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

Three-dimensional reconstruction, taphonomic and petrological data suggest that the oldest record of bioturbation is a body fossil coquina

Fossil material assigned to Nenoxites from the late Ediacaran Khatyspyt Formation of Arctic Siberia (550–544 Ma) has been presented as evidence for bioturbation prior to the basal Cambrian boundary. However, that ichnological interpretation has been challenged, and descriptions of similar material from other global localities support a body fossil origin. Here we combine x-ray computed tomography, scanning electron microscopy and petrographic methods to evaluate the body or trace fossil nature of Nenoxites from the Khatyspyt Formation. The fossilized structures consist of densely packed chains of three-dimensionally preserved silicic, bowl-shaped elements surrounded by distinct sedimentary halos, in a dolomitized matrix. Individual bowl-shaped elements can exhibit diffuse mineralogical boundaries and bridging connections between elements, both considered here to result from silicification and dolomitization during diagenesis. This new morphological and petrological evidence, in conjunction with recent studies of the late Ediacaran tubular taxa Ordinilunulatus and Shaanxilithes from China, suggest that the Khatyspyt specimens most probably reflect a coquina deposit of Shaanxilithes-like body fossils. Our data support the possibility of Shaanxilithes-like organisms representing total group eumetazoans

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

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

The developmental biology of <i>Charnia</i> and the eumetazoan affinity of the Ediacaran rangeomorphs

Molecular timescales estimate that early animal lineages diverged tens of millions of years before their earliest unequivocal fossil evidence. The Ediacaran macrobiota (~574 to 538 million years ago) are largely eschewed from this debate, primarily due to their extreme phylogenetic uncertainty, but remain germane. We characterize the development of Charnia masoni and establish the affinity of rangeomorphs, among the oldest and most enigmatic components of the Ediacaran macrobiota. We provide the first direct evidence for the internal interconnected nature of rangeomorphs and show that Charnia was constructed of repeated branches that derived successively from pre-existing branches. We find homology and rationalize morphogenesis between disparate rangeomorph taxa, before producing a phylogenetic analysis, resolving Charnia as a stem-eumetazoan and expanding the anatomical disparity of that group to include a long-extinct bodyplan. These data bring competing records of early animal evolution into closer agreement, reformulating our understanding of the evolutionary emergence of animal bodyplans

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