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Ediacara biota

The Ediacara (formerly Vendian) biota are ancient lifeforms of the Ediacaran Period, which represent the earliest known complex multicellular organisms. They appeared soon after the Earth thawed from the Cryogenian period's extensive glaciers, and largely disappeared soon before the rapid appearance of biodiversity known as the Cambrian explosion, which saw the first appearance in the fossil record of the basic patterns and body-plans that would go on to form the basis of modern animals. Little of the diversity of the Ediacara biota would be incorporated in this new scheme, with a distinct Cambrian biota arising and usurping the organisms that dominated the Ediacaran fossil record.

The organisms of the Ediacaran Period first appeared around and flourished until the cusp of the Cambrian , when the characteristic communities of fossils vanished. While rare fossils that may represent survivors have been found as late as the Middle Cambrian (510 to 500 million years ago), the earlier fossil communities disappear from the record at the end of the Ediacaran, leaving only controversial fragments of once-thriving ecosystems, if anything. Multiple hypotheses exist to explain this disappearance, including preservation bias, a changing environment, the advent of predators, and competition from other lifeforms.

Some Ediacaran organisms might have been closely related to groups that would rise to prominence later; for instance, Kimberella shows some similarity to molluscs, and other organisms have been thought to show bilateral symmetry, though this is controversial. Most microscopic fossils are morphologically distinct from later lifeforms: they resemble discs, mud-filled bags, or quilted mattresses. Classification is difficult, and the assignment of some species even at the level of kingdom — animal, fungus, protist or something else — is uncertain: one paleontologist has even gained support for a separate kingdom Vendozoa (now renamed Vendobionta). Their strange form and apparent disconnectedness from later organisms have led some to consider them a "failed experiment" in multicellular life, with later multicellular life independently re-evolving from unrelated single-celled organisms.

History

The first Ediacaran fossils discovered were the disc-shaped Aspidella terranovica, in 1868. Their discoverer, A. Murray, a geological surveyor, found them useful aids for correlating the age of rocks around Newfoundland. However, since they lay below the "Primordial Strata", the Cambrian strata that were then thought to contain the very first signs of life, it took four years for anybody to dare propose they could be fossils. Elkanah Billings' proposal was dismissed by his peers on account of their simple form, and they were instead declared gas escape structures, inorganic concretions, or even tricks played by a malicious God to promote unbelief. No similar structures elsewhere in the world were then known, and the one-sided debate soon fell into obscurity. In 1933, Georg Gürich discovered specimens in Namibia, but the firm belief that life originated in the Cambrian led to them being assigned to the Cambrian Period, and no link to Aspidella was made. In 1946, Reg Sprigg noticed "jellyfishes" in the Ediacara Hills of Australia's Flinders Ranges but these rocks were believed to be Early Cambrian, so while the discovery sparked some interest, little serious attention was garnered.

It was not until the British discovery of the iconic Charnia in 1957 that the pre-Cambrian was seriously considered as containing life. This frond-shaped fossil was found in England's Charnwood Forest, and due to the detailed geologic mapping of the British Geological Survey there was no doubt that these fossils sat in Precambrian rocks. Palæontologist Martin Glaessner finally made the connection between this and the earlier finds, and with a combination of improved dating of existing specimens and an injection of vigour into the search, many more instances were recognised.

However, all specimens discovered until 1967 were in coarse-grained sandstone that prevented preservation of fine details, making interpretation difficult. S.B. Misra's discovery of fossiliferous ash-beds at the Mistaken Point assemblage in Newfoundland changed all this, as the delicate detail preserved by the fine ash allowed the description of features that were previously invisible.

