The Asselian palynoflora from the Autunian series (Muse oil-shale beds, Autun Basin, France): new insights on early Permian equatorial palaeoflora and palaeoenvironments

The Muse oil-shale beds (Muse OSB) of the Permian Muse Formation in the Autun Basin (France) have received attention for their exceptionally well-preserved flora and fauna throughout many decades. Recent CA-ID-TIMS U/Pb radiometric datings place the Muse section in the lower Asselian (~298.6 Ma), leading to a re-examination of the palynoflora using an alternative palynological method. Twelve productive samples provide a detailed taxonomic and quantitative evaluation of the palynoflora. The new data revealed now two distinct palynological assemblages, Muse-A in the lower part and Muse-B in the middle-upper part of the Muse section, assigned to a peat swamp forest environment dominated by cordaitalean conifers, alongside medullosalean cycads and callistophytalean seed ferns, with an undergrowth of lycophytes and ferns. Additionally, a forest dominated by walchian conifers (Voltziales) would be present but more distant from the lake. Vegetation dynamics between these associations are generally stable, except for an increase in Cordaitales at the expense of Voltziales. This variation, observed throughout the Autun Basin, seems to be affected by seasonal climate-driven changes. The co-occurrence in both assemblages of Permian xerophytic elements accompanied by Carboniferous wetland flora suggests a seasonally dry climate, supporting hygrophyte plant survival, persisting for at least 300 ky after the Carboniferous–Permian boundary. The phytoplankton community consisted of freshwater green algae, increasing towards the upper part of the section, coeval with increased explosive volcanic activity. Wind-transported volcanic ash likely provided nutrient input, enhancing lacustrine phytoplankton growth.