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A research program to study palaeoenvironmental changes during the PETM: The paleoflora study of Rivecourt and Le Quesnoy outcrops (Oise, France).
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International audience. Two main Paleogene lignite deposits are known from the Paris Basin. Rivecourt (Paleocene) and Le Quesnoy (Eocene) are temporally and spatially close and frame the Paleocene-Eocene boundary. These two localities show a high diversity of fruits and seeds. Preliminary studies highlighted that numerous families of plants were present in these two assemblages and show that there are differences in floristic composition. To date, paleocarpology investigations have been concentrated on Menispermaceae from the Le Quesnoy site. However, more comprehensive taxonomic comparisons between these two assemblages could yield new data and perspectives to understand the impact of global warming on the paleoflora. With these considerations, we conducted a study on Icacinaceae endocarps from both localities. These endocarps are numerous, diverse and exceptionally preserved. Icacinaceae are frequent in Paleogene lignite assemblages from Europe (mainly from the London Clay) and North America. Furthermore, the types of ornamentations are easily recognizable. We described five new morphotypes from Rivecourt site which all have a pair of subapical horn-like protrusions, a reticular ornamentation, and the vascular bundle embedded in endocarp. This latter character is synapomorphic of the Iodes genus while the presence of subapical horns is characteristic of some modern Asian Iodes. In contrast, we found only three morphotypes of Iodes from Le Quesnoy site with only one of them shared with Rivecourt. Nevertheless, the main morphotype from Le Quesnoy (about 70% of the specimens) has no subapical horns. Thus, this form may morphologically closer to the species currently in Africa. In this study we demonstrate abrupt changes in the composition of Icacinaceae across the Paleocene-Eocene boundary; in the Paleocene, the species show morphological features characteristic of some modern Asian species, whereas in the Eocene a mix of Asian and African morphological features is present. We suggest that these successional floras in the Paris Basin sites are probably linked to global warming and the expansion of the megathermal flora during the Eocene. This analysis is the first step of a broader comparative study of these two sites aiming to address the following questions: Do we find the same trend in other groups? Do we have a congruent pattern of affinities through time and space? Do we observe an increase or decrease of biodiversity in relation of global warming? In addition to fruits and seeds, woods and flowers have been found in both sites, including flowers embedded in amber from the Le Quesnoy site. The study of all fossil types will provide us with a more comprehensive understanding of the diversity of these Paleocene floras.
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