RECONSTRUCTING THE LOCAL VEGETATION AND SEASONALITY OF THE LOWER EOCENE BLUE RIM SITE OF SOUTHWESTERN WYOMING USING FOSSIL WOOD

摘要

Premise of research. Permineralized wood along with fossil leaves, reproductive structures, and dispersed pollen and spores are preserved in the uppermost Lower Eocene (~49.0 Ma) Blue Rim escarpment of the Bridger Formation in southwestern Wyoming. Each component of the flora provides a slightly different view of the diversity and environment, but the wood assemblage provides additional information on the taxonomic composition, forest structure, and local seasonality and climate. Methodology. Fossil wood specimens, including occasional in situ stumps, were collected from Bridger strata exposed at Blue Rim, north of Green River, Wyoming. Thin sections in the three standard orientations were prepared of each specimen and examined by light microscopy. Specific gravity and vulnerability index values were calculated for well-preserved specimens. Tree height was estimated from diameter measurements obtained from 10 specimens. Pivotal results. Seven wood types are recognized in the assemblage, including a single specimen of Pinaceae, the only conifer macrofossil recovered at Blue Rim to date, and six angiosperms. Angiosperm woods have affinities to Canellaceae, Fabaceae, and Anacardiaceae. The specimen of Canellaceae is the only Blue Rim wood with scalariform perforation plates, predominately uniseriate rays, and mostly solitary vessels. A new Caesalpinioid species, Peltophoroxylon diversiradii, lacks storied structure and has conspicuous paratracheal axial parenchyma. The most abundant wood type has radial canals and prismatic crystals in the ray cells and is assigned to Edenoxylon parviareolatum (Anacardiaceae). Conclusions. While the wood assemblage has significantly lower taxonomic diversity compared to the leaves, reproductive structures, and dispersed palynoflora, it includes some taxa not otherwise recognized from Blue Rim. These trees, reaching ~16-28 m tall, would have supported co-occurring climbers in the same flora. Diffuse porosity, generally absent or indistinct growth rings, rare scalariform perforation plates, and the high vulnerability indices of the Blue Rim woods suggest that climate conditions were warm with limited seasonality.