| Summary: | The Canadian High Arctic during the late Paleocene–early Eocene was home to an ecosystem composed of temperate and tropical flora and fauna. The fossil-bearing deposits on Ellesmere and Axel Heiberg islands contain the best of record of these, now extinct, high latitude ecosystems. These fossil flora are preserved in fluvial sediments indicative of a flood plain environment, with evidence of crevasse splay and deltaic deposits, interbedded with coal sequences, and rare layers of volcanic ash. The paleolatitude of these islands during the early Paleogene has been estimated at approximately ~75-80°N. These fossil floras have been placed stratigraphically in the upper Paleocene–lower Eocene, possibly occurring during or following one of the early Eocene hyperthermals (e.g. the Paleocene-Eocene Thermal Maximum or PETM, or the Eocene Thermal Maximum 2 or ETM2). Reported here is a comprehensive assessment of the character and composition of these now extinct high latitude forests. Paleoclimate is assessed using physiognomic analysis of 3 leaf megafloras (Split Lake, Stenkul Fiord and Strathcona Fiord), and nearest living relative (Bioclimatic Analysis) on the palynoflora from the Margaret Formation from Ellesmere Island. Physiognomic approaches correlate modern leaf morphology with modern climatic parameters in order to reconstruct paleoclimate from fossil leaf assemblages. Nearest living relative methods estimate climate from fossil floras by association to modern floras and utilizing the climatic envelopes of the modern plants. The results of these analyses indicate high summer precipitation (228-249 cm/yr) and warm mean annual temperatures (10-12 °C) in the Arctic during the early Eocene, which in part corroborates prior floristic, faunal, and isotopic early Eocene Arctic studies. Nevertheless, these results contradict a prior isotopic wood analysis that suggested seasonal precipitation or monsoonal conditions were a feature of these high latitude environments. Instead, present data are consistent with prior modeling studies that showed equable precipitation (i.e., precipitation distributed equally across the summer and winter seasons) for these northern ecosystems. The equable precipitation regime, distributed across the summer light season and winter dark season, would have required a permanent to semi-permanent polar cloud cap, which may have contributed to regional warmth in the Arctic during the early Eocene. In addition, a comprehensive morphotype catalogue of fossil plants is provided. This work utilizes multiple fossil localities from Ellesmere and Axel Heiberg islands, and forms a systematic framework that establishes an early Paleogene polar flora from High Arctic latitudes in Canada. A total of 62 ‘dicot’ angiosperm morphotypes, three monocotyledonous angiosperms, 13 gymnosperms, and five pteridophyte morphotypes are described and discussed in the context of the early Paleogene world. This work presents a significant contribution to the understanding of northern-polar biodiversity and environments during the warm greenhouse climate of the early Paleogene. Finally, reported here are the first quantitative megafloral diversity estimates from Stenkul Fiord, Ellesmere Island, Canada, utilizing two purpose-made census-sampled collections supported by horizon-specific palynological analysis. Recent U-Pb geochronology places the fossil collections stratigraphically near the PETM and ETM2 hyperthermal events of the early Eocene. The fossil megafloras and palynofloras were analyzed using interpolation/extrapolation coverage-based rarefaction analysis using iNEXT in the R program, and compared against modern forests. Results of these analyses demonstrate that the early Eocene paleoarctic supported diverse forests ecosystems with floral diversity similar to modern mid-latitude broadleaf forests from North America, but that diversity was restricted as a result of photic seasonality. Furthermore, evidence shows that these ecosystems underwent floristic change probably related to the transient hyperthermal events.
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