TEMPORAL AND LATITUDINAL FLORA GRADIENTS IN THE LATE JURASSIC MORRISON FORMATION

The Late Jurassic Morrison Formation is a terrestrial, continent-scale (New Mexico, USA to Alberta, Canada) deposit in the Western Interior Basin (WIB) of North America. Sedimentary analysis, fossils, and climate proxies generally indicate that the formation represents a predominantly savannah-like ecosystem with lakes, wetlands and swamps. Given the latitudinal (>20° present-day) and temporal (at least Kimmeridgian–Tithonian), expanse of the Morrison, strong floral gradients in the formation are expected. However, the Morrison has been largely overlooked in this regard, particularly in terms of plant megafossils, because it is composed of heavily oxidized sediments and few fossil floras have been identified. Previous biostratigraphic and biogeographic palynofloral analysis has indicated little temporal variation but some latitudinal gradation in plants across the formation. Plant megafossil analyses generally support this result but previous studies have used museum specimens with little stratigraphic or taxonomic control. In this study, five systematically described floras (Como Bluff, Temple Canyon, Mygatt-Moore, Montezuma Creek and the Belt Flora) are examined by their taxonomic composition, stratigraphic position, and geographic placement within the Morrison Formation. Initial results show that while there are a few overlapping elements, the Upper and Lower Morrison show marked differences in taxonomic composition. These include fewer cycad and more conifer species in the Lower Morrison, and a taxonomically distinct fern flora between the Upper and the Lower Morrison. However, the localities are too tightly spaced geographically to determine the presence or absence of a latitudinal gradient. Future megafloral work on the Morrison should concentrate on precise stratigraphic placement of floras, facies association of floral elements, continued systematic descriptions of well-known sites, and fieldwork in select regions to expand the latitudinal coverage of the fossil plants in the formation.