NEW OBSERVATIONS ON THE EARLY DEVONIAN FLORAS OF THE BEARTOOTH BUTTE FORMATION (WYOMING)

The Beartooth Butte Formation (Pragian-Emsian, 410-400 Ma) contains the only extensive Early Devonian plant assemblages in western North America. The unit is discontinuous and consists of dolomitic channel fill deposits interpreted as estuarine or fluvial. Plant fossils occur at two localities in northern Wyoming forming allochthonous assemblages that also include fish and eurypterid remains. The plant material is preserved as coalified compressions (sometimes highly oxidized) or impressions and is characterized by a relatively high degree of fragmentation. Previous studies of the Beartooth Butte Formation flora have revealed considerable diversity (including Renalia, Sawdonia, Gosslingia, Rebuchia, Drepanophycus, Leclercqia, Psilophyton and Sphondylophyton), but many of the taxa have not been published formally and some need reevaluation. An ongoing reexamination of the flora, aimed at filling this gap, is adding to the known diversity. Among the new findings are specimens with slender axes (<1mm) bearing lateral stalked sporangia at irregular intervals. Several small specimens (<1 cm) display a rosette-like habit with branching appendages radiating from a central point and reminiscent of bryophyte gametophytes. The assemblages also contain small, delicate fossils with slender isotomously branching axes and branched sporangia, as well as minute cooksonioid specimens. Our findings emphasize the need for thorough re-examination of this poorly characterized fossil flora, which will add to knowledge of Early Devonian plant diversity and biogeography.