Hybridization is a key mechanism for increased diversification and speciation among modern plants and is especially important in certain families such as Rosaceae and Anacardiaceae. This mechanism is found to occur most commonly in areas of ecological disturbance, distribution, and migration. Resulting hybrid individuals have characteristic intermediate or mosaic patterns that combine those of the two parents in morphologically distinct and identifiable ways. The diverse latest Early Eocene flora of Republic, Washington, USA, is among the most collected of the Okanagan Highlands floras. This fossil flora includes relatives of modern taxa that are known to hybridize and produce characteristic patterns of leaf morphology and venation (Rosaceae; Anacardiaceae). In this study, we document patterns found in representative fossil leaves that are comparable to those indicative of hybridization in the modern relatives. We use the genera Sorbus L. (Rosaceae) and Rhus L. (Anacardiaceae) as models to demonstrate potential hybridization in the latest Early Eocene.

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