TERRES

The invasion of the land by plants and animals (“terrestrialization”: Ordovician - Devonian) is one of the most significant evolutionary events in the history of life on Earth, and correlates in time with periods of major palaeoenvironmental perturbations. The development of a vegetation cover on the previously barren land surfaces impacted the global biogeochemical cycles as well as the geological processes of erosion and sediment transport. The terrestrialization process includes the rise of major new groups of animals, such as arthropods and tetrapods, the latter numbering some 24000 living species, including humans. Mass extinction and radiation events observed in the marine fossil record correlate significantly with bioevents recorded in the terrestrial realm, providing evidence of strong terrestrial-marine teleconnections. The evolution of early land plants also correlates with a dramatic decline in CO2 concentration in the atmosphere, testifying to a first-order disturbance in the global carbon cycle. The onset of the end-Devonian glaciation after a protracted period of “greenhouse” climatic conditions appears also to be causally linked to the invasion of the continents by land plants. The proposed research aims to a better understanding of the mechanisms of the origin of land plants, early terrestrial ecosystem evolution and the associated palaeoenvironmental consequences by means of interdisciplinary (palaeobiological and biogeochemical) analyses of fossil material and the host sediments. Microscopic spores occur abundantly in many types of sediments and are the only direct evidence of the earliest land plants. Their study will permit to track the first evolutionary steps of the parent plants, their biodiversification pathways, and their patterns of diffusion onto the continents. Acritarchs constitute the microfossil remains of photosynthetic oceanic plankton, the primary producers at the very base of the Palaeozoic trophic web. The acritarchs also include probably the ancestors of land plants, as transitional organisms between acquatic algae and fully terrestrial plants. Spores and acritarchs are often found associated in marine sedimentary rocks, thus allowing a direct correlation between bioevents occurring in the terrestrial flora, and in the oceanic plankton. These analyses can be completed by the palaeobiological study of land plant macrofossils of Devonian age and their biogeographical distribution. Moreover, plant macroremains carry invaluable information on morphology and structure of early land plants, and, consequently, on how they exploited/adapted to their environments. The origin of tetrapods is undoubtedly linked to the saturation by land plants of the ecospace corresponding to the fluvial and continental marginal environments. Biogeochemical analyses will be performed on concentrates and/or single-elements of organic-walled microfossils in order to ascertain variation in carbon isotopic composition. These data can be exploited to place constraints on carbon cycling and used as proxies for atmospheric CO2 concentration. The integration of the different datasets (palaeobiological, geochemical, sedimentological), will allow an improved understanding of the palaeobiological mechanisms of the terrestrialization processes and associated global change. Palaeobiological, palaeobiogeographical, and geochemical data will be integrated in numerical models of climate and biogeochemical cycles in order to gain further insight on the co-evolution of life and its environments during a period of profound ecological and environmental changes.