Ocean circulation in the Toarcian (Early Jurassic), a key control on deoxygenation and carbon burial on the European Shelf

The Toarcian Oceanic Anoxic Event (T-OAE, ∼183 My) was a long-lasting episode of ocean deoxygenation during the Early Jurassic. The event is related to a period of global warming and characterized by major perturbations to the hydrological and carbon cycles with high rates of organic matter burial in shelf seas. Ocean circulation during the Toarcian and its influence on marine biogeochemical cycles are still not fully understood. Here,we assess the spatial extent of anoxia in the NW Tethys Ocean during the T-OAE, the relationship with ocean circulation and the impact on organic carbon burial, using new and existing sedimentary records from the European Epicontinental Shelf (EES) in combination with general circulation model results. We demonstrate that bottom waters on the southwestern part of the shelf were mainly oxic during the T-OAE, while those in the northeastern basins were mostly anoxic or even sulfidic. Results for two ocean-atmosphere models (FOAM and MITgcm) suggest the presence of a strong clockwise gyre over the EES, which brought oxygenated equatorial waters from the Tethys Ocean to the southern shelf. The northward limb of the gyre was significantly weakened due to the rough bathymetry of the northern shelf, making this relative small region highly sensitive to local ocean stratification. These sluggish ocean dynamics promoted bottom water anoxia and enhanced burial of organic carbon in the northeastern basins, which accounted for 3–5% of the total carbon extracted from the ocean-atmosphere system as recorded by the positive carbon isotope shift

Early Cambrian fuxianhuiids from China reveal origin of the gnathobasic protopodite in euarthropods

Euarthropods owe their evolutionary and ecological success to the morphological plasticity of their appendages. Although this variability is partly expressed in the specialization of the protopodite for a feeding function in the post-deutocerebral limbs, the origin of the former structure among Cambrian representatives remains uncertain. Here, we describe Alacaris mirabilis gen. et sp. nov. from the early Cambrian Xiaoshiba Lagerstätte in China, which reveals the proximal organization of fuxianhuiid appendages in exceptional detail. Proximally, the post-deutocerebral limbs possess an antero-posteriorly compressed protopodite with robust spines. The protopodite is attached to an endopod with more than a dozen podomeres, and an oval flap-shaped exopod. The gnathal edges of the protopodites form an axial food groove along the ventral side of the body, indicating a predatory/scavenging autecology. A cladistic analysis indicates that the fuxianhuiid protopodite represents the phylogenetically earliest occurrence of substantial proximal differentiation within stem-group Euarthropoda illuminating the origin of gnathobasic feeding

Reconstructing the diet of a 505-million-year-old arthropod: Sidneyia inexpectans from the Burgess Shale fauna.

21 pagesInternational audienceThe feeding ecology of the 505-million-year-old arthropod Sidneyia inexpectans from the middle Cambrian (Series 3, Stage 5) Burgess Shale fauna (British Columbia, Canada) is revealed by three lines of evidence: the structure of its digestive system, the fossilized contents of its gut and the functional anatomy of its appendages. The digestive tract of Sidneyia is straight, tubular and relatively narrow in the trunk region. It is enlarged into a pear-shaped area in the cephalic region and stretches notably to form a large pocket in the abdomen. The mouth is ventral, posteriorly directed and leads to the midgut via a short tubular structure interpreted as the oesophagus. Anteriorly, three pairs of glands with internal, branching tubular structures open into the digestive tract. These glands have equivalents in various Cambrian arthropod taxa (e.g. naraoiids) and modern arthropods. Their primary function was most likely to digest and assimilate food. The abdominal pocket of Sidneyia concentrates undigested skeletal elements and various residues. It is interpreted here as the functional analogue of the stercoral pocket of some extant terrestrial arachnids (e.g. Araneae, Solifugae), whose primary function is to store food residuals and excretory material until defecation. Analysis of the gut contents indicates that Sidneyia fed largely on small ptychopariid trilobites, brachiopods, possibly agnostids, worms and other undetermined animals. Sidneyia was primarily a durophagous carnivore with predatory and/or scavenging habits, feeding on small invertebrates that lived at the water-sediment interface. There is no evidence for selective feeding. Its food items (e.g. living prey or dead material) were grasped and manipulated ventrally by its anterior appendages, then macerated into ingestible fragments and conveyed to the mouth via the converging action of strong molar-like gnathobases. Digestion probably took place within the anterior midgut via enzymes secreted in the glands. Residues were transported through the digestive tract into the abdominal pocket. The storage of faeces suggests infrequent feeding. The early diagenetic three-dimensional preservation of the digestive glands and abdominal pocket may be due to the capacity of Sidneyia to store Phosphorus and Calcium (e.g. spherites) in its digestive tissues during life as do, for example, modern horseshoe crabs

