REVIEW OF EARLY TRIASSIC THYLACOCEPHALA

Thylacocephala (Euarthropoda: Eucrustacea?) is a group of enigmatic fossil euarthropods, known from at least the Silurian to the Cretaceous. The Triassic is considered to be the period during which thylacocephalans were the most diversified with 17 species reported from 19 localities in nine countries. However, Thylacocephala were assumed to be rare during the Early Triassic until recent discoveries in Japan, Nevada and Idaho, U.S.A.       Our study focuses on thylacocephalans from the Early Triassic, especially from Madagascar and Idaho. The revision of previously known taxa from Madagascar provides new important information. A new kind of ornamentation is reported for Paraostenia ambatolokobensis nov. comb., previously assigned to Ostenocaris ambatolokobensis. In addition, Ankitokazocaris acutirostris and Paraostenia ambatolokobensis are only the third and fourth thylacocephalan taxa for which possible cephalic appendages are described.  New occurrences of Thylacocephala in Nevada and Idaho, U.S.A., lead to the description of one new taxon and to the reassignement of Parisicaris triassica to Ankitokazocaris triassica nov. comb. Those occurrences provide a significant contribution to the knowledge of Thylacocephala taxonomic diversity and geographic distribution during the Early Triassic. An important revision of Early Triassic thylacocephalan taxa from Japan and China is also performed, including Ankitokazocaris chaohuensis, Ankitokazocaris parva nov. comb., Ankitokazocaris utatsuensis nov. comb. and Paraostenia sp. Overall, the taxonomic diversity of Triassic thylacocephalans has likely been slightly over-estimated (17 species before, 15 now). However, the Triassic represents the richest period in terms of thylacocephalan-bearing outcrops (19 localities from nine countries)

Gladius-bearing coleoids from the Upper Cretaceous Lebanese Lagerstätten: diversity, morphology, and phylogenetic implications

Gladius-bearing coleoids are rare in the fossil record. For the Cretaceous period, these cephalopods are mainly recorded in a few Lagerstätten in Lebanon (Haqel, Hajoula, En Nammoura, and Sahel Aalma). Here, we study 16 specimens of gladius-bearing coleoids from these Upper Cretaceous Lebanese Lagerstätten to investigate their taxonomic diversity. Besides two species that were already reported (Dorateuthis syriaca and Glyphiteuthis libanotica), one new species is identified in the Cenomanian site of Hajoula: Rachiteuthis acutali n. sp., as well as another form of Glyphiteuthis from En Nammoura. Several studied specimens exhibit well-preserved soft-part characters. Among them, we document for the first time two transverse rows of sessile suckers in D. syriaca and we confirm the absence of tentacles, as well as the presence of a crop in this species. This strongly supports the phylogenetic proximity of D. syriaca with modern vampyropods rather than with modern decabrachians. In turn, the similarity in gladius morphology between this taxon and modern squids is regarded as convergen

External controls on the distribution, fabrics and mineralization of modern microbial mats in a coastal hypersaline lagoon, Cayo Coco (Cuba).

45 pagesInternational audienceActive, carbonate-mineralizing microbial mats flourish in a tropical, highly evaporative, marine-fed lagoonal network to the south of Cayo Coco Island (Cuba). Hypersaline conditions support the development of a complex sedimentary microbial ecosystem with diverse morphologies, a variable intensity of mineralization and a potential for preservation. In this study, the role of intrinsic (i.e. microbial) and extrinsic (i.e. physicochemical) controls on microbial mat development, mineralization and preservation was investigated. The network consists of lagoons, forming in the interdune depressions of a Pleistocene aeolian substratum; they developed due to a progressive increase in sea-level since the Holocene. The hydrological budget in the Cayo Coco lagoonal network changes from west to east, increasing the salinity. This change progressively excludes grazers and increases the saturation index of carbonate minerals, favouring the development and mineralization of microbial mats in the easternmost lagoons. Detailed mapping of the easternmost lagoon shows four zones with different flooding regimes. The microbial activity in the mats was recorded using light–dark shifts in conjunction with microelectrode O2 and HS− profiles. High rates of O2 production and consumption, in addition to substantial amounts of exopolymeric substances, are indicative of a potentially strong intrinsic control on mineralization. Seasonal, climate-driven water fluctuations are key for mat development, mineralization, morphology and distribution. Microbial mats show no mineralization in the permanently submersed zone, and moderate mineralization in zones with alternating immersion and exposure. It is suggested that mineralization is also driven by water-level fluctuations and evaporation. Mineralized mats are laminated and consist of alternating trapping and binding of grains and microbially induced magnesium calcite and dolomite precipitation. The macrofabrics of the mats evolve from early colonizing Flat mats to complex Cerebroid or Terrace structures. The macrofabrics are influenced by the hydrodynamic regime: wind-driven waves inducing relief terraces in windward areas and flat morphologies on the leeward side of the lagoon. Other external drivers include: (i) storm events that either promote (for example, by bioclasts covering) or prevent (for example, by causing erosion) microbial mat preservation; and (ii) subsurface degassing, through mangrove roots and desiccation cracks covered by Flat mats (i.e. forming Hemispheroids and Cerebroidal structures). These findings provide in-depth insights into understanding fossil microbialite morphologies that formed in lagoonal settings

