A re-examination of the nama-type Vendian organism Rangea schneiderhoehni

The need to re-examine Rangea has been motivated by two factors: first, by the recent progress in the understanding of three-dimensional mouldic preservation of Vendian fossils, and second, by discoveries of this taxon outside Gondwana albeit in the same sedimentary environment as seen\ud in Namibia. Several important features are revealed, including the in situ posture in the sediment, the\ud double-layered quilted structure, the tripartite stemless body and the mucous-supported sheath in the sediment. It is suggested that Rangea represents an infaunal organism, and that the similarity with other members of the Nama-type biota reflects convergence in functional and fabricational constraints in relation to infaunal life habit

Sesgo tafonómico en los datos de distribución de Cloudina en Siberia

Cloudina-morph fossils in Siberia have been traditionally regarded as a taphonomic mode of Anabarites tests inserted one into another under specific hydrodynamic conditions. Clusters of telescoped conical tests are ubiquitous in the Kessyusa Group and coeval strata across Siberia and not all of them can be easily interpreted as a result of simple mechanical stacking. It remains to be confirmed whether any of these clusters actually represents a life association of a Cloudina-morph structure.En Siberia los morfotipos de Cloudina han sido tradicionalmente considerados como una variedad tafonómica de conchas de Anabarites, insertadas una dentro de otra, bajo condiciones hidrodinámicas específicas. Las asociaciones de conchas cónicas telescópicas son omnipresentes en el Grupode Kessyusa y en estratos contemporáneos a lo largo de Siberia, y no todos ellos pueden ser fácilmente interpretados como resultado de un simple apilamiento mecánico. Queda por ver si alguna de estas bioacumulaciones representa una asociación de vida de una estructura morfotípica de tipo Cloudina

A reassessment of the problematic Ediacaran genus Orbisiana Sokolov 1976

The genus Orbisiana was established in 1976 by B.S. Sokolov based on a collection of seven specimens within core material from a borehole drilled through the Ediacaran-age Gavrilov Yam Formation of the Moscow Basin, Russia. Here we reassess the original material for the type species Orbisiana simplexSokolov 1976, which was long considered to be lost; fix the holotype of the type species; and revise the original diagnosis of the genus and species. Pyritisation of the fossils, which are preserved in finely laminated shales, allows three-dimensional morphological characterisation of this taxon using X-ray microtomography (μCT). Morphological and taphonomic analyses of the type material and additional three-dimensionally preserved specimens from the Verkhovka Formation (Vendian of the White Sea area) suggest that Orbisiana simplex consisted of submillimetric to millimetric globular chambers arranged in compact, grape-like clusters, or forming sinuous to linear aggregates. Occasionally, aggregates can bifurcate, with no appreciable change in chamber dimensions or shape. The phylogenetic affinity of Orbisiana remains uncertain, but its chambered construction, putative agglutinated structure of the chamber walls, and compact, occasionally branching chamber arrangement are shared with agglutinated tests of the Ediacaran genus Palaeopascichnus. Our reassessment and systematic study of the genus Orbisiana sheds new light on one of the least studied members of the late Ediacaran macroscopic biota.NER

La aparición de Dickinsonia en facies no marinas

Dickinsonia in the central Urals occurs in a succession transitional from marginal marine to non-marine. Even within this sequence Dickinsonia appears to be restricted to the least marine facies interpreted as a coastal lagoon surrounded by tidal flats. Another puzzling aspect is the complete absence of other Ediacaran taxa that are usually found together with Dickinsonia in other fossil localities. In fact, the Sinii Kamen Member of the Cherny Kamen Formation yielded an assemblage of Ediacaran macrofossils including aspidellamorph and mawsonitomorph holdfasts, frondomorphs, palaeopascichnids, and concentric ring structures of microbial origin; however, fossils of Dickinsonia have never been found in this association.En los Urales centrales, Dickinsonia se encuentra en medios de transición entre sustratos continentales y marinos marginales, generalmente de tipo laguna costera rodeada por llanuras mareales. Otro aspecto llamatico es la ausencia de otros taxones ediacáricos asociados con Dickinsonia, relativamente abundantes en otros medios. De hecho, el Miembro de Sinii Kamen de la Formación de Cherny Kamen ha librado una asociación de macrofósiles ediacáricos, que incluye anclajes de tipo aspidellamorfo y mawsonitomorfo, frondomorfos, palaeopascichnidos y estructuras en anillos concéntricos de origen microbiano, con los que no se asocia Dickinsonia