Poor communication, combined with the difficulty in correlating globally distinct formations, led to a plethora of different names for the biota. In 1960, the French name "Ediacarien" — after the Ediacaran Hills in Southern Australia, which take their name from aborigine Idiyakra, "water is present" — was added to the competing terms "Sinian" and "Vendian", for terminal-Precambrian rocks, names that were also applied to the lifeforms. "Ediacaran" and "Ediacarian" were subsequently applied to the epoch or period of geologic time and its corresponding rocks. In March 2004, the International Union of Geological Sciences ended the inconsistency by formally naming the terminal period of the Neoproterozoic after the Australian locality.

Preservation

All but the smallest fraction of the fossil record consists of the robust skeletal matter of decayed corpses. Hence, since Ediacaran biota had soft bodies and no skeletons, their abundant preservation is surprising. The absence of burrowing creatures living in the sediments undoubtedly helped; since after the evolution of these organisms in the Cambrian, soft-bodied impressions were usually disturbed before they could fossilize.

Microbial mats

Microbial mats are areas of sediment stabilised by the presence of colonies of microbes, which secrete sticky fluids or otherwise bind the sediment particles. They appear to migrate upwards when covered by a thin layer of sediment, but this is an illusion caused by the colony's growth; individuals do not, themselves, move. If too thick a layer of sediment is deposited before they can grow or reproduce through it, parts of the colony will die, leaving behind fossils with a characteristically wrinkled "elephant skin" texture. Most Ediacaran strata with the "elephant skin" texture characteristic of microbial mats contain fossils, and Ediacaran fossils are almost never found in beds that do not contain these microbial mats. Although microbial mats were once widespread, the evolution of grazing organisms in the Cambrian vastly reduced their numbers, and these communities are now limited to inhospitable refugia where predators cannot survive long enough to eat them.

Fossilisation

The preservation of these fossils is one of their great fascinations to science. As soft-bodied organisms they would normally not fossilise. Unlike later soft-bodied fossil biota (such as the Burgess Shale, or Solnhofen Limestone) the Ediacara biota is not found in a restricted environment subject to unusual local conditions: they were a global phenomenon. The processes that were operating must have been systemic and worldwide. There was something very different about the Ediacaran Period that permitted these delicate creatures to be left behind. It is thought that the fossils were preserved by virtue of rapid covering by ash or sand, trapping them against the mud or microbial mats on which they lived. Ash beds provide more detail, and can readily be precisely dated to the nearest million years or better by means of radiometric dating. However, it is more common to find Ediacaran fossils under sandy beds deposited by storms or high-energy, bottom-scraping ocean currents known as turbidites. Soft-bodied organisms today almost never fossilise during such events, but the presence of widespread microbial mats probably aided preservation by stabilising their impressions in the sediment below.

What is preserved?

The rate of cementation of the overlying substrate, relative to the rate of decomposition of the organism, determines whether the top or bottom surface of an organism is preserved. Most disc-shaped fossils decomposed before the overlying sediment was cemented, and the ash or sand slumped in to fill the void, leaving a cast of the underside of the organism.

Conversely, quilted fossils tend to decompose after the cementation of the overlying sediment; hence their upper surfaces are preserved. Their more resistant nature is reflected in the fact that in rare occasions, quilted fossils are found within storm beds, the high-energy sedimentation not having destroyed them as it would have the less-resistant discs. Further, in some cases, the bacterial precipitation of minerals formed a "death mask", creating a mould of the organism.

Morphology

Forms of Ediacaran fossil
The earliest discovered potential embryo, preserved within an acanthomorphic acritarch. The term 'acritarch' describes a range of unclassified cell-like fossils.
Cyclomedusa, a disc-shaped fossil that has been interpreted as a microbial artefact. Metric scale.
A cast of the quilted Charnia, the first accepted complex Precambrian organism. Charnia was once interpreted as a relative of the sea-pens.
Spriggina, a possible precursor to the trilobites, may be one of the predators that led to the demise of the Ediacaran fauna and subsequent diversification of animals.

A late Ediacaran trace fossil preserved on a bedding plane.

