The Archaeopterid Forests of Lower Frasnian (Upper Devonian) Westernmost Laurentia: Biota and Depositional Environment of the Maywood Formation in Northern Wyoming as Reflected by Palynoflora, Macroflora, Fauna, and Sedimentology

Premise of research. The flora of the Maywood Formation, one of only three Devonian floras previously recognized in western North America, is known only from a brief report focused on stratigraphy and has never been characterized in more detail. A detailed assessment of this flora and associated animal fossils has implications for the age and depositional environments of the Maywood Formation and for Devonian plant biogeography. Methodology. Fieldwork at the Cottonwood Canyon (Wyoming) exposure of the Maywood Formation produced a measured section characterizing the sedimentology of the unit and samples that we analyzed for palyno-morph, macrofloral, and faunal content using standard methods. Pivotal results. The palynological assemblage is dominated by archaeopterid progymnosperm spores, lacks unequivocally marine components, indicates the low burial depth and temperature (ca. 537C) of the unit, and supports an early Frasnian age. Plant macro-and mesofossils including charcoal, adpressions, sporangia, and spore packages reflect a vegetation with quasi-monodominant archaeopterids but also including (unidentified) plants that produced the seed megaspore Spermasporites (for which the Cottonwood Canyon occurrence represents a geographic range extension). Scales indicate the presence of sarcopterygian and tetrapodomorph fishes. Sedimentary facies, palynofacies, and plant macrofossil taphonomy are consistent with a lagoon or lake margin environment on a carbonate platform disconnected from the open marine realm. Conclusions. The arid carbonate platform of the western margin of early Frasnian Laurentia hosted a fire-prone vegetation cover heavily dominated by archaeopterid progymnosperms. The Maywood Formation preserves fossil assemblages reflecting this vegetation at Cottonwood Canyon (Wyoming), in lagoonal or lacustrine deposits that also host microconchid tube worms and fish. The parent plant of the seed megaspore Spermasporites, present in this vegetation, was widely distributed all across Euramerica.