Sébastien Clausen

A New View of the Cambrian Arthropod Fuxianhuia

Abstract Fuxianhuia from the Chengjiang fauna is one of the least derived Cambrian arthropods. A supposed pair of hook-shaped post-antennal appendages (Chen et al. 1995) was later reinterpreted as a pair of gut diverticula situated in a carapace fold (Waloszek et al. 2005). The latter interpretation of the diverticular nature of this structure is verified, but we show that the diverticula are situated inside a head capsule. This capsule carries a pair of antennae. It is shown that the head has also a pair of ordinary locomotory limbs. It is also demonstrated that a number of body segments have more than one pair of legs. There is indication of a possible pair of small eyes in addition to the large pair. The gut may include sediment from the substrate, which indicates that sediment was ingested. Fuxianhuiids share with many lobopodians characters such as a lack of specialisation between and within the ventral appendages. The exopod rises as a lateral fold on a straight limb axis, indicating an origin as a secondary addition to a uniramous limb. The terms Arthropoda s.l. (or pan-Arthropoda) and Arthropoda s.s. are replaced by Aiolopoda Hou & Bergström, 2006, and Arthropoda (in the original sense). The origination of the arthropods is discussed.

An early Cambrian fauna of skeletal fossils from the Emyaksin Formation, northern Siberia.

An assemblage of mineralised skeletal fossils containing molluscs, hyoliths, halkieriids, chancelloriids, tommotiids, lobopodians, paleoscolecids, bradoriids, echinoderms, anabaritids, hyolithelminths, hexactinnelid, and heteractinid sponges is described from the early Cambrian Emyaksin Formation exposed along the Malaya Kuonamka and Bolshaya Kuonamka rivers, eastern flanks of the Anabar Uplift, northern Siberian Platform. The sampled succession is attributed to the Tommotian—Botoman Stages of Siberia and correlated with Stage 2 of Series 1—Stage 4 of Series 2 of the IUGS chronostratigraphical scheme for the Cambrian. Carbon isotope chemostratigraphy is applied herein for regional correlation. The fauna contains the earliest Siberian and probably global first appearances of lobopodians, paleoscolecids, and echinoderms, and includes elements in common with coeval faunas from Gondwana, Laurentia, and Baltica. For the first time from Siberia, the latest occurrence of anabaritids is documented herein from the Atdabanian Stage. Problematic calcium phosphatic sclerites of Fengzuella zhejiangensis have not been previously known from outside China. The sellate sclerites, Camenella garbowskae and mitral sclerites, C. kozlowskii are unified within one species, C. garbowskae. In addition to more common slender sclerites, Rhombocorniculum insolutum include broad calcium phosphatic sclerites. A number of fossils described herein demonstrate excellent preservation of fine details of skeletal microstructures. Based on new microstructural data, sclerites of Rhombocorniculum are interpreted as chaetae of the type occurring in annelids. A new mollusc Enigmaconus? pyramidalis Kouchinsky and Vendrasco sp. nov. and a hyolith Triplicatella papilio Kouchinsky sp. nov. are described.

Major geodynamic and sedimentary constraints on the chronostratigraphic correlation of the lower-middle Cambrian transition in the western Mediterranean region

The sedimentary rocks of the lower-middle Cambrian transition in the western Mediterranean region have recorded a superposition of extensive tectonic, volcanic, epeirogenic, and eustatic events that led to a complex sequence framework that, in some cases, makes detailed chronostratigraphic correlations difficult. This paper summarizes and updates the relationships between event stratigraphy, fluctuations of relative sea level, setting of major stratigraphic discontinuities, unconformities, and condensed levels, and succession of benthic community replacements displayed by outcrops located in the Iberian Peninsula (Iberian, Cantabrian and Córdoba platforms), Moroccan Atlas (Souss Basin), and the Montagne-Noire and Sardinian platforms. The resulting mosaic of inter-related geodynamic processes is correlated by trilobite, archaeocyath-and acritarch-based chronostratigraphic scales, taking as reference for the base of the Middle Cambrian in West Gondwana the immigration of paradoxidid trilobites.

Stem structure and evolution in the earliest pelmatozoan echinoderms

Abstract Echinoderm skeletal debris from the Early–Middle Cambrian boundary Micmacca Breccia of Morocco includes the oldest known holomeric columnals. The original calcite of these ossicles is coated and replaced by iron oxides, occasionally overlain by a late coating of silica, and preserves with high fidelity fine details of their three-dimensional microstructure. Irrespective of their external morphology, columnals can be divided into three groups based on the distribution of stereom microfabrics, which we suggest indicates the presence of at least three different holomeric stemmed taxa. One of these columnal types has well-developed galleried stereom perpendicular to its articulation facets, a sure sign that long, penetrative collagen bundles bound columnals together, as in modern stemmed crinoids. This columnal morphology also shows a primitive type of interlocking articulation, which we term parasymplexy and which may have helped to counter torsional stresses. The two other columnals either lacked fibrous connective tissue or had shallow, non-penetrative fibers between columnals.

