Land Plants of the Northern Rocky Mountains Before the Appearance of Flowering Plants

Abstract

Evidence of land plants before the appearance of flowering plants near the end of the Early Cretaceous is spotty. Seas covered much of the land during the Paleozoic and early Mesozoic, but certain areas were exposed, perhaps as islands. Evidence of Devonian vegetation from Beartooth Butte, Wyoming includes rhyniophytes, trimerophytes, zosterophylls, and lycopsids. Fossils from several different deposits show Late Mississippian to Pennsylvanian age vegetation like that of coal swamps to the east. The floras include lepidodendrids, calamites, ferns, seed ferns, and cordaites, with conifers present at some localities. Triassic and Early Jurassic land vegetation is poorly known in the region, as most strata are marine; but Late Jurassic sediments reflect the general emergence of the land concurrent with uplift along the Nevadan orogenic zone at the western edge of the present northern Rocky Mountains. The interior basin supported bryophytes, horsetails, cycads, caytonialeans, ginkgoes, and conifers, with bennettitaleans and ferns dominating. This was followed in the Early Cretaceous by similar flora but with the swamp vegetation dominated by conifers. These plants apparently grew under a generally arid but seasonally wet frost-free climate. It was into this environment that the first flowering plants of the region migrated. The object of this paper is to give an overview of the types of vegetation that occurred in the northern Rocky Mountain region before the appearance of flowering plants there. This region extends along the Rocky Mountains from central Colorado and Utah north into southern Alberta and British Columbia. The time span involved extends from the Early Devonian Siegenian, about