De la nécessité du MEB en paléopalynologie

Une des faiblesses de l’étude des pollens la plus fréquemment citées est la faible résolution taxonomique obtenue par microscopie à lumière transmise (MLT). Cette difficulté empêche l’établissement d’une interprétation paléoécologique détaillée. Le MEB (microscope électronique à balayage) est un outil à la disposition des scientifiques depuis presque 40 ans, mais il est malgré tout rarement utilisé en routine pour les recherches en paléopalynologie. La principale raison invoquée en est que la technique du grain par grain consomme beaucoup de temps, ce qui n’est pas forcément le cas. En combinant les techniques MLT et MEB, les grains de pollens fossiles peuvent être identifiés avec plus d’exactitude, et il devient possible, en outre, de distinguer des taxons similaires, mais botaniquement distincts. Dans ce sens, la paléopalynologie peut fournir aux phylogénéticiens et paléoclimatologistes une pléthore de données utiles.One of the commonly quoted weaknesses of pollen analysis is its poor taxonomic resolution, which can be achieved with the Light Microscope (LM). This prevents detailed palaeoecological interpretations from being made. Although the Scanning Electron Microscope (SEM) has been widely available for almost 40 years, it is rarely used in routine palaeopalynological research. The usual reason given is that single-grain techniques are too time-consuming. However, this need not be the case. By combining LM and SEM, fossil pollen grains can be identified more accurately. Moreover, it is possible to distinguish between similar, but botanically distinct, taxa. In this way, palaeopalynology can supply phylogeneticists and palaeoclimatologists with a plethora of useful data.</p

Menatanthus mosbruggeri gen. nov. et sp. nov. – A flower with in situ pollen tetrads from the Paleocene maar lake of Menat (Puy-de-Dôme, France)

The Paleocene Fossil-Lagerstätte Menat in France is well known for its wealth of outstandingly well preserved fossil insects and plants. Despite being known for more than a century, the palaeoflora, which is regarded as typical for the late Thanetian by some authors, has largely been neglected since the 1940s. New excavations and surveys yielded exceptionally well-preserved plant material, including a minute, heptamerous flower bearing in situ pollen tetrads, comparable to tetrads of the modern ericacean genus Kalmia L, in its anthers. The only known modern ericacean genus which is characterised by heptamerous flowers is Bejaria Mutis ex L., a basal relative of the tribe Phyllodoceae within Ericaceae, which also includes the genus Kalmia in a relatively basal position. However, heptamerous flowers also occur very rarely (mostly interpreted as teratologies) in a number of other modern Ericaceae, but also in various other modern angiosperm families. Due to the unique combination of a heptamerous flower with Kalmia-type pollen tetrads within the anthers, the new taxon Menatanthus mosbruggeri gen. nov. et sp. nov. is erected. The lack of morphological data from the flower itself and the fact that comparable pollen tetrads can be produced by a number of modern families, however, prevent an assignment of the new taxon to any known angiosperm family.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Alexander von Humboldt-Stiftung http://dx.doi.org/10.13039/10000515

A silicified wood from the Early Cretaceous sediments in the Kaligandaki Valley, west central Nepal

A silicified wood has been discovered from the Tethyan Cretaceous (Berriasian) deposits belonging to the Kagbeni Formation of north Central Nepal. The wood exhibits anatomical features which are well in accordance with Araucarioxylon nepalense described by Barale et al. (1976) from another locality in the Kagbeni Formation near Kagbeni in the Thakkhola Valley in Central Nepal. It is a pycnoxylic wood with mostly uniseriate and rarely biseriate bordered pits on radial tracheid walls. According to recent taxonomic opinions this type of wood should not be treated as Araucarioxylon, but as Agathoxylon Hartig. Thus we propose the name Agathoxylon nepalense comb. nov. for this type of wood. The sandstones of the Kagbeni Formation have been interpreted as delta-deposits, with a major flow direction from the south. This suggests that the wood originated from the northern margin of Indian sub-continent

Leaf physiognomy records the Miocene intensification of the South Asia Monsoon

Our understanding regarding the onset and development of the modern South Asia monsoon (SAM) is still incomplete due to its complex nature and differing views about its relationship with major orographic features such as the Himalaya and Tibetan Plateau. Climate data derived from some terrestrial and marine sediments from the Neogene suggests the onset and intensification of the SAM to a near-modern state occurred during the Miocene, while modelling and other terrestrial proxies point to a much earlier origin for the proto-East Asia monsoon (EAM) and proto-SAM. Angiosperm leaves, particularly dicot leaves, provide a good indication of prevailing climatic conditions as a result of key adaptations in their leaf structure. Here we use Climate Leaf Analysis Multivariate Program (CLAMP) in conjunction with general circulation models to decode the Lower (~13–11 Ma) and Middle (9.5–6.8 Ma) Siwalik climate signal inherent in the physiognomy of fossil leaves. The reconstructed climate data indicates that the Middle Siwalik was warmer and wetter than the Lower Siwalik, particularly in the cooler part the year. The leaf physiognomy of Lower and Middle Siwalik assemblages is indistinguishable to that of the modern leaf assemblages, which are influenced by today's SAM climate. This shows that the SAM was already well established as an independent domain during the late middle Miocene (~13–11 Ma) and has remained nearly the same from the perspective of leaf adaptations since then. A quantitative monsoon intensity index indicates an intensified monsoon during the late Miocene (9.5–6.8 Ma), a finding replicated by climate modelling