Terreneuvian Orthothecid (Hyolitha) Digestive Tracts from Northern Montagne Noire, France; Taphonomic, Ontogenetic and Phylogenetic Implications

More than 285 specimens of Conotheca subcurvata with three-dimensionally preserved digestive tracts were recovered from the Terreneuvian (early Cambrian) Heraultia Limestone of the northern Montagne Noire, southern France. They represent one of the oldest occurrences of such preserved guts. The newly discovered operculum of some complete specimens provides additional data allowing emendation of the species diagnosis. Infestation of the U-shaped digestive tracts by smooth uniseriate, branching to anastomosing filaments along with isolated botryoidal coccoids attests to their early, microbially mediated phosphatisation. Apart from taphonomic deformation, C. subcurvata exhibits three different configurations of the digestive tract: (1) anal tube and gut parallel, straight to slightly undulating; (2) anal tube straight and loosely folded gut; and (3) anal tube straight and gut straight with local zigzag folds. The arrangement of the digestive tracts and its correlation with the mean apertural diameter of the specimens are interpreted as ontogenetically dependent. The simple U-shaped gut, usually considered as characteristic of the Hyolithida, developed in earlier stages of C. subcurvata, whereas the more complex orthothecid type-3 only appears in largest specimens. This growth pattern suggests a distinct phylogenetic relationship between these two hyolith orders through heterochronic processes.

PELMATOZOAN ECHINODERMS AS COLONIZERS OF CARBONATE FIRMGROUNDS IN MID-CAMBRIAN HIGH ENERGY ENVIRONMENTS

Abstract New echinoderm holdfast discoveries from Gondwana demonstrate that pelmatozoans have been cementers able to attach to carbonate firmgrounds since the basal middle Cambrian. Echinoderms were thus colonizing shallow, high-energy environments well before the appearance of the first true carbonate hardgrounds in the Furongian. Morphological innovations and adaptations to firmground media ( = substrates) were first developed in softground, clayey, offshore environments where echinoderms cemented to shell fragments. This preadaption allowed echinoderms to quickly and effectively exploit the newly emerging hardground habitats in the Furongian to Early Ordovician.

A middle Cambrian fauna of skeletal fossils from the Kuonamka Formation, northern Siberia

An assemblage of mineralized skeletal fossils containing molluscs, hyoliths, chancelloriids, protoconodonts, lobopods, paleoscolecids, bradoriids, echinoderms and hexactinellid sponges is described from the middle Cambrian part of the Kuonamka Formation, exposed along the Malaya Kuonamka and Bolshaya Kuonamka rivers, northern Siberian Platform. The sampled succession is attributed to the Kuonamkites and lower Tomagnostus fissus–Paradoxides sacheri biozones of the Amgan Stage of Siberia, correlated with Series 3, Stage 5—lower Drumian Stage of the IUGS chronostratigraphical scheme for the Cambrian. This work complements descriptions of molluscs from the same samples published by Gubanov et al. (2004) with additional material. It contains forms in common with coeval faunas from Australia, China, Western Gondwana, Avalonia, Laurentia and Baltica, increasing potential for global biostratigraphic correlation and understanding of palaeogeographic connections.

Chronostratigraphic and palaeogeographic significance of an early Cambrian microfauna from the Heraultia Limestone, northern Montagne Noire, France