Biological Soil Crusts as Modern Analogues for the Archean Continental Biosphere: Insights from Carbon and Nitrogen Isotopes

5 pagesInternational audienceStable isotope signatures of elements related to life such as carbon and nitrogen can be powerful biomarkers that provide key information on the biological origin of organic remains and their paleoenvironments. Marked advances have been achieved in the last decade in our understanding of the coupled evolution of biological carbon and nitrogen cycling and the chemical evolution of the early Earth thanks, in part, to isotopic signatures preserved in fossilized microbial mats and organic matter of marine origin. However, the geologic record of the early continental biosphere, as well as its evolution and biosignatures, is still poorly constrained. Following a recent report of direct fossil evidence of life on land at 3.22 Ga, we compare here the carbon and nitrogen isotopic signals of this continental Archean biosphere with biosignatures of cyanobacteria biological soil crusts (cyanoBSCs) colonizing modern arid environments. We report the first extended δ13C and δ15N data set from modern cyanoBSCs and show that these modern communities harbor specific isotopic biosignatures that compare well with continental Archean organic remains. We therefore suggest that cyanoBSCs are likely relevant analogs for the earliest continental ecosystems. As such, they can provide key information on the timing, extent, and possibly mechanism of colonization of the early Earth's emergent landmasses

A new Early Triassic crinoid from Nevada questions the origin and palaeobiogeographical history of dadocrinids

Knowledge of the early evolution of post-Palaeozoic crinoids mainly relies on the well-preserved and abundant material sampled in Triassic Konservat-Lagerstätten such as those from the Anisian Muschelkalk (Middle Triassic) of the Germanic Basin. These crinoid-bearing Lagerstätten have been central to understanding the rapid evolution and diversification of crinoids after the dramatic Permian/Triassic Boundary biological crisis that led the class to near-extinction. The Encrinida are the emblematic crinoids of the Triassic. They are mainly known from rich fossil deposits where their abundant ossicles are at the origin of the extensive crinoidal limestone beds of the German Upper Muschelkalk. So far, they were first represented in the Middle Triassic by the family Dadocrinidae and genus Dadocrinus. In the present work, a new species Dadocrinus montellonis sp. nov., is described based on a well-preserved, almost complete articulated specimen from the Spathian (Lower Triassic) of Nevada (USA). The new species differs from other species of Dadocrinus by its palaeobiogeographic position but also by its earlier stratigraphic occurrence and ancestral morphology. It represents the first reported occurrence of Dadocrinus outside the Germanic Basin prior to the Middle Triassic and also the oldest firm evidence of its presence in the Early Triassic (middle–late Spathian). This discovery sheds new light on the origin of post-Palaeozoic crinoids. It suggests a much wider distribution than commonly assumed for the genus Dadocrinus and implies that the first dadocrinids originated either in the Panthalassa or Tethys oceans, and then dispersed over long distances in a relative short period of time

New thylacocephalans from the Early Triassic Paris Biota (Bear Lake County, Idaho, USA).

International audienceTwo new genera and species of thylacocephalans (Arthropoda, Thylacocephala), Parisicaris triassica Charbonnier and Ligulacaris parisiana Charbonnier, are described from the early Spathian Paris Biota. These new occurrences are the first reports of thylacocephalans from Triassic rocks in North America. They considerably enlarge the spatiotemporal distribution of these enigmatic arthropods and highlight their relatively high generic richness during the Early Triassic. It also confirms that the Triassic was the taxonomically richest period for Thylacocephala

Pelagic ecosystem recovery after end-Permian mass extinction.

