A tree without leaves

The puzzle presented by the famous stumps of Gilboa, New York, finds a solution in the discovery of two fossil specimens that allow the entire structure of these early trees to be reconstructed

Diversity of large woody lignophytes preceding the extinction of Archaeopteris: new data from the middle Tournaisian of Thuringia (Germany)

International audienceAnatomically preserved axes representing three lignophyte species occur in the middle Tournaisian deposit of Kahlleite in Thuringia. One is characterized by a small oval eustele, short uniseriate rays, and alternate distichous phyllotaxy. It is assigned to the progymnosperm genus Protopitys. The two others share a eustelic primary vascular system comprising a parenchymatous pith and numerous xylem strands in a peripheral position. The secondary xylem comprises rays that are mostly uniseriate and rarely exceed 20 cells in height. One is referred to as Eristophyton sp.; the second, characterized by ray cells showing a wide range of sizes and shapes is assigned to Aporoxylon primigenium. These records extend the stratigraphical range of Protopitys and Eristophyton down to the middle Tournaisian and confirm their great longevity through most of the Mississippian. They suggest that the diversity of putative arborescent lignophytes co-occurring with Archaeopteris around the D/C boundary but surviving successfully above this limit has been underestimated

Diversity patterns of the vascular plant group Zosterophyllopsida

International audienceZosterophylls were prominent components of the Early Devonian Eophytic flora. However, the diversity studies of early vascular plants have paid little attention to the particular dynamics of this group. To address this concern, we prepared a relational database management system at the species-level to explore the fossil record of the Zosterophyllopsida. From this, a set of diversity metrics and sampling-corrected (so-called residual diversity) curves were used to analyze the temporal and spatial dimensions of palaeodiversity. Results highlight that the diversity dynamics of the Zosterophyllopsida is characterized by a sustained increase of species diversity from the Silurian-Devonian boundary to the end-Pragian followed by a large depletion during the Emsian and a small recovery phase in the Eifelian, after which only minor fluctuations are observed until the end-Devonian. Residual diversity reveals that both the heyday and the main fall of diversity are not driven by sampling. In addition, the taxic composition of five putative palaeophytogeographic units, Laurussia, Siberia, northwestern Gondwana, Kazakhstan and northeastern Gondwana (i.e. Australia, China and the Shan-Thai block) was compared. The high level of endemicity shown by each unit confirms the phytogeographic differentiation and the occurrence of geographical barriers preventing massive floral exchanges between the corresponding regions for the Late Silurian-Early Devonian time interval. Statistical analyses were conducted on the three largest datasets, those corresponding to Laurussia, Siberia and northeastern Gondwana. New evidence indicates that the diversity dynamics of the group followed the same pattern in these regions, showing greatest diversity during the Pragian and sustained levels of extinction thereafter. By contrast, residual diversity reveals regional patterns. The radiation of the Zosterophyllopsida may have stopped earlier in northeastern Gondwana and Siberia than Laurussia. We propose that the onset of the extinctions resulted from the competitive replacement of the zosterophyllopsids by increasingly diversified lycopsids and basal euphyllophytes whose evolution would have been favoured by external factors, possibly the sea-level changes observed in the Pragian. This research was funded by Project ANR-2010-BLAN-607-02 "TERRES"

Paleontology in France: 200 years in the footsteps of Cuvier and Lamarck

International audienceDue to its richness in fossil localities and Fossil-Lagerstätten, France played a major role in the 18th and 19th centuries in establishing paleontology and biostratigraphy as scientific disciplines. The French naturalist and zoologist Cuvier (1769-1832) established the fields of comparative anatomy and paleontology, and proposed the concept of 'catastrophism' in geology. The naturalist Lamarck (1744-1829) is considered the founder of invertebrate paleontology and biostratigraphy and an early pioneer in the studies of evolution, developing the idea of 'transformism' and creating the word 'fossil', while his successor Blainville (1777-1850) was the first to use the word 'paleontology'. Based on this rich heritage, numerous French scientists strengthened paleontology as an important discipline during the 19th and 20th centuries. Paleontology was present at the universities of most major French cities, as documented by the rich collections in over 50 natural history museums and university collections. The most significant paleontological collection is that housed in the Muséum National d'Histoire Naturelle (MNHN) at Paris that currently hosts the largest research unit in paleontology of France with over 100 scientists, curators and technicians. The second largest collection (and the largest in terms of invertebrate fossils) is housed at the University of Lyon1, where the most important university paleontology research team is present. About 250 professional paleontologists are currently working in research units that are mostly affiliated to the Centre National de Recherche Scientifique (CNRS), in public or private museums, or in the numerous natural parcs. A significant generation change took place in the early 2000s, with the retirement of the paleontologists recruited in the 1960s and 1970s, that were often specialized in alpha-taxonomy and stratigraphy, and the arrival of a young generation of scientists that attempts to answer more 'modern' questions, such as global (climate) change, biodiversity, or evolution. This new generation of paleontologists faces modified funding schemes with project-based supporting structures in a more and more competitive environment. In the present paper we attempt to summarize the current situation of paleontology as a discipline in the very complex academic and scientific context of France. After a short overview on the history of French paleontology in the last centuries, a synopsis on institutions and funding agencies is presented briefly. The major research departments and their research themes are then described, together with the most important collections, the paleontological associations, journals, and databases, etc. Paleontological training possibilities and job opportunities, in particular in academia, are next documented, concluding with a summary of the prospects of the discipline

Investigating Devonian trees as geo-engineers of past climates: linking palaeosols to palaeobotany and experimental geobiology

