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The Icacinaceae from the Palaeogene of the Paris Basin: diversity, affinities and changes through the PETM

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Rio, Cédric Del | Franceschi, Dario De

Edité par CCSD -

International audience. The Icacinaceae Miers is a family of tree, shrubs and lianas with an extant pantropical distribution. This group is well known in the fossil record from Paleocene and Eocene, particularly in Northern Hemisphere, although fossil evidence from Southern Hemisphere has recently been increased. In the Paris Basin, Icacinaceae are represented by numerous fossil endocarps from five main localities. Among these, two sites in Oise, Rivecourt (late Thanetian) and Le Quesnoy (early Ypresian) are geographically and temporally very close. The Paleocene-Eocene thermal maximum (PETM) event occurs (temporally) between these two fossil outcrops. Thus, the study of the Icacinaceae fossil in those localities provides a good opportunity to elucidate the impact of the PETM on this family in the Paris Basin. Based on 266 fossil specimens attributed to Icacinaceae, from the five Paris Basin localities, we conducted detailed anatomical (using SEM) and morphological studies. The fossil remains are mainly lignitic endocarp, but other endocarp preservation as quartzite - sandstone imprints, as well as flower and pollen preserved in amber has been studied. The study of the fossils from the Paris Basin allows to distinguish twelve species from only one extant genus (Iodes) and three extinct genera (Palaeophytocrene Reid and Chandler, Icacinanthium Del Rio & De Franceschi and Icacinicarytes Pigg, Manchester & Devore). This diversity within the Paris Basin is comparable to other major Palaeogene outcrops as the London Clay or the Clarno Formation. All species are related to a tropical climber groups. The climbers are abundant in the riverbanks or channels and thus the representation of only climbers of this family could be a taphonomic bias. In this context, the overall ancient Icacinaceae diversity could be underestimated. Among Icacinaceae fossil from the Paris Basin, there are some evidences of affinities with extant Indo-Malaysian vegetation, which seems to be a relict area of wider Paleocene and Eocene Eurasian vegetation. No clear extinction is shown through the PETM, but a floral enrichment across the PETM is demonstrated in particular with a new Iodes form at early Eocene, possibly part of incoming new floral assemblages arrived together with new faunal elements.

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