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The Horseshoe Canyon Formation in southern Alberta, Canada, preserves abundant plant fossils from the upper Campanian to upper Maastrichtian. However, studies describing the macroflora are taxonomically out of date, and limited work has been done to utilise the flora for palaeoclimatic reconstruction. Fifty-eight plant taxa are documented from the Morrin Bridge locality within the lower Maastrichtian Horsethief Member of the Horseshoe Canyon Formation. The assemblage is composed of gymnosperms, including diverse Cupressaceae, cycadophytes, and ginkgophytes, with rare ferns, and a relatively diverse suite of angiosperms, including dicot leaves of Platanaceae, Cercidiphyllaceae, Trochodendraceae, Betulaceae, and two monocots. Leaf physiognomy-based palaeoclimate reconstructions suggest a mean annual temperature of 12.4 ± 2.7 °C, warmest quarter temperature of 20.0 ± 2.5 °C, and coldest quarter temperature of 3.8 ± 3.2 °C, a mean annual precipitation of 1,282 (−405/+585) mm∙yr–1, relative humidity of 81.8 ± 9.3 %, and wettest month and driest month precipitation was 155 (−57/+90) mm and 71 (−39/+90) mm, respectively. This indicates a warm-temperate, humid environment, with modest temperature seasonality and minor precipitation seasonality. These results align with palaeosol data and sedimentological indicators of saturated floodplain conditions transitioning to more fluvial systems.
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