Truncated bimodal latitudinal diversity gradient in early Paleozoic phytoplankton

International audienceThe latitudinal diversity gradient (LDG)-the decline in species richness from the equator to the poles-is classically considered as the most pervasive macroecological pattern on Earth, but the timing of its establishment, its ubiquity in the geological past, and explanatory mechanisms remain uncertain. By combining empirical and modeling approaches, we show that the first representatives of marine phytoplankton exhibited an LDG from the beginning of the Cambrian, when most major phyla appeared. However, this LDG showed a single peak of diversity centered on the Southern Hemisphere, in contrast to the equatorial peak classically observed for most modern taxa. We find that this LDG most likely corresponds to a truncated bimodal gradient, which probably results from an uneven sediment preservation, smaller sampling effort, and/or lower initial diversity in the Northern Hemisphere. Variation of the documented LDG through time resulted primarily from fluctuations in annual sea-surface temperature and long-term climate changes

Phylogenetic hypothesis for early Pliensbachian ammonites

Phylogenetic hypothesis for early Pliensbachian ammonites of the western Tethys provided in parenthetical format for the entire dataset and for each subdataset analyzed in this stud

Supplementary table 3. Results of GEE models testing the relationship between species latitudinal midpoint and range size for species with at least two occurrences

Results of GEE models testing the relationship between species latitudinal midpoint and range size for species with at least two occurrence

Supplementary table 1. Datasets analyzed to test for the existence of a Rapoport effect in early Pliensbachian ammonites of the western Tethys

Datasets analyzed to test for the existence of a Rapoport effect in early Pliensbachian ammonites (first Excel sheet), and for the three chronozones of this substage (following Excel sheets

Supplementary table 2. Results of GEE models testing the relationship between species latitudinal midpoint and range size for trans-province species and for NWE species without Beaniceras luridum

Results of GEE models testing the relationship between species latitudinal midpoint and range size for trans-province species and for NWE species without Beaniceras luridu

Phylogenetic conservatism of species range size is the combined outcome of phylogeny and environmental stability.

12 pagesInternational audienceAimTo test for the phylogenetic conservatism of geographic range size and to explore the effect of the environment on this potential conservatism.LocationThe western Tethys Ocean and its surroundings (present-day Europe, the Middle East and North Africa) during the early Pliensbachian (Early Jurassic).MethodsUsing 104 localities and 1,765 occurrences of ammonite species, we estimated geographic range sizes using the extent of occurrence and the latitudinal range. The phylogenetic conservatism of range sizes was tested using Moran's I index which measures phylogenetic autocorrelation, and Pagel's λ which indicates whether a phylogeny correctly predicts covariance patterns among taxa on a given trait according to a Brownian evolution model. We conducted these analyses for two neighbouring provinces with contrasting environmental features (Mediterranean and Northwest European). We also explored scale effects by considering the whole western Tethys and two temporal resolutions (chronozone and sub-stage).ResultsA marked difference in phylogenetic signal is observed between Mediterranean (MED) and Northwest European (NWE) species; the range size of MED species is more frequently phylogenetically conserved than that of NWE species. No phylogenetic conservatism of species range size is observed during the last chronozone of the early Pliensbachian which is characterized by numerous palaeoclimatic and palaeoenvironmental changes.Main conclusionsSpecies range size may be partly determined by phylogeny, but this phylogenetic conservatism is modulated by spatio-temporal environmental stability. The phylogenetic signal of species range size may be labile through time within the same lineage and may differ between contemporaneous species of the same group. This lability stems from the fact that species range size results from a complex interplay of intrinsic and extrinsic factors

Supplementary figure 1. Species-level phylogenetic hypothesis for ammonites of the western Tethys, for the entire early Pliensbachian and for each chronozone of this substage

Species-level phylogenetic hypothesis for early Pliensbachian ammonites (214 sp; after Hardy et al. 2012) together with their province(s) of occurrence (A); and subtrees for species of the Jamesoni (B; 132 sp), Ibex (C; 103 sp) and Davoei (D; 45 sp) chronozones. Inter-species phylogenetic distances correspond to branch lengths between each species pair in the phylogeny. Abbreviations: MED, Mediterranean; NWE, Northwest European; sp, species