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Ammonoïdes du Smithien (Trias inférieur) du Nord‐Ouest du Guangxi (Chine du Sud) et modélisation biogéographique de la récupération biotique des ammonoïdes après l'extinction de masse Permien/Trias

The Permo-Triassic mass extinction (ca. 252 Ma) drastically affected the evolution of life, resulting in the decimation of more than 90% of marine species. Pre-crisis levels of marine ecosystem complexity were not reached until Middle Triassic time. Ammonoids (Cephalopoda) recovered much faster than other marine shelled invertebrates.The Early Triassic is an appropriate period to study how climate and oceanic conditions influenced the recovery of marine organisms, especially in terms of taxonomic richness and paleobiogeography. Indeed, Early Triassic ammonoids represent an ideal case of an almost monophyletic clade evolving in a stable paleogeographical framework, which was largely influenced by other parameters such as sea surface temperature (SST), currents, water chemistry, etc.First, in order to improve our understanding of the principal constraints controlling the dynamics of the ammonoid recovery, we constructed a “geophyletic model”, in which SST and currents are the “forcing” parameters applied to the biogeographical dispersal of a randomly generated clade. Validation of the results of the “geophyletic model” was based on a comparison with the distribution patterns of present-day Atlantic foraminifers. Next, we applied the “geophyletic model” to Early Triassic paleogeography to simulate the spatial and temporal variations of ammonoid diversity during the recovery, in response to “forcing” parameters such as SST and currents. The model primarily demonstrates that the edification and shape of a marine latitudinal gradient of taxonomic richness is largely governed in a non-linear fashion by the shape and magnitude of the SST gradient.Second, our simulation results were compared to the Early Triassic ammonoid record. Based on a refined global data set at the basin level, we investigate the paleobiogeographical global latitudinal and longitudinal diversity patterns in terms of climatic changes during the Early Triassic. During this period, the global first order trend in increasing ammonoid diversity was accompanied by a progressive change from cosmopolitan to latitudinally-restricted distributions. This change led to theemergence of a pronounced latitudinal diversity gradient during most of the Smithian and Spathian stages, which entails increased steepness of the SST gradient during the late Early Triassic. However, two brief episodes of ammonoid cosmopolitanism combined with low global diversity interrupted the first order increasing trend at the very beginning and very end of the Smithian. The analysis of endemicity indicates a rapid biogeographical maturing and structuring of faunas concomitant with the edification of the latitudinal diversity gradient. The distribution of taxa also reveals a pattern of latitudinal belts of faunal compositions across Panthalassa. Thus, Early Triassic ammonoid recovery in time and space is interpreted as having been largely controlled by the evolution of SST gradients.The third part of this dissertation focuses on the taxonomy and biostratigraphic distribution of Smithian ammonoids from South China. With an equatorial paleoposition at the boundary between Tethys and Panthalassa, South China occupies a key position for the reconstruction of biogeographic patterns. A thorough bed by bed sampling provides for the first time a detailed stratigraphic distribution of Smithian ammonoids in this area, which is by far the most complete succession in the Tethys. A new local zonation is established and correlated with other successions from mid- and highpaleolatitudes.L'extinction de masse permo‐triasique (~252 Ma) a décimé plus de 90% des espèces marines vivant à cette époque. La récupération biotique consécutive était jusqu'alors considérée comme extrêmement lente (~10 millions d'années). Dans ce contexte, les ammonoïdes (céphalopodes) du Trias inférieur constituent un ensemble phylogénétiquement bien contraint d'organismes marins abondant dans le registre fossile, et donc particulièrement appropriés à l'étude de l'impact des conditions climatiques et océaniques (températures[SST], courants, etc.) sur les dynamiques macroécologiques et biogéographiques post‐crise. Afin d'appréhender l'impact relatif de ces paramètres, nous avons élaboré un modèle de simulation 2D, dit modèle « géophylétique », où les SST et les courants forcent la dispersion géographique d'un ensemble phylogénétique généré aléatoirement. Les simulations indiquent que la forme et l'intensité du gradient latitudinal de diversité résultent de la combinaison de deux effets « milieu de domaine », l'un géographique et l'autre thermique, en lien avec la forme et l'intensité du gradient de SST : plus celui‐ci est marqué, plus le gradient de diversité est accentué. Ces résultats sont comparés aux données fossiles, augmentées de plusieurs nouvelles faunes du Smithien de Chine du Sud, présentées ici sous une forme monographique. Celles‐ci indiquent que les ammonoïdes sont, après la crise permo‐triasique, parmi les organismes marins les plus rapides à se re‐diversifier et à reconquérir l'océan mondial (< ~2 ma). Le début du Trias inférieur est marqué par un gradient latitudinal de diversité très faible, avec de très nombreux genres cosmopolites. Ce gradient de diversité s'accentue progressivement durant le Trias inférieur, en lien avec une augmentation de la différentiation latitudinale et du degré d'endémisme des faunes. Cette accentuation correspond vraisemblablement au développement d'un fort gradient latitudinal de SST au cours du Trias inférieur