The Ediacaran biota exhibited a vast range of morphological characteristics. Size ranged from millimetres to metres; complexity from "blob-like" to intricate; rigidity from sturdy and resistant to jelly-soft. Almost all forms of symmetry were present. These organisms differed from earlier fossils by displaying an organised, differentiated multicellular construction and centimetre-plus sizes.

These disparate morphologies can be broadly grouped into form taxa: "Embryos" : Recent discoveries of Precambrian multicellular life have been dominated by reports of embryos, particularly from the Doushantuo Formation in China. Some finds generated intense media excitement though some have claimed they are instead inorganic structures formed by the precipitation of minerals on the inside of a hole. Other "embryos" have been interpreted as the remains of the giant sulfur-reducing bacteria akin to Thiomargarita, a view which is highly contested yet gradually gaining supporters.

Microfossils dating from — just 3 million years after the end of the Cryogenian glaciations — may represent embryonic 'resting stages' in the life cycle of the earliest known animals.

An alternative proposal is that these structures represent adult stages of the animals of this period. Discs : Circular fossils, such as Ediacaria, Cyclomedusa, and Rugoconites led to the initial identification of Ediacaran fossils as cnidaria, which include jellyfish and corals. Further examination has provided alternative interpretations of all disc-shaped fossils: none is now confidently recognised as jellyfish. Alternate explanations include holdfasts, protists and sea anemones; the patterns displayed where two meet have led to many apparent individuals being recognised as microbial colonies. Useful diagnostic characters are often lacking because only the underside of the organism is preserved by fossilization. Bags : Fossils such as Pteridinium preserved within sediment layers resemble "mud-filled bags". The scientific community is a long way from reaching a consensus on their interpretation. Quilted organisms : The organisms considered in A. Seilacher's revised definition of the Vendobionta share a "quilted" appearance, and resembled an inflatable mattress. Sometimes, these quilts would be torn or ruptured prior to preservation: such damaged specimens provide valuable clues in the reconstruction process. For example, the three (or more) petaloid fronds of Swartpuntia germsi could only be recognised in a posthumously damaged specimen — usually, multiple fronds were hidden as burial squashed the organisms flat.

This "rangeomorph" class of organism, including the famous Charnia and Charniodiscus, is both the most iconic of the Ediacaran biota, and the most difficult to place within the existing tree of life. The quilted structure may be derived from a shared common ancestor (synapomorphy), but if it represents the most ecologically sensible form for an organism to take, different lineages may have converged in their morphology (convergent evolution). Non-Ediacaran Ediacarans : Some Ediacaran organisms have more complex details preserved, which has allowed them to be interpreted as possible early forms of living phyla, excluding them from some definitions of the Ediacaran biota.

The earliest such fossil is the reputed bilaterian Vernanimalcula, claimed by some, however, to represent the infilling of an egg-sac or acritarch. Later examples, almost universally accepted as bilaterians, include the mollusc-like Kimberella, Spriggina (pictured), and the shield-shaped Parvancorina, whose affinities are currently debated.

A suite of fossils known as the Small shelly fossils are represented in the Ediacaran, most famously by Cloudina, a shelly tube-like fossil that often shows evidence of predatory boring, suggesting that whilst predation may not have been common in the Ediacaran Period, it was at least present.

Representatives of modern taxa existed in the Ediacaran, some of which are recognisable today. Sponges, red and green algæ, protists and bacteria are all easily recognisable, with some pre-dating the Ediacaran by thousands of millions of years. Trace fossils : The only Ediacaran burrows are horizontal, on or just below the surface. Such burrows imply the presence of motile organisms with heads, which would probably have had a bilateral symmetry. This could place them in the bilateral clade of animals. Putative "burrows" dating as far back as may have been made by animals which fed on the undersides of microbial mats, which would have shielded them from a chemically unpleasant ocean; however their uneven width and tapering ends make a biological origin difficult to defend. The burrows observed imply simple behaviour, and the complex, efficient feeding traces common from the start of the Cambrian are absent. Some Ediacaran fossils, especially discs, have been interpreted tentatively as trace fossils, but this hypothesis has not gained widespread acceptance. As well as burrows, some trace fossils have been found directly associated with an Ediacaran fossil. Yorgia and Dickinsonia are often found at the end of long pathways of trace fossils matching their shape; the method of formation of these disconnected and overlapping fossils largely remains a mystery. The potential mollusc Kimberella is associated with scratch marks, perhaps formed by a radula, further traces from appear to imply active crawling or burrowing activity.

