Charnia at 50: Developmental models for Ediacaran fronds

Correct interpretation of the Ediacara biota is critical to our understanding of the dramatic events at the base of the Cambrian. We review here the history of thought and examine new laser images of the holotype of Charnia masoni Ford, 1958, of the Ediacara biota, in terms of growth and development. Growth and development are argued to provide critical tools for understanding this and other enigmatic fossil groups. We show that Charnia cannot be related to the modern cnidarian group, the sea pens, with which it has for so long been compared, because they have opposite growth polarities. This is shown by our work on material collected by HMS challenger. Recent evolutionary studies also show that sea pens are a highly derived group of actively burrowing cnidarians that are likely to have evolved later than the Palaeozoic. The traditional paradigm of translating Phanerozoic animal phyla back into the Ediacaran is therefore questioned. © 2008 The Palaeontological Association

Charnia and sea pens are poles apart

Charnia from the Ediacara biota is here examined in terms of its growth and development. The Ediacara biota comes from the critical period of evolution just before the Cambrian Explosion and is key to our understanding of the origin of animal life. We show that Charnia cannot be related to the modern cnidarian group the sea pens (Pennatulacea) with which it has for so long been compared, as generative zones cannot be homologized between these forms

Dickinsonia from Ediacara: A new look at morphology and body construction

During groundbreaking fieldwork on the taphonomy and sedimentology of the famous fossil deposits at the Ediacara sheep station [Goldring, R., Currnow, C.N., 1967. The stratigraphy and facies of the Late Precambrian at Ediacara, South Australia. Journal of the Geological Society of Australia 14, 195-214.], Roland Goldring formed one of the few collections of original Ediacara fossil materials that can still be accessed outside of Australia. This material, collected from the type locality at Ediacara, is now housed in the Natural History Museum at Oxford, and includes excellent specimens of the enigmatic fossil Dickinsonia costata [Sprigg, R.C., 1949. Early Cambrian 'jellyfishes' of Ediacara, South Australia, and Mount John, Kimberly District, Western Australia. Transactions of the Royal Society of South Australia 73, 72-99.]. This first detailed description of the Oxford Dickinsonia material, and its associated wrinkle structures, is discussed with relevance to some wider problems facing Ediacaran taxonomy. Serial photographic reconstruction techniques are used to produce high resolution analyses of these fossils. Attention is drawn to features such as relief inversion of the upper surface, in which a 'spatulate B segment' is typically preserved in positive hyporelief while the rest of the fossil is mainly preserved in negative hyporelief. Segments can also appear to bifurcate and merge-a feature which, together with the glide plane of symmetry, is inconsistent with the bilaterian model often argued for this taxon. Controversial and conflicting claims for the presence of circular muscles, a rigid outer cuticle, an axial gut and even radial tentacles can all be explained by a single hypothesis that involves a simple, possibly coelenterate-grade, organism which had a distinctive, hydraulic body structure that underwent stretching, strain, collapse and contraction as the internal turgor pressure in the segments changed, especially during death and burial. © 2008 Elsevier B.V. All rights reserved

Evolutionary relationships within the Avalonian Ediacara biota: new insights from laser analysis

We report new high-resolution laser scanning of the type material for the earliest, complex Ediacaran genera Charnia, Bradgatia, Charniodiscus and Ivesheadia from Charnwood, UK, and compare these with Beothukis mistakensis gen. et sp. nov. and the recently described taxa Charnia wardi, Charnia antecedens and Fractofusus spp. from broadly coeval strata in Newfoundland. We use the laser and other techniques to map the similarities and differences in morphology between these Ediacaran rangeomorphs. Key features are suggested to include the number of growth axes, the number and placement of growth tips, the presence of radiating or subparallel axes for the first- and higher-order branches, the extent of displayed or undisplayed leaf-like 'rangeomorph' architecture, and the extent of furling of the margins of these leaf-like elements. These features are then used to propose suggested homologies between these taxa, leading to a preliminary phylogenetic hypothesis for the evolution of the Avalonian Ediacara biota. © 2009 Geological Society of London

