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"

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

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

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

Ovule et arborescence, deux innovations végétales majeures au Dévonien

International audienceL’origine de l’ovule, à partir des sporanges nus et déhiscents des groupes ancestraux est loin d’être résolue. Elle requiert une différenciation sexuelle des spores, un raccourcissement du cycle biologique ainsi que l’acquisition par les mégasporanges de caractères morphologiques liés à la capture du pollen et à leur protection. Le travail réalisé avec les paléobotanistes de l’Université de Liège sur Runcaria, un proto-ovule du Dévonien Moyen de Belgique, permet de proposer une hypothèse sur les modalités évolutive de cette innovation qui a permis aux végétaux terrestres de coloniser les milieux secs.L’arborescence est apparue au Dévonien Moyen chez les Cladoxylopsida, une classe proche des fougères. Si la morphologie externe des premières Cladoxylopsida arborescentes est bien documentée, leur structure interne qui renseigne sur les modalités de leur croissance et de leur fonctionnement est mal connue. L’analyse anatomique de spécimens récoltés dans le sud marocain a permis d’établir une stratégie de l’arborescence basée sur un développement important des tissus primaires et une contribution faible ou nulle des tissus secondaires. Ces données, couplées à l’analyse quantitative de spécimens des collections de l’Université de Liège, ont été utilisées pour simuler la croissance de Pseudosporochnus, une des Cladoxylopsida arborescentes les plus répandues. Ce travail a permis d’estimer la quantité de carbone accumulée dans cette plante au cours de son développement et dans une des premières forêts de l’histoire terrestre.Divers travaux ont popularisé l’idée que l’apparition des premières forêts aurait conduit à une baisse sensible du CO2 atmosphérique et à un refroidissement global à la fin du Dévonien. Des simulations réalisées avec le modèle GEOCLIM-Slave Jr. et faisant intervenir la distribution spatiale de types fonctionnels de plantes confirment la contribution des plantes à la réduction du CO2 atmosphérique. Elles montrent, cependant, que la réduction d’albedo résultant de l’évolution de la couverture végétale aurait entrainé un découplage du CO2 et des températures, ces dernières restant inchangées durant une grande partie du Dévonien

First ovules integument: what roles ?

International audienceThe Famennian is a period of increased taxonomic diversity for the first representatives of the major plant groups, especially the spermatophytes (i.e. the seed plants) that, today, represent the most abundant and diversified plant group on Earth [1,2]. Early spermatophytes evolved a unique ovule-centered reproductive syndrome that allowed them to germinate and grow rapidly in disturbed habitats and, secondarily, to colonize dry habitats unfavorable to the reproduction of their free-sporing predecessors [3,4] (Fig. 1). In these ovulate organisms, the female gametophyte grows and produces gametes inside the indehiscent megasporangium (nucellus) that differentiates a pollen chamber retaining the pollen grains in its apical part. An integument encloses the nucellus. Like the Runcaria protovule that preceded them by 20 million years, Famennian ovules were all surrounded by a cupule, except perhaps Spermolithus and Warsteinia [5–7]. The latter, however, have been found isolated and may also have had a cupule remaining on the plant that produced them

Premiers arbres. Le modèle Archaeopteris

Les premiers organismes végétaux autoportants et dépassant 2 m de haut sont apparus il y a un peu plus de 370 millions d’années, à la fin du Dévonien moyen. Trois groupes inventent l’arborescence de façon convergente à cette époque : les Lycopsida, les Cladoxylopsida et les progymnospermes. Tous partagent une reproduction à partir de spores et un mode de ramification terminale, impliquant la division égale ou inégale des apex. Archaeopteris, chez les progymnospermes, regroupe les arbres les plus hauts (40 m de hauteur maximale estimée) et les plus répandus à la surface du globe à la fin du Dévonien. Il se démarque par un certain nombre d’innovations telles que l’hétérosporie, la possession de feuilles et celle de bois assurant la double fonction de support et de conduction. Des analyses récentes indiquent qu’Archaeopteris avait, de plus, inventé un syndrome de ramification latérale comparable à la ramification axillaire des plantes à graines dont il est le plus proche parent connu actuellement. Certaines particularités anatomiques du système vasculaire suggère qu’il pouvait se bouturer ou se marcotter. Cet ensemble de caractères « modernes » pourraient expliquer le succès d’Archaeopteris jusqu’à la limite Dévonien/Carbonifère, son extinction coïncidant avec la radiation des premières plantes à graines