Classification and interpretation

Classification of the Ediacarans is difficult, and hence a variety of theories exist as to their placement on the tree of life.

Cnidarians

Since the most primitive eumetazoans — multi-cellular animals with tissues — are recognised as cnidarians, the first attempt to categorise these fossils designated them as jellyfish and sea-pens. However, detailed study of their growth pattern has discounted this hypothesis.

"The dawn of animal life"

Martin Glaessner proposed in The dawn of animal life (1984) that the Ediacara biota were recognisable stem group members of modern phyla, but were unfamiliar because they had yet to evolve the characteristic features we use in modern classification. Adolf Seilacher responded by suggesting that the Ediacaran sees animals usurping giant protists as the dominant life form.

In 1986 Mark McMenamin claimed that Ediacarans did not possess an embryonic stage, and thus could not be animals. He believed that they independently evolved a nervous system and brains, meaning that "the path toward intelligent life was embarked upon more than once on this planet", though this idea has not been widely accepted.

New phylum

Seilacher most famously suggested that the Ediacaran organisms represented a unique and extinct grouping of related forms descended from a common ancestor (clade) and created the kingdom Vendozoa, named after the now-obsolete Vendian era. He later excluded fossils identified as metazoans and relaunched the phylum "Vendobionta".

He described the Vendobionta as quilted cnidarians lacking stinging cells. This absence precludes the current cnidarian method of feeding, so Seilacher suggested that the organisms may have survived by symbiosis with photosynthetic or chemoautotrophic organisms.

Lichens

Gregory Retallack's hypothesis that Ediacaran organisms were lichens has failed to gain wide-spread acceptance. He argues that the fossils are not as squashed as jellyfish fossilised in similar situations, and their relief is closer to petrified wood. He points out the chitinous walls of lichen colonies would provide a similar resistance to compaction, and claims the large size of the organisms — sometimes over a metre across, far larger than any of the preserved burrows — also hints against a classification with the animals.

Other interpretations

Almost every possible phylum has been used to accommodate the Ediacaran biota at some point, from algæ, to protists known as foraminifera, to fungi to bacterial or microbial colonies, to hypothetical intermediates between plants and animals.

Origin

It took almost 4 billion years from the formation of the Earth for the Ediacaran fossils to first appear, 655 million years ago. Whilst putative fossils are reported from , the first uncontroversial evidence for life is found , and cells with nuclei certainly existed by : why did it take so long for forms with an Ediacaran grade of organisation to appear?

It could be that no special explanation is required: the slow process of evolution simply required 4 billion years to accumulate the necessary adaptations. Indeed, there does seem to be a slow increase in the maximum level of complexity seen over this time, with more and more complex forms of life evolving as time progresses, with traces of earlier semi-complex life such as Nimbia, found in the Twitya formation, possibly displaying the most complex morphology of the time.

The alternative train of thought is that it was simply not advantageous to be large until the appearance of the Ediacarans: the environment favoured the small over the large. Examples of such scenarios today include plankton, whose small size allows them to reproduce rapidly to take advantage of ephemerally abundant nutrients in algal blooms. But for large size never to be favourable, the environment would have to be very different indeed.

A primary size-limiting factor is the amount of atmospheric oxygen. Without a complex circulatory system, low concentrations of oxygen cannot reach the centre of an organism quickly enough to supply its metabolic demand.