Ecología de Dickinsonia en llanuras mareales

Specimens of Dickinsonia from the Central Urals are characterised by clear bilateral symmetry. Taking into account observations from Australian specimens, we consider that the so called ‘glide reflection symmetry’ in these fossils is a taphonomic phenomenon. The size frequency distribution plot shows the predominance of smaller individuals in the studied population of Dickinsonia from the Central Urals. Assuming that the age of an individual is manifested in the body size, there is a significant predominance of juvenile individuals in the population. Three possible scenarios can be envisaged: (i) the population has a large number of juvenile individuals as the result of high survivorship rate in the intertidal zone; (ii) the population teems with juvenile forms because it is buried immediately after hatching; (iii) assuming that Dickinsonia was an actively motile organisms, that abundance of juvenile individuals could be explained by their inability to escape burial (although it is difficult to imagine that some of the mature individuals are buried with signs of escape behaviour); and (iv) the population could be interpreted as a fossilised ‘nesting ground’ for Dickinsonia in the intertidal zone where juvenile forms underwent maturation before migrating back to the subtidal zone. The study population can be characterised as expanding or stable; therefore, the intertidal setting can be described as favourable for these organisms.Los ejemplares de Dickinsonia de los Urales Centrales se caracterizan por una clara simetría bilateral. Tomando en cuenta las observaciones de especímenes australianos, consideramos que la llamada’simetría de reflexión por deslizamiento’ en estos fósiles es un fenómeno tafonómico. El gráfico de distribución de frecuencias de tamaño muestra el predominio de individuos juveniles en la población estudiada de Dickinsonia de los Urales Centrales. Asumiendo que la edad de un individuo se manifiesta por su tamaño, existe un predominio significativo de individuos juveniles en la población. Se pueden prever tres escenarios posibles: (1) la población tiene un gran número de individuos juveniles como resultado de la alta tasa de supervivencia en la zona intermareal; (2) la población está dominada por formas juveniles porque está enterrada inmediatamente después de la eclosión; (3) suponiendo que Dickinsonia fuera un organismo móvil activo, la abundancia de individuos juveniles podría explicarse por su incapacidad para escapar del entierramiento (aunque es difícil imaginar que algunos de los individuos maduros estén enterrados con signos de comportamiento de escape); y (4) la población podría interpretarse como un “lugar de nidificación” fosilizado para Dickinsonia en la zona intermareal, donde las formas juveniles maduraron antes de migrar a la zona submareal. La población de estudio puede caracterizarse como en expansión o estable; por lo tanto, el entorno intermareal puede describirse como favorable para estos organismo

Anatomy of the Ediacaran rangeomorph Charnia masoni.

The Ediacaran macrofossil Charnia masoni Ford is perhaps the most iconic member of the Rangeomorpha: a group of seemingly sessile, frondose organisms that dominates late Ediacaran benthic, deep-marine fossil assemblages. Despite C. masoni exhibiting broad palaeogeographical and stratigraphical ranges, there have been few morphological studies that consider the variation observed among populations of specimens derived from multiple global localities. We present an analysis of C. masoni that evaluates specimens from the UK, Canada and Russia, representing the largest morphological study of this taxon to date. We describe substantial morphological variation within C. masoni and present a new morphological model for this species that has significant implications both for interpretation of rangeomorph architecture, and potentially for existing taxonomic schemes. Previous reconstructions of Charnia include assumptions regarding the presence of structures seen in other rangeomorphs (e.g. an internal stalk) and of homogeneity in higher order branch morphology; observations that are not borne out by our investigations. We describe variation in the morphology of third and fourth order branches, as well as variation in gross structure near the base of the frond. The diagnosis of Charnia masoni is emended to take account of these new features. These findings highlight the need for large-scale analyses of rangeomorph morphology in order to better understand the biology of this long-enigmatic group.NER

