PALYNOLOGICAL EVIDENCE FOR CONIFER DOMINANCE AND ARID CLIMATE IN THE LATE JURASSIC MORRISON FORMATION, U.S.A

The Late Jurassic Morrison Formation extends across the Western Interior of the U.S.A. and as equivalent formations into Canada. Its rich and diverse vertebrate fauna is well known, but its vegetation remains poorly understood. Sparse plant megafloras, consisting of leaves or permineralized axes, include cycads, cycadeoids, tree ferns, conifers and gingkophytes. Plant fossils suggest a relatively wet climate for Morrison times, but this contradicts abundant sedimentological evidence for seasonally dry climates throughout the southern and central part of the formation. Parrish et al. (2004) also support inference of a seasonally dry climate based on plant taphonomic data, and further suggest that ferns dominated the landscape, with arborescent plants confined to wetter habitats. Here we provide evidence from 10 palynological samples collected from upper Morrison strata in New Mexico, Arizona, Utah, Colorado and Wyoming for drier conditions in the south and central parts of the formation and conifer dominance throughout. The putative xerophytic conifer Classopollis (Cheirolepidiaceae) is dominant in Arizona and New Mexico, and fern spores (considered a proxy for wetter conditions), are low in both frequency and taxonomic diversity. Classopollis is replaced in dominance by members of the conifer families Araucariaceae and Taxodiaceae on the Colorado Plateau and by Pinaceae and Podocarpaceae in Wyoming. Fern abundance and diversity increase northward, suggesting wetter conditions, but spores remain subordinate throughout the formation. Fern morphotypes include members of the archaic families Dipteridaceae, Cyatheaceae, Dicksoniaceae, Osmundaceae and Schizaeaceae, as well as unknown Filicales. Other components of the palynofloras, such as Gnetales, cycads and cycadeoids, lycophytes, sphenophytes and bryophytes, are uncommon or extremely rare. Plant megafloras display only partial overlap with palynofloras, suggesting that palynomorphs and plant megafossils are sampling different parts of the landscape. More data are needed in order to confirm the observed trends. However, building on the pioneering study by Litwin et al. (1998), our analysis confirms palynology as an additional source of evidence for better understanding of Morrison age vegetation and climate.