We present the rationale for a cross-disciplinary investigation addressing the ‘Devonian plant hypothesis’ which proposes that the evolutionary appearance of trees with deep, complex rooting systems represents one of the major biotic feedbacks on geochemical carbon cycling during the Phanerozoic. According to this hypothesis, trees have dramatically enhanced mineral weathering driving an increased flux of Ca2+ to the oceans and, ultimately, a 90% decline in atmospheric CO2 levels through the Palaeozoic. Furthermore, experimental studies indicate a key role for arbuscular mycorrhizal fungi in soil–plant processes and especially in unlocking the limiting nutrient phosphorus in soil via Ca-phosphate dissolution mineral weathering. This suggests co-evolution of roots and symbiotic fungi since the Early Devonian could well have triggered positive feedbacks on weathering rates whereby root–fungal P release supports higher biomass forested ecosystems. Long-standing areas of uncertainty in this paradigm include the following: (1) limited fossil record documenting the origin and timeline of the evolution of tree-sized plants through the Devonian; and (2) the effects of the evolutionary advance of trees and their in situ rooting structures on palaeosol geochemistry. We are addressing these issues by integrating palaeobotanical studies with geochemical and mineralogical analyses of palaeosol sequences at selected sites across eastern North America with a particular focus on drill cores from Middle Devonian forests in Greene County, New York State

Bedrock erosion by root fracture and tree throw: A coupled biogeomorphic model to explore the humped soil production function and the persistence of hillslope soils

In 1877, G. K. Gilbert reasoned that bedrock erosion is maximized under an intermediate soil thickness and declines as soils become thinner or thicker. Subsequent analyses of this “humped” functional relationship proposed that thin soils are unstable and that perturbations in soil thickness would lead to runaway thinning or thickening of the soil. To explore this issue, we developed a numerical model that simulates the physical weathering of bedrock by root fracture and tree throw. The coupled biogeomorphic model combines data on conifer population dynamics, rootwad volumes, tree throw frequency, and soil creep from the Pacific Northwest (USA). Although not hardwired into the model, a humped relationship emerges between bedrock erosion and soil thickness. The magnitudes of the predicted bedrock erosion rates and their functional dependency on soil thickness are consistent with independent field measurements from a coniferous landscape in the region. Imposed perturbations of soil erosion during model runs demonstrate that where bedrock weathering is episodic and localized, hillslope soils do not exhibit runaway thinning or thickening. The pit-and-mound topography created by tree throw produces an uneven distribution of soil thicknesses across a hillslope; thus, although episodes of increased erosion can lead to temporary soil thinning and even the exposure of bedrock patches, local areas of thick soils remain. These soil patches provide habitat for trees and serve as nucleation points for renewed bedrock erosion and soil production. Model results also suggest that where tree throw is a dominant weathering process, the initial mantling of bedrock is not only a vertical process but also a lateral process: soil mounds created by tree throw flatten over time, spreading soil over bedrock surfaces

Considering river structure and stability in the light of evolution: Feedbacks between riparian vegetation and hydrogeomorphology

River ecological functioning can be conceptualized according to a four-dimensional framework, based on the responses of aquatic and riparian communities to hydrogeomorphic constraints along the longitudinal, transverse, vertical and temporal dimensions of rivers. Contemporary riparian vegetation responds to river dynamics at ecological timescales, but riparian vegetation, in one form or another, has existed on Earth since at least the Middle Ordovician (c. 450 Ma) and has been a significant controlling factor on river geomorphology since the late Silurian (c. 420 Ma). On such evolutionary timescales, plant adaptations to the fluvial environment and the subsequent effects of these adaptations on aspects of fluvial sediment and landform dynamics resulted in the emergence, from the Silurian to the Carboniferous, of a variety of contrasted fluvial biogeomorphic types where water flow, morphodynamics and vegetation interacted to different degrees. Here we identify several of these types and describe the consequences for biogeomorphic structure and stability (i.e. resistance and resilience), along the four river dimensions, of feedbacks between riparian plants and hydrogeomorphic processes on contrasting ecological and evolutionary timescales.This is the author's accepted manuscript and will be under embargo until the 18th of September 2015. The final version is available from Wiley at onlinelibrary.wiley.com/doi/10.1002/esp.3643/abstrac

Stepwise evolution of Paleozoic tracheophytes from South China: contrasting leaf disparity and taxic diversity

During the late Paleozoic, vascular land plants (tracheophytes) diversified into a remarkable variety of morpho- logical types, ranging from tiny, aphyllous, herbaceous forms to giant leafy trees. Leaf shape is a key determinant of both function and structural diversity of plants, but relatively little is known about the tempo and mode of leaf morphological diversification and its correlation with tracheophyte diversity and abiotic changes during this re- markable macroevolutionary event, the greening of the continents. We use the extensive record of Paleozoic tra- cheophytes from South China to explore models of morphological evolution in early land plants. Our findings suggest that tracheophyte leaf disparity and diversity were decoupled, and that they were under different selec- tive regimes. Two key phases in the evolution of South Chinese tracheophyte leaves can be recognized. In the first phase, from Devonian to Mississippian, taxic diversity increased substantially, as did leaf disparity, at the same time as they acquired novel features in their vascular systems, reproductive organs, and overall architecture. The second phase, through the Carboniferous–Permian transition, saw recovery of wetland communities in South China, associated with a further expansion of morphologies of simple leaves and an offset shift in morphospace occupation by compound leaves. Comparison with Euramerica suggests that the floras from South China were unique in several ways. The Late Devonian radiation of sphenophyllaleans contributed signif- icantly to the expansion of leaf morphospace, such that the evolution of large laminate leaves in this group oc- curred much earlier than those in Euramerica. The Pennsylvanian decrease in taxic richness had little effect on the disparity of compound leaves. Finally, the distribution in morphospace of the Permian pecopterids, gigantopterids, and equisetaleans occurred at the periphery of Carboniferous leaf morphospace