The architecture of Ediacaran Fronds

Ongoing discoveries of new rangeomorph fossils from the Ediacaran of Avalonia allow us to put forward a unified and approachable scheme for the description and phylogenetic analysis of frondose genera and their species. This scheme focuses upon the branching morphology of rangeomorph units. Our system has the advantage of being applicable at all visible scales of subdivision and is suitable for the study of isolated fragmentary specimens. The system is also free from hypothesis about biological affinity and avoids tectonically influenced features such as shape metrics. Using a set of twelve character states within this unified scheme, we here present emended diagnoses for Beothukis, Avalofractus, Bradgatia, Hapsidophyllas, Fractofusus, Trepassia and Charnia, together with a more extensive taxonomic treatment of the latter genus. For those forms that fall within the morphological spectrum between Trepassia and Beothukis, we introduce Vinlandia gen. nov. It is hoped that this scheme will provide a robust framework for future studies of rangeomorph ontogeny and evolution. © The Palaeontological Association

Effaced preservation in the Ediacara biota and its implications for the early macrofossil record

Abstract: Ediacaran structures known as 'pizza discs' or Ivesheadia have long been considered enigmatic. They are amongst the oldest known members of the Ediacara biota, apparently restricted to the Avalonian successions of Newfoundland and the UK, c. 579-560Ma. Here, we suggest that these impressions are taphomorphs, resulting from the post-mortem decay of the frondose Ediacaran biota. Ediacaran fossils range from well-preserved, high-fidelity variants to almost completely effaced specimens. The effaced specimens are inferred to have undergone modification of their original morphology by post-mortem microbial decay on the sea floor, combined with sediment trapping and binding. In this style of preservation, morphological details within the organism became variously subdued as a function of the extent of organic decay prior to casting by overlying sediments. Decay and effacement were progressive in nature, producing a continuum of grades of preservation on Ediacaran bedding planes. Fossils preserved by such 'effaced preservation' are those that have suffered these processes to the extent that only their gross form can be determined. We suggest that the lack of detailed morphology in effaced specimens renders such fossils unsuitable for use as type material, as it is possible that several taxa may, upon degradation and burial, generate similar morphological taphomorphs. We here reinterpret the genus Ivesheadia as a taphomorph resulting from extensive post-mortem decay of frondose organisms. Blackbrookia, Pseudovendia and Shepshedia from beds of comparable age in England are likewise regarded as taphomorphs broadly related to Charnia or Charniodiscus spp. To reflect the suggestion that such impressions are likely to be taphomorphs, and not taxonomically discrete, we propose the term ivesheadiomorphs to incorporate all such effaced taphonomic expressions of Ediacaran macrofossil taxa in Avalonian assemblages. Our recognition of effaced preservation has significant implications for Ediacaran taxonomy, and consequently for measures of Ediacaran diversity and disparity. It is implied that Avalonian assemblages preserve both organisms that were alive and organisms that were already dead at the time of burial. As such, the fossil assemblages cannot be taken to represent census populations of living organisms, as in prior interpretations. © The Palaeontological Association

Understanding ancient life: how Martin Brasier changed the way we think about the fossil record

Critical to our understanding of life on Earth is the ability to judge the validity of claims of very ancient ‘fossils’. Martin Brasier’s most important contribution to this debate was to establish a framework within which to discuss claims of the ‘oldest’ life. In particular, Brasier et al. (2002) made it clear that the burden of proof must fall on those making the claim of ancient life, not those refuting it. This led to his formulation of the concept of the continuum of morphologies produced by life and non-life, and the considerable challenges of differentiating biogenesis from abiogenesis. Martin Brasier developed a set of criteria for distinguishing life from non-life, and extended the use of many new high-resolution analytical techniques to palaeontological research. He also applied this null hypothesis way of thinking to the origin of animals and the Cambrian explosion (Brasier 2009), leading to him being involved in the development of a series of nested null hypotheses, his “cone of contention”, to analyse enigmatic fossils more generally. In short, Martin Brasier taught us how to formulate biological hypotheses in deep time, established the rules for how those hypotheses should be tested, and championed a host of novel analytical techniques to gather the data required. As a consequence, future discussions of enigmatic specimens and very old fossils will be greatly enriched by his contributions.JBA thanks the ongoing support of Oxford University’s Museum of Natural History and Department of Zoology. DW was funded by the European Commission and the Australian Research Council (FT140100321). AGL is supported by the Natural Environment Research Council [grant number NE/L011409/1]. NM is supported by the Centre for Excellence in Palaeosciences at the University of Witwatersrand and Rhodes University, South Africa. DMc is supported by an NSERC DG award