The paleoflora of the Eocene Okanagan Highlands includes all plant and fungi fossils preserved in the Eocene Okanagan Highlands Lagerstätten. The highlands are a series of Early Eocene geological formations which span an 1,000 km (620 mi) transect of British Columbia, Canada and Washington state, United States and are known for the diverse and detailed plant fossils which represent an upland temperate ecosystem immediately after the Paleocene-Eocene thermal maximum, and before the increased cooling of the middle and late Eocene to Oligocene. The fossiliferous deposits of the region were noted as early as 1873, with small amounts of systematic work happening in the 1880-90s on British Columbian sites, and 1920-30s for Washington sites. A returned focus and more detailed descriptive work on the Okanagan Highlands sites revived in the 1970s. The noted richness of agricultural plant families in Republic and Princeton floras resulted in the term "Eocene orchards" being used for the paleofloras.
Paleoflora The Eocene Okanagan Highlands hosted a diverse mix of temperate and tropical paleobiotic elements, with the forests having the first significant proportions of temperate plants in North America. The paleobotanical community was a mixture of plants found in subtropical evergreen and temperate deciduous forests. Included in the forest were a number of important modern temperate flowering plant families such as Betulaceae, Rosaceae, and Sapindaceae, plus the conifer family Pinaceae. Study of the deciduous plants from the highlands has documented the occurrence of heteromorphic leaves derived from sun versus shade conditions and long shoot or short shoot buds. The paleobotanical community of the Republic area has been noted as the most diverse floral community of the Okanagan highlands, with some estimates ranging to over 68 families and 134 genera being present. The noted richness of Rosaceae fossils along with other important agricultural plant families found in the Republic and Princeton floras, including the genera that contain modern apples, blackberries, cherries, and serviceberries resulted in Wes Wher and Donald Hopkins (1994) coining the term "Eocene orchards". Fossil evidence from both Sorbus/Crataegus and Rhus leaves in the Republic sites indicate the area was a center for species overlap and active hybridization events.
Extent The majority of the lake deposits are compression fossils in lake bed sediments grouped informally into "Northern", "Central", and "Southern" sites. The Northern sites are composed of unnamed Ootsa Group formations which outcrop as the "Driftwood shales" near Smithers, British Columbia, sites now considered lost in the Quesnel, British Columbia area, and the "Horsefly shales", of an unnamed formation and unnamed group which outcrop around Horsefly, British Columbia. The Central sites represent Kamloops Group formations with the McAbee Fossil Beds, Tranquille River site and Falkland site, all in the Tranquille Formation, the Quichena site and Stump Lake site in the Coldwater Beds and outcrops of the Chu Chua Formation near Barriere, British Columbia. The Southern sites include the Princeton Group Allenby Formation sites surrounding Princeton, British Columbia, such as "Nine Mile Creek", "One Mile Creek", "Pleasant Valley", "Thomas Ranch", "Vermilian Bluffs", and "Whipsaw Creek". The most southerly of the Okanagan Highlands lakes, the Klondike Mountain Formation in Northern Ferry County, Washington include the "Boot Hill", "Corner Lot", "Gold Mountain", "Knob Hill", and "Mount Elizabeth" localities. Closely correlated with the Klondike Mountain Formation are the Penticton groups Kettle River, Marama and Marron Formations in the Boundary District along the Canada-United States border. There is debate as to the affiliation of the, now lost, Quesnel outcrops with the Greater Okanagan Highlands. Archibald et al. (2018) in the monograph overview of the Highlands Hymenoptera included them as part of the series. However the certainty for the placement was later questioned by Archibald and Cannings (2022) who opted to tentatively exclude Quesnel from the highlands while discussing the history of field collecting in the region.
Chert and amber
Additionally two important non-compression biotas are present in the Eocene Okanagan Highlands. A permineralized chert flora, the Princeton Chert is found along the Similkameen River interbedded with coal deposits of the Ashnola shale unit, Allenby Formation known for anatomically preserved plants. In the Central sites, subbituminous coal of the Hat Creek Coal Formation around Hat Creek hosts an amber biota, the Hat Creek amber which preserves many small insects and plant fragments that would likely not be found in the compression biotas.
Diatoms A small group of algae taxa in the protist order Ochrophyta have been described from the highlands. The Horsefly shales preserve and are placed in the family Aulacoseiraceae, a member of the diatom order Bacillariophyceae,. The third algae was first identified as a living paleoendemic Mallomonas species M. intermedia now restricted to warmer climates in North America and Europe. Genetic study of living specimens of M. intermedia from Europe and North America by Pael Škaloud and team reveled the North American specimens to be a distinct but related species, with the two species diverging in the Miocene. This resulted in the Horsefly specimens being considered an undescribed Mallomonas species ancestral to the whole species group the modern taxa belong to. In association with the identified diatom genera, undescribed fragilarioid diatoms, paraphysomonad golden-algae, Heliozoans, and euglyphid salicaceous amoebae have been reported.
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