Remarkable preservation of microbial mats in Neoproterozoic siliciclastic settings : Implications for Ediacaran taphonomic models

The authors thank Duncan McIlroy and Alex Liu for their discussions, help, comments and field support, the National Trust for access to Longmyndian localities, and the staff of the British Geological Survey Palaeontology unit and the Oxford University Museum of Natural History for their assistance with access to materials. The comments and suggestions of two anonymous reviewers and Nora Noffke significantly improved the manuscript.Peer reviewedPostprin

Anatomy of the Ediacaran rangeomorph Charnia masoni

The Ediacaran macrofossil Charnia masoni Ford is perhaps the most iconic member of the Rangeomorpha: a group of seemingly sessile, frondose organisms that dominates late Ediacaran benthic, deep‐marine fossil assemblages. Despite C. masoni exhibiting broad palaeogeographical and stratigraphical ranges, there have been few morphological studies that consider the variation observed among populations of specimens derived from multiple global localities. We present an analysis of C. masoni that evaluates specimens from the UK, Canada and Russia, representing the largest morphological study of this taxon to date. We describe substantial morphological variation within C. masoni and present a new morphological model for this species that has significant implications both for interpretation of rangeomorph architecture, and potentially for existing taxonomic schemes. Previous reconstructions of Charnia include assumptions regarding the presence of structures seen in other rangeomorphs (e.g. an internal stalk) and of homogeneity in higher order branch morphology; observations that are not borne out by our investigations. We describe variation in the morphology of third and fourth order branches, as well as variation in gross structure near the base of the frond. The diagnosis of Charnia masoni is emended to take account of these new features. These findings highlight the need for large‐scale analyses of rangeomorph morphology in order to better understand the biology of this long‐enigmatic group

Upper Vendian in the east, northeast and north of East European Platform: Depositional processes and bio­tic evolution

Subject. Analysis of lithogeochemical proxies in the Upper Vendian mudstones reveals little if any variation in depositional environment for the Redkinian, Belomorian and Kotlinian regional stages in the east, northeast and north of East European Platform. The coeval macrobiota, in contrast, demonstrates significant macroevolutionary and macroecological transformations. Thus, the Avalon-type ecological association consisting of frondomorphs and vendobionts evolved in low-energy inner shelf during the Redkinian, the Belomorian Stage is characterised by diversification of frondomorphs, migration of vendobionts into relatively high-energy depositional settings (shoreface and prodelta), and emergence of tribrachiomorphs and bilateralomorphs, where as the Kotlinian Stage is marked by a sharp decline in taxonomic diversity of soft-bodied organisms (the Kotlinian Crisis). We don’t know to what degree, if at all, depositional parameters as palaeogeodynamics, palaeoclimate, sediment composition, volcanic activity influenced the Ediacaran biota, but these agents were not responsible for the above mentioned biotic transformations. Materials and methods. We suggest that intrinsic factors such as ecological interactions could be the primary trigger of the Kotlinian crisis. This conclusion has been reached based on the study of composition of major rock-forming oxides, rare- and trace elements in fine-grained aluminosiliciclastic rocks (argillites, shales and silt-rich mudstones). Geological samples were collected in outcrops of the Asha Group of South Urals and Sylvitsa Group of Central Urals, as well as from the drill core of the Keltma-1 (Vychegda Trough) and Tuchkino-1000 (Southeast White Sea area) boreholes. We also used the data on chemical composition of mudstones from the Staraya Russa and Vasil’evsky Ostrov formations form the southern slope of the Baltic Shield. Results. With this information in hand we could assess, with varying degree of confidence, such parameters as a degree of recycling of the material supplied into the late Vendian Mezen Basin; sediment provenance; composition of the substrate that microbial mats and soft-bodied organisms lived on in different parts of the basin; and palaeogeodynamic environment at the time when different groups of soft-bodies organisms were emerging