Abundant and diverse Small Shelly Fossils (SSFs) have been reported in Northern Montagne Noire (Southern France) since the last 20th century from early Cambrian phosphatic carbonates called Heraultia Limestone. This assemblage includes Watsonella crosbyi, a mollusc with high-potential for the definition of the base of the Cambrian Stage 2, as recently emphasized by the Cambrian Stage Subdivision Working Group of International Commission of Stratigraphy. Nevertheless, the Heraultia Limestone is traditionally considered Cambrian Stage 3 to 4 based on questionnable lithological correlations among different tectonostratigraphic units. The focus of this paper is to re-evaluate the age and the paleogeographical significance of the Heraultia Limestone in the light of recent advances in SSF systematic and biostratigraphy. Reassessed assemblage is dominated by molluscs (helcionellids, ?polyplacophors and other problematic taxa), and abundant orthothecid hyoliths, problematic tubes; few problematica are also present. Twenty-eight species and three morphotypes are described. Two species and one genus are new (Obscurania tormoi Devaere, sp. nov.; Alaconcha Devaere, gen. nov. and Alaconcha rugosa Devaere, sp. nov.), seven are reported for the first time. Among previously mentionned species, 13 are reassigned, 7 confirmed, and 9 unrecovered from the studied sampled. The global stratigraphic range of each species is established based on updated inventory of known occurrences. It argues for a Terreneuvian (Nemakit-Daldynian/Tommotian according to the Siberian stratigraphic chart) age of the microfossil assemblage. The Watsonella crosbyi–Oelandiella korobkovi Interval Zone is defined and is correlated with base of Cambrian Stage 2 (Tommotian) of Siberia, China, Mongolia, and Avalonia. The Northern Montagne Noire would accordingly witness for one of the earliest, isolated but consequent Tommotian carbonate-platform on the Western Gondwana margin. As a result, present tectonic and palaeogeographic models have to be emended, and factors that favored such isolated platforms should be further investigated. DEVAERE ET AL.: FRENCH CAMBRIAN MICROFAUNA 2 Léa Devaere. UMR 8217 Géosystèmes CNRS-Université LILLE 1, F59655 Villeneuve d’Ascq, France, lea.devaere@ed.univ-lille1.fr. Sébastien Clausen. UMR 8217 Géosystèmes CNRS-Université LILLE 1, F59655 Villeneuve d’Ascq, France, sebastien.clausen@univ-lille1.fr. Michael Steiner. Department of Earth Sciences, Freie Universität Berlin, Malteserstrasse 74-100, Haus D, Berlin, 12249, Germany, steiner@zedat.fu-berlin.de. J. Javier Álvaro. Centro de Astrobiología (CSIC/INTA), Ctra. de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Spain, alvarobjj@cab.inta-csic.es. Daniel Vachard. UMR 8217 Géosystèmes CNRS-Université LILLE 1, F59655 Villeneuve d’Ascq, France, daniel.vachard@univ-lille1.fr.

Botoman (Lower Cambrian) turbid- and clear-water reefs and associated environments from the High Atlas, Morocco

Abstract Exposures of the Botoman (Lower Cambrian), Lemdad and Issafen formations on the Lemdad syndine, southern High Atlas, provide an excellent example of the interactions between tectonic events, magmatic activity and carbonate productivity. The major factors that controlled the nucleation of carbonate factories on the Botoman High Atlas platform were: (i) synsedimentary tectonism, as normal faulting resulted in tilting of fault blocks causing irregular topographies and subsequent sharp erosion; (ii) volcanism, because pyroclastic influx smothered carbonate factories except in distal areas of the platform or during quiescent episodes of volcanic activity; and (iii) the influence of successive shoaling parasequences. The Botoman reefs exhibit a wide range of external morphologies, including tabular (biostromes) and domal (bioherms and patches) boundstones, which do not exceed 3.5 m of thickness. Although archaeocyathan-microbial reefs only developed under clearwater conditions, microbial reefs grew also under turbid-water conditions. Domal and digitate stromatoids, Girvanella crusts, Epiphyton bushes and thromboid-stromatoid intergrowths document the ability of some microbial communities to develop heterotrophic strategies when submitted to a moderate terrigenous input. Turbidity was a major ecological factor that constrained development of filter/suspension-feeder and phototrophic organisms, but not necessarily of benthic non-phototrophic microbial communities.

Re-evaluating the palaeobiology and affinities of the Ctenocystoidea (Echinodermata)

Synopsis Ctenocystoids are a poorly understood group of fossil deuterostomes, with little known about their morphology, palaeobiology or affinities. A three‐dimensional specimen of the ctenocystoid Ctenocystis utahensis from the Middle Cambrian Spence Shale (western USA) was imaged using X‐ray microtomography and digitally reconstructed. This allowed the species to be described in greater detail than previously possible, providing a number of morphofunctional insights. Two openings are identified: (i) an anterior opening controlled by moveable plates in the distinctive ctenoid apparatus and (ii) a wide posterior periproct. A putative third body opening (hydropore and/or gonopore) is not present, contrary to previous reconstructions. Ctenocystis utahensis was most probably epibenthic, facing into water currents during life. Suspended food particles entered paired lateral grooves within the ctenoid apparatus through gaps between neighbouring ctenoid plates; when the ctenoid apparatus was open, material could pass from these lateral grooves to an internal chamber, which led into the main body cavity. Our reconstruction demonstrates that the plates of the ctenoid apparatus and the lateral feeding grooves are not homologous with paired brachioles, refuting suggestions that ctenocystoids are derived blastozoans. Instead, it is probable that ctenocystoids are basal stem‐group echinoderms, sharing several plesiomorphies (a bilateral body plan, pharyngeal gill slits and a paired water vascular system) with the latest common ancestor of echinoderms plus hemichordates. Reconstructing the anatomy of ctenocystoids in three‐dimensions is critical to a full understanding of their palaeobiology and phylogenetic position.