On the early earth, reactive elements such as iron and uranium existed in a reduced form; these would react with any free oxygen produced by photosynthesising organisms. Oxygen would not be able to build up in the atmosphere until all the iron had rusted (producing banded iron formations), and other reactive elements had also been oxidised. Donald Canfield detected records of the first significant quantities of atmospheric oxygen just before the first Ediacaran fossils appeared — and the presence of atmospheric oxygen was soon heralded as a possible trigger for the Ediacaran radiation. Oxygen seems to have accumulated in two pulses; the rise of small, sessile (stationary) organisms seems to correlate with an early oxygenation event, with larger and mobile organisms appearing around the second pulse of oxygenation. The resolution of the fossil record is too low to make this assertion definite, and current research seeks to accurately determine the role that oxygen may have played.

Periods of intense cold have also been suggested as a barrier to the evolution of multicellular life. The earliest known embryos, from China's Doushantuo Formation, appear just a million years after the Earth emerged from a global glaciation, suggesting that ice cover and cold oceans may have prevented the emergence of multicellular life. Potentially, complex life may have evolved before these glaciations, and been wiped out. However, the diversity of life in modern Antarctica has sparked disagreement over whether cold temperatures increase or decrease the rate of evolution.

In early 2008 a team analysed the range of basic body structures ("disparity") of Ediacaran organisms from three different fossil beds: Avalon in Canada, to ; White Sea in Russia, to ; and Nama in Namibia, to , immediately before the start of the Cambrian. They found that, while the White Sea assemblage had the most species, there was no significant difference in disparity between the three groups, and concluded that before the beginning of the Avalon timespan these organisms must have gone through their own evolutionary "explosion", which may have been similar to the famous Cambrian explosion .

Disappearance

The low resolution of the fossil record means that the disappearance of the Ediacarans remains something of a mystery. There appears to have been a relatively abrupt disappearance at the end of the Ediacaran period; reports of Cambrian "Ediacarans" are not universally accepted. The cause — and reality — of this disappearance is open to debate.

Preservation bias

The sudden vanishing of Ediacaran fossils at the Cambrian boundary could simply be because conditions no longer favoured the fossilisation of Ediacaran organisms, which may have continued to thrive unpreserved. However, if they were common, more than the occasional specimen might be expected in exceptionally preserved fossil assemblages (Konservat-Lagerstätten) such as the Burgess Shale and Chengjiang — unless such assemblages represent an environment never occupied by the Ediacaran biota, or unsuitable conditions for their preservation.

Predation and grazing

It is suggested that by the Early Cambrian, organisms higher in the food chain caused the microbial mats to largely disappear. If these grazers first appeared as the Ediacaran biota started to decline, then it may suggest that they destabilised the microbial substrate, leading to displacement or detachment of the biota; or that the destruction of the mat destabilised the ecosystem, causing extinctions.

Alternatively, skeletonised animals could have fed directly on the relatively undefended Ediacaran biota. However, if the interpretation of the Ediacaran age Kimberella as a grazer is correct then this suggests that the biota had already had limited exposure to "predation".

There is however little evidence for any trace fossils in the Ediacaran Period, which may speak against the active grazing theory. Further the onset of the Cambrian Period is defined by the appearance of a worldwide trace fossil assemblage, quite distinct from the activity-barren Ediacaran Period.

Competition

It is possible that increased competition due to the evolution of key innovations amongst other groups, perhaps as a response to predation, drove the Ediacaran biota from their niches. However, this argument has not successfully explained similar phenomena. For instance, the bivalve molluscs' "competitive exclusion" of brachiopods was eventually deemed to be a coincidental result of two unrelated trends.

Change in environmental conditions

While it is difficult to infer the effect of changing planetary conditions on organisms, communities and ecosystems, great changes were happening at the end of the Precambrian and the start of the Early Cambrian. The breakup of the supercontinents, rising sea levels (creating shallow, "life-friendly" seas), a nutrient crisis, fluctuations in atmospheric composition, including oxygen and carbon dioxide levels, and changes in ocean chemistry (promoting biomineralisation) could all have played a part.

Assemblages

Ediacaran-type fossils are recognised globally in 25 localities and a variety of depositional conditions, and are commonly grouped into three main types, named after typical localities. Each assemblage tends to occupy its own region of morphospace, and after an initial burst of diversification changes little for the rest of its existence.

Avalon-type assemblage

The Avalon-type assemblage is defined at Mistaken Point in Canada, the oldest locality with a large quantity of Ediacaran fossils. The assemblage is easily dated because it contains many fine ash-beds, which are a good source of zircons used in the uranium-lead method of radiometric dating. These fine-grained ash beds also preserve exquisite detail. Constituents of this biota appear to survive through until the extinction of all Ediacarans at the base of the Cambrian.

The biota comprises deep sea dwelling rangeomorphs such as Charnia, all of which share a fractal growth pattern. They were probably preserved in situ (without post-mortem transportation), although this point is not universally accepted. The assemblage, while less diverse than the Ediacara- or Nama-types, resembles Carboniferous suspension-feeding communities, which may suggest filter feeding — by most interpretations, the assemblage is found in water too deep for photosynthesis. The low diversity may reflect the depth of water — which would restrict speciation opportunities — or it may just be too young for evolution to rich biota. Opinion is currently divided between these conflicting hypotheses.

Ediacara-type assemblage

The Ediacara-type assemblage is named after Australia's Ediacara Hills, and consist of fossils preserved in areas near the mouths of rivers (prodeltaic facies). They are typically found in interbedded sandy and silty layers formed below the normal base of wave-related water motion, but in waters shallow enough to be affected by wave motion during storms. Most fossils are preserved as imprints in microbial mats, but a few are preserved within sandy units.

Biota ranges
Sub-biota graphical timeline • Sub-biota graphical timeline • [edit]

Axis scale: millions of years ago, dated with U/Pb of zircons

Nama-type assemblage

The Nama assemblage is best represented in Namibia. Three-dimensional preservation is most common, with organisms preserved in sandy beds containing internal bedding. Dima Grazhdankin believes that these organisms represent burrowing organisms, while Guy Narbonne maintains they were surface dwellers. These beds are sandwiched between units comprising interbedded sandstones, siltstones and shales, with microbial mats, where present, usually containing fossils. The environment is interpreted as sand bars formed at the mouth of a delta's distributaries.

Significance of assemblages

In the White Sea region of Russia, all three assemblage types have been found in close proximity. This, and the faunas' considerable temporal overlap, makes it unlikely that they represent evolutionary stages or temporally distinct communities. Since they are globally distributed — described on all continents except Antarctica — geographical boundaries do not appear to be a factor; the same fossils are found at all palæolatitudes (the latitude where the fossil was created, accounting for continental drift) and in separate sedimentary basins.

It is most likely that the three assemblages mark organisms adapted to survival in different environments, and that any apparent patterns in diversity or age are in fact an artefact of the few samples that have been discovered — the timeline (right) demonstrates the paucity of Ediacaran fossil-bearing assemblages. An analysis of one of the White Sea fossil beds, where the layers cycle from continental sea-bed to inter-tidal to estuarine and back again a few times, found that a specific set of Ediacaran organisms was associated with each environment.

As the Ediacaran biota represent an early stage in multicellular life's history, it is unsurprising that not all possible modes of life are occupied. It has been estimated that of 92 potentially possible modes of life — combinations of feeding style, tiering and motility — no more than a dozen are occupied by the end of the Ediacaran. Just four are represented in the Avalon assemblage. The lack of large-scale predation and vertical burrowing are perhaps the most significant factors limiting the ecological diversity; the emergence of these during the Early Cambrian allowed the number of lifestyles occupied to rise to 30.