A Morphotype Catalogue, Floristic Analysis and Stratigraphic Description of the Aspen Shale Flora(Cretaceous–Albian) of Southwestern Wyoming

Abstract

We describe 28 fossil plant morphotypes from the Aspen Shale flora (Cretaceous: middle to late Albian) in southwestern Wyoming. This impression flora includes 6 ferns, 1 sphenopsid, 2 conifers, 17 dicotyledonous angiosperm (dicot) leaves and 2 dicot reproductive structures. The Aspen Shale megaflora is most similar to that of Subzone IIB of the Potomac Group of the eastern United States. Analysis of the Aspen Shale sedimentology and botanical composition shows occupation of open, paludal sites by a succession of progressively more complex plant communities. Like other middle Cretaceous floras, these data suggest that early angiosperms were weedy, herbaceous to shrubby, early successional competitors to ferns on open substrates. The description and illustration of the Aspen Shale morphotypes is presented as an example of how an entire flora can be described and analyzed before full taxonomic determinations have been made.