A NEW EARLIEST PALEOCENE, MOUNTAIN PROXIMAL, MATURE FOREST FLOOR FOSSIL LEAF SITE IN THE DENVER BASIN, COLORADO, USA: IMPLICATIONS FOR THE ORIGINS OF RAINFORESTS

North America’s Western Interior Basin Cretaceous floras were diverse and heterogenous, while Paleocene floras were depauperate and homogeneous. Forest diversity did not recover until the Early Eocene, ~10 Ma after the K-Pg extinction. This paradigm shifted with the discovery of the heterogeneous and high diversity Early Paleocene Castle Rock rainforest (ca. 64 Ma) in the Denver Basin, Colorado. The remarkable composition and heterogeneity of the Castle Rock flora is hypothesized to result from two factors: 1) the proximity of the site to the topographic front on the western edge of the Basin resulting in intensified orographic rainfall, and 2) its unusual taphonomic setting, where overbank deposits buried a mature forest floor. Understanding the origin of the Castle Rock flora requires a fossil site of the same geographic and taphonomic setting closer to the K-Pg. A series of new mountain proximal, earliest Paleocene (~10 m above the K-Pg boundary; ca. 65.9 Ma), mature forest floor deposits (including in situ tree stumps) preserving megaflora were excavated in the Denver Basin. Over 1,500 fossil leaves were censused from 6 quarry sites spanning a ~50 meter exposure that preserved 4 distinct forest floor horizons. Together, the flora contain 12 angiosperms, 2 ferns, and 3 seeds/reproductive parts. The taxa include characteristic Early Paleocene plants including dominant dicots“Rhamnus” goldiana, Platanus raynoldsii, and “Zizyphus” sp., minor dicots “Ficus” planicostata, “Artocarpus” lessigiana, cf. “Vitis” stantoni, Paleonelumbo macroloba, and three unidentified species. Monocots include Sabalites sp. and cf. Paloreodoxites plicatus; ferns include Allantodiopsis erosa and Onoclea sp. Leaf sizes indicate a rainfall of ~2.9 m +126/-88 cm (1 SE). The “Rhamnus” goldiana specimens exhibit a leaf physiognomy similar to “Rhamnus” leaves from the Castle Rock site. The new site shows a high spatial heterogeneity in the dominance of taxa between quarries. Also, leaves resembling “Vitis” stantoni indicate a possible range extension for this taxon across the K-Pg. Together, the new site indicates that while diversity was markedly low, spatial heterogeneity in forests and “rainforest” leaf physiognomy occurred near the K-Pg boundary. The high diversity in mountain proximal floras appeared later in the Denver Basin.