Une explication morphogénétique de la loi de covariation de Buckman

Nous montrons ici que les cas d’extrême variabilité découverts au 19 siècle et décrits chez les ammonites comme résultant de la loi de covariation de Buckman [1887] dépendent essentiellement de la géométrie interne des coquilles de ces organismes. Cette géométrie peut être caractérisée en termes de degré de courbure ventrale et latérale et ces paramètres contrôlent la concentration de morphogènes présents dans l’épithelium qui sécrète la coquille. Les ornements les plus saillants sont présents là où les tours sont les plus incurvés et les régions plates de la coquille sont presque lisses. Ces observations sont testées ici dans le cadre des modèles de réaction-diffusion développés par Meinhardt (1995)

Diversité des familles de radiolaires au cours du temps

L’examen de la biodiversité des radiolaires, au niveau de la famille au cours du Phanérozoïque révèle quelques tendances générales connues chez d’autres groupes d’organismes, surtout dans le plancton, alors que d’autres tendances leur sont particulières. La crise permo-triasique, l’une des plus importantes dans l’évolution des organismes marins, est marquée chez les radiolaires par l’extinction de deux familles (Albaillellaria et Latentifistularia) vers la fin du Permien, mais surtout par une énorme diversification des spumellaires et nassellaires au Trias inférieur et moyen. La diversité des radiolaires s’accroît du Cambrien au Jurassique, reste relativement stable au Crétacé et décroît légèremen

Sea-level changes and ammonite faunal turnover during the Lias/Dogger transition in the western Tethys

The aim of this paper was to investigate the possible connections between ammonite faunal turnover and the eustatic events recorded in Tethyan sequences during the middle Toarcian/early Bajocian time interval. For this we have analysed the biostratigraphic ranges, at the subzone level, of approximately 600 ammonite species belonging to 160 genera from several selected sections of the western Tethys (Mediterranean and Submediterranean provinces). The analysis of taxon ranges enabled us to plot curves for ammonite faunal turnovers, inter-subzonal distance, and diversity. Comparing the mentioned curves with Tethyan sequences [Hardenbol et al., 19981, we find that sea-level changes correlate well with origination and extinction events and faunal diversity. Most of the faunal turnovers correlate with stratigraphic events. Extinction events with their corresponding decrease in diversity correlate with regressive intervals and with major or minor sequence boundaries. Origination events and their corresponding increase in diversity were clearly connected with transgressions in Tethyan sequences. In several cases, the major sequence boundary and the subsequent transgressive phase correlate with major ammonite faunal turnover, whereas minor or medium sequence boundaries generally gave rise to minor or medium turnovers

Preliminary work on the compression behaviour of canola straw to high density products

In the United Kingdom (UK), the total area of canola harvested increased between 2000 and 2008 from 332,000 ha to 598,000 ha, respectively. Currently there is not a significant market for canola straw in the UK, and consequently development of a market for canola straw would add value to the gross margin of the crop at farm level. As a biomass waste product, canola straw could be used as a fuel to generate heat through combustion. However, straw exhibits a low bulk density which makes its transport more expensive than the transfer of natural gas or petroleum. Reducing the cost of collection, transport and storage of biomass through densification is thus critical to developing a sustainable infrastructure capable of working with significant quantities of raw material. This paper focuses on a preliminary study of the fundamental behaviour of canola straw under compression in closed cylindrical dies, in order to design efficient equipment for compression of biomass to solve the biomass collection, transportation and storage problem. To fulfill the aim of this paper, two main objectives were studied: 1) Definition of the pressure-density curve of the compression of canola straw and 2) Analysis of the effect of applied pressure on the measured die density and the relaxed densities and specific energies required to produce the wafers. As a conclusion of this preliminary work it has been demonstrated that canola straw can be used to produce high density products without the need of adding binders or lubricants. The specific energy required to produce the wafers, as well as the final density of the wafers, was affected by the applied pressure. Specific energies to produce 50 mm diameter wafers varied from 15.0 to 57.2 MJ t-1, depending on the pressure applied