Abstract :
[en] The Lower Cretaceous is a period during which several groups of embryophytes still present in current ecosystems appear in the fossil record and/or diversify, such as Pinaceae, Cupressaceae and angiosperms. At the beginning of the Lower Cretaceous, gymnosperms still constituted the main component of arborescent strata (Gynkgoales, Cycadales). The Pinaceae, the most diverse conifer family today, diversified during the Early Cretaceous period, and will likely reach its maximum diversity during the Late Cretaceous. This diversification is mainly revealed by the numerous ovulated cones (female reproductive organ), found in the sedimentary deposits of the Cretaceous of Europe. In the Cretaceous, 27 species were described, including 9 in England, 8 in France and 10 in Belgium. The latter come from different historical localities where Wealden facies sediments outcrop, dated from the Barremian to the Albian (129.4 ─ 100.5 Ma). This unique fossil record is distinguished by its abundance (640 fossil cones and 2114 isolated scales), its quality of preservation (the cones are lignitized and perfectly preserved anatomically) and its diversity, which makes it one of the most exceptional collections for Lower Cretaceous Pinaceae. This material is therefore regularly cited or used in the literature, whether for systematic studies or phylogenetic studies. However, since these species were mainly studied in the first half of the 20th century, they were described according to the species concepts of the time, concepts that have now been widely revised and questioned. Revising all of this material has proved necessary in order to study the evolutionary dynamics of the group, in particular by contextualizing it with the diversification of angiosperms. The revision of this material will allow to refine the understanding of the evolution and the diversification of these first Pinaceae during a key period of their evolutionary history, but also to provide updated data for future analyzes on the evolution of this family. The taxonomic review of this material combines anatomical descriptions and linear and geometric morphometric analyzes in order to characterize the external form but also nano- and micro-X-ray tomography in order to characterize the internal structures of these fossil objects in a non-invasive way. In order to better understand and delimit the concept of morphotaxon for this type of fossil object, linear and geometric morphometric analyzes were used to quantify the intra- and interspecific morphological disparity of female cones of fossil and current Pinaceae. This work has made it possible to quantify the maximum shape variation of fossil species, and to compare it to the variation in intraspecific shape of the cones of current Pinaceae. The shape variability of the fossil ovulated cones is not different from that of the current Pinaceae, and this from the beginning of their diversification; moreover, I show that the intraspecific disparity can be reasonably quantified as soon as about fifteen individuals are analyzed. This opens many perspectives in the quantification of the diversity and disparity of fossil assemblages. Based on these new morphometric proxies, the diversity of Belgian Pinaceae has been revised. This demonstrates that the significant intra-specific variability observed in certain historically described morphotaxa (e.g., Pityostrobus andraei (COEMANS) SEWARD) was overestimated and several morphotypes representing different species were grouped together. In fine, this revision highlights an underestimation of the diversity of Belgian Pinaceae in the Lower Cretaceous. The richness of this fossil record does not stop only at the specific diversity since the quality of preservation also allows the description of traces of interactions between Pinaceae and animals, as well as the description of differential taphonomic processes according to the locality of origin. Following these reinterpretations, ten Belgian taxa are re-encoded in the morphological matrix of Pinaceae. A character on the shape of the scales of ovulate cones is modified, making it possible to code with more precision the external morphology of the cones. An updated phylogeny of fossil and current Pinaceae is proposed. The question of the origin of the Pinaceae family is also addressed with the fossil forms belonging to the genus Schizolepidopsis, of which a new specimen is described in the Lower Jurassic of Belgium. This thesis work demonstrates that the diversity of Pinaceae in Western Europe is greater than that previously described, and further supports an important diversification of Pinaceae during the Lower Cretaceous. This work underlines the importance of revising these taxa described in the 20th century, and invites to produce a similar work on the other taxa of Pinaceae of Europe of the Lower Cretaceous (in particular in France and in England) in order to bring new important information during this key period of their evolution.
