Vegetational change during the Middle-Late Pennsylvanian transition in western Pangaea

We present the first analysis of vegetational change in far western equatorial Pangaea (New Mexico, USA) during the Middle–Late Pennsylvanian transition (determined by conodonts and fusulinids) of the Late Paleozoic Ice Age. The study is based on the largest database assembled from this region: 28 of 44 quantitatively analysed floras from 14 of 26 stratigraphic levels. Most sampled floras are ‘mixed’, both below and above the boundary, including both hygromorphic and mesomorphic/xeromorphic taxa. The taxonomic data were recalibrated morphometrically focusing on foliar traits of lamina width and venation. All data were examined using stratigraphic credible intervals, capture–mark–recapture analyses, and resampling analyses. Results indicate no substantive taxonomic turnover across the boundary. This stands in marked contrast to patterns in mid-Pangaean coal basins where there is a large wetland vegetational turnover. However, plant and physical geological data indicate that immediately following the boundary in New Mexico, and for approximately half of the Missourian Stage, floras previously dominated by hygromorphs become overwhelmingly dominated by mesomorphic/xeromorphic taxa. Although expressed differently, the western Pangaean physical and palaeobotanical patterns parallel those from mid-Pangaean coal basins and suggest a widespread environmental change.