Results for Ediacaran biota
On this page:
 
Sci-Tech Encyclopedia:

Ediacaran biota

A widely distributed group of soft-bodied marine organisms that are preserved as fossils in rocks of latest Proterozoic age (600–543 million years ago; Ma). The biota characterizes a geological period, known as the Ediacarian or the Vendian, which precedes the widespread appearance of animals with mineralized skeletons. The name Ediacara refers to an abandoned mining area about 380 mi (600 km) north of Adelaide, South Australia.

Although Ediacaran fossils were described in the 1930s from southwest Africa (Namibia), it was the discovery in 1946 of abundant fossil “jellyfish” at Ediacara that sparked international interest in this biