DISPERSED PALYNOMORPH AND CUTICLE ASSEMBLAGES FROM THE LATE JURASSIC MORRISON FORMATION, SOUTHEASTERN UTAH: FLORISTIC AND PALEOCLIMATE IMPLICATIONS

The Late Jurassic Morrison Formation is world renowned for its rich vertebrate fossils, but the plants remain poorly understood. We report here on two palynomorph assemblages from the Brushy Basin Member located east of Bears Ears National Monument, southeastern Utah. One site is located south of Black Mesa and was collected from a bed that grades into a horizon with an ash dated at 150.67 +- 0.32 Ma. The second site, located north of Black Mesa and near the top of the member, is exceptional in including within a 6 meter section an ash bed with pristine zircons, reasonably well preserved palynomorphs and cuticles, plant megafossils, including the ginkgophyte Czekanowskia, ferns and conifers, and metazoans. Plant megafossils are rare throughout the Morrison Formation, and such a diverse assemblage of terrestrial fossils combined with a datable ash is extremely unusual if not unique. The other fossils and ash are under study and will be described in a future report. Dispersed cuticles require more study, but are clearly coniferous. The enigmatic pollen form Exesipollenites dominates both assemblages (64-69% of 300 grains counted). Exesipollenites was likely produced by a wind-pollinated tree, possibly Cupressaceae. Definite Cupressaceae is represented by Perinopollenites (3-7%). Diverse saccate pollen comprises much of the remaining samples (13-15%), representing Pinaceae, ?Podocarpaceae, and ?Voltziales. Spores of ferns and lycophytes display low abundance (3-5%) and diversity. Rare but floristically significant elements include Cycadopites (=Czekanowskia, Ginkgoaceae or Bennettitales), Ephedripites (= Gnetales) and Vitreisporites pallidus (Caytonia). Paleoclimate signals from these assemblages are mixed. Exesipollenites is frequently co-dominant with the xerophytic conifer Classopollis, but in these two samples the latter is rare. On the other hand, Araucariaceae, usually co-dominant in the Brushy Basin Member and indicating more mesic conditions, is rare here. Low diversity and abundance of ferns also suggests drier conditions. Overall, these assemblages appear intermediate between New Mexico, at the southern extend of the Morrison, and those from northern Utah, Colorado and Wyoming, reinforcing the concept of a latitudinal gradient of increasing southward aridity in the Morrison.