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Detailed palaeontologic and taphonomic techniques to reconstruct an earliest Paleocene fossil flora: An example from southwestern North Dakota, USA

Abstract : A stratigraphic section in the basal Fort Union Formation (Paleocene) in southwestern North Dakota was used to study in detail the post-crisis recovery as well as to reconstruct the local environment and its evolution using sedimentology, palynology, fossil floras and vertebrate data. This report will focus on the flora from this site, corresponding to the first appearance data for Paleocene floral recovery, just above the Cretaceous­Tertiary (K/T) Boundary. The studied flora consist of an assemblage of tightly stacked leaves preserved as carbon imprints (also called leaf mats), a preservation condition that makes the extraction of each individual leaf difficult to achieve directly on site. As a result, a new technique was tested, allowing the study of every leaf preserved, their sedimentological context as well as their position relative to each other. A large block of matrix including the leaf mat was jacketed in plaster and was taken to the museum laboratory for analysis under controlled conditions. Preparation consisted of removing sediments at a millimeter scale and recording of placement and orientation of all fossil materials for three-dimensional reconstructions. Using this technique, a description and census of more than 300 leaf specimens was possible within an area of only 0.5 m2. The general sedimentological context indicates that the leaves were deposited in a near-stream environment associated with short-term flood events. Detailed information on depositional environment was gathered both by cutting a stratigraphic column from the Hell Creek/Fort Union formational contact up through the basal 4 m of the Fort Union Formation and by studying sediments and leaf preservation mode in detail within the leaf mat. Significant changes in taxonomic abundances correlated with different lithologies was observed, and a leaf species new to the study area was reported. The new methodology proves to be an efficient way to recover additional taphonomic and paleoenvironmental information from leaf mats necessary to understand the depositional dynamics of a fossiliferous leaf site, as well as to improve the record of taxonomic census. In a biostratigraphical prospective, the specimens recovered represent a low-diversity Fort Union flora composed exclusively of dicots that do not exist in the Hell Creek Formation. Preliminary palynological analysis reveal a Cretaceous age for the entire stratigraphic section, implying that the studied leaf mat is part of the FU0 megafloral zone (as defined by the occurrence of a Fort Union flora with Cretaceous palynomorphs). However, this Cretaceous age attribution for the entire section is questioned due to the occurrence of Paranymphaea crassifolia (part of the Paleocene FUI megafloral zone) within another leaf mat located 266 cm above the base of the coal representing the formational contact, and the occurrence of Paleocene PU1 mammals, reported from sediment within the leaf block interval.
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Antoine Bercovici, Jacqueline Wood, Dean Pearson. Detailed palaeontologic and taphonomic techniques to reconstruct an earliest Paleocene fossil flora: An example from southwestern North Dakota, USA. Review of Palaeobotany and Palynology, Elsevier, 2008, 151 (3-4), pp.136-146. ⟨10.1016/j.revpalbo.2008.03.004⟩. ⟨insu-00372010⟩



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