefly sites are part of a larger fossil site system collectively known as the Eocene Okanagan Highlands. The highlands, including the Early Eocene formations between Driftwood Canyon at the north and Republic at the south, have been described as one of the "Great Canadian Lagerstätten" based on the diversity, quality and unique nature of the paleofloral and paleofaunal biotas that are preserved. The highlands temperate biome preserved across a large transect of lakes recorded many of the earliest appearances of modern genera, while also documenting the last stands of ancient lines. The Klondike Mountain Formation represents the pinpoint spot along an upland lake system series that was surrounded by a warm temperate ecosystem with nearby volcanism The paleoforest surrounding the lakes have been described as precursors to the modern temperate broadleaf and mixed forests of Eastern North America and Eastern Asia. Based on the fossil biotas, the lakes were higher and cooler then the coeval coastal forests preserved in the Puget Group and Chuckanut Formation of Western Washington, which are described as lowland tropical forest ecosystems. Estimates of the paleoelevation range between higher than the coastal forests. This is consistent with the paleoelevation estimates for the lake systems, which range between , which is similar to the modern elevation of , but higher. Estimates of the mean annual temperature have been derived from climate leaf analysis multivariate program (CLAMP) analysis of the Republic paleoflora, while bioclimatic and mutual climate range analyses (BA/MCRA) was used for the Kishenehn paleoflora. The CLAMP results after multiple linear regressions for Republic gave a mean annual temperature of approximately , with the leaf margin analysis giving . BA/MCRA on the Republic flora suggested a mean annual temperatures around . BA/MCRA results for the Kishenehn flora returned , with wider temperature change between summer and winter, with average annual low temperatures around , averaging around , lower than Republic estimates. The bioclimatic analysis for Republic and Kishenehn suggest mean annual precipitation amounts of and respectively. The Kishenehn paleolake is thought to have been surrounded by a dense forest environment, which is supported by the presence of taxa like Juniperus which are often found in low light environments, and faunal elements such as the long-bodied cranefly †Cyttaromyia lynnae and a number of arboreal mammals. The forest experienced distinct seasonality, unlike the older Republic flora, having a warm summer in which the majority of the annual precipitation happened, and a cold winter, with temperatures dipping below freezing. The climate of the surrounding lake basin is suggested to have possibly been moderated by a black-body effect from the lake, in which the water mass provided warmer average temperature levels. Smith et al. do note that their climate estimates may be problematic, with the annual low temperatures being problematic due to fossils of both subtropical insects and teeth from undescribed crocodilians coming from the formation. == References ==