The Paleocene Megaflora of the Western Interior of North America: Tapping the Potential of One of the Most Extensive Terrestrial Fossil Records

The most extensive exposures of Paleocene terrestrial strata in the world are present in the Laramide basins of New Mexico, Colorado, Utah, Wyoming, Montana, North Dakota, and South Dakota, and in Saskatchewan and Alberta. Collection of fossil plants in these basins began in the 1860s. Today, the Paleocene megaflora of the Rocky Mountains is an impressive record of over a thousand localities spanning a contiguous region of more than a million square kilometers. It is arguably the largest and most continuous fossil floral sequence of any age. The geographic and temporal density of Paleocene megafloral sites in these rocks provides the opportunity to assess regional vegetation over a ten-million-year period in a range of paleolatitudes, paleoelevations, local sedimentological facies, and position within depositional basins relative to adjacent mountains and seaways. Deposited in a greenhouse world, bracketed by the K-T boundary and the PETM, associated with vertebrate and invertebrate faunas and palynofloras, this record holds tremendous potential for elucidating critical events in Earth history. Significant challenges exist: 1) many of the historical localities are not precisely located nor their specimens curated; 2) temporal resolution of most sites is relatively poor; 3) taxonomic definition of megafloral elements has not been modernized nor standardized; and 4) foliar physiognomic analysis of paleoclimate is subject to large errors. Progress is being made by: 1) re-collecting old sites with new censusing techniques; 2) enhancing and testing biostratigraphic age estimates with magnetostratigraphy and radiometric dating; 3) using floral morphotyping as a first step toward resolved taxonomy in concert with computer databases that organize high resolution images of both type specimens and multiple examples of morphotypes; and 4) developing new physiognomic tests and using larger sample sizes. It is now becoming possible to resolve regional phytogeography at a temporal resolution of 100 k.y.