Prunus and Oemleria (Rosaceae) flowers from the late early Eocene Republic flora of northeastern Washington state, USA.

International Journal of Plant Sciences (Impact Factor: 1.54). 01/2011; 172:948-958. DOI: 10.1086/660880

ABSTRACT Two genera of Rosaceae are described from the latest early Eocene Republic flora of northeastern Washington State, United States. Prunus cathybrownae sp. nov. (Rosaceae: subfamily Spiraeoideae, tribe Amygdaleae sensu Potter et al.) is based on eight flowers, including one containing in situ pollen and two immature fruits. Flowers are actinomorphic, perigynous, and pentamerous, with a campanulate hypanthium bearing five sepals. The gynoecium is unicarpellate and consists of a distally flared, bilobed stigma; an elongate style; and an ellipsoid, bilaterally asymmetric ovary. Two whorls of stamens—an inner one in which stamens are reflexed and an outer whorl of extended stamens—are both inserted into the hypanthium. Pollen from the outer whorl is 20 μm in diameter and tricolporate with a striately ornamented exine; clusters of smaller, presumably immature grains 7 μm long and 4 μm wide lacking distinctive ornamentation were recovered from the inner whorl. Immature fruits differ from the flowers in either having a senescent style or lacking one entirely and having fewer remnants of perianth parts and a larger and more symmetrical ovary. A second flower, Oemleria janhartfordae sp. nov. (Rosaceae: subfamily Spiraeoideae, tribe Osmaronieae sensu Potter et al.) is actinomorphic and perigynous and has five free pistils that each resemble the solitary pistil of Prunus. These include a flattened, bilobed stigma and an ellipsoid, bilaterally asymmetric ovary. The fossil has the laterally fixed, elongate style characteristic of the genus. These fossils are the oldest known floral examples of these two genera and demonstrate that both Prunus and Oemleria were present in the latest early Eocene in western North America. The Okanogan Highlands floristic province provides the earliest fossil evidence to date for the first major radiation of the Rosaceae, an important mostly temperate, mostly Northern Hemisphere family.

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