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

The influence of environmental setting on the community ecology of Ediacaran organisms

The broad-scale environment plays a substantial role in shaping modern marine ecosystems, but the degree to which palaeocommunities were influenced by their environment is unclear. To investigate how broad-scale environment influenced the community ecology of early animal ecosystems we employed spatial point process analyses to examine the community structure of seven bedding-plane assemblages of late Ediacaran age (558–550 Ma), drawn from a range of environmental settings and global localities. The studied palaeocommunities exhibit marked differences in the response of their component taxa to sub-metre-scale habitat heterogeneities on the seafloor. Shallow-marine palaeocommunities were heavily influenced by local habitat heterogeneities, in contrast to their deep-water counterparts. Lower species richness in deep-water Ediacaran assemblages compared to shallow-water counterparts across the studied time-interval could have been driven by this environmental patchiness, because habitat heterogeneities correspond to higher diversity in modern marine environments. The presence of grazers and detritivores within shallow-water communities may have promoted local patchiness, potentially initiating a chain of increasing heterogeneity of benthic communities from shallow to deep-marine depositional environments. Our results provide quantitative support for the “Savannah” hypothesis for early animal diversification – whereby Ediacaran diversification was driven by patchiness in the local benthic environment