WARM-WET LATE JURASSIC COASTAL VEGETATION FROM WESTERN NORTH AMERICA

Two Late Jurassic macroflora from the Pacific region, originally described in the early 1900s, are reappraised. These flora are the Monte de Oro flora from northern California (late Oxfordian-early Kimmeridgian) and the Riddle flora from southwestern Oregon (late Tithonian). The preservation of floral elements and co-occurrence of marine invertebrates in the same formations suggest that the assemblages are parautochthonous in nature and from near-shore environments. Considering the paleogeographic configurations and stratigraphic positions, the two flora represent sediments from volcanic arcs along the Pacific subduction zone of western North America during the Late Jurassic.

Cycadophytes are dominant in both flora: Cycadales and Nilssoniales in the Monte de Oro flora and Bennettitales and Nilssoniales in the Riddle flora. High species diversity of macrophytic Ctenis (a cycadalean foliar genus with anastomosing veins) in the Monte de Oro flora is notable. The co-dominance of ginkgophytes together with cycadophytes in both flora is unusual. Scale-leaf conifers, which had global distribution in the low to mid-latitude during the Late Jurassic, are insignificant or absent. Ferns are not abundant; taxonomically, temperate osmundaceous and dicksoniaceous ferns are found together with more thermophilic schizaceous and dipteridaceous taxa. Pteridosperms are poorly represented, except for rare findings of Caytoniales in both floras and Ptilozamites in the Monte de Oro flora.

This floristic composition is significantly different from the Morrison flora, the only other known Late Jurassic flora of North America. The Morrison Formation is Kimmeridgian to early Tithonian in age, and is well known for its many iconic dinosaurs, including Allosaurus, Diplodocus, and Stegosaurus. The flora is rich in conifers and ferns, and is reconstructed as predominantly herbaceous, savannah-like vegetation. In contrast, the two flora from the Pacific coast represent warm and wet environments with higher precipitation than the inland Morrison flora. These flora provide important insights into plant community structures and phytogeography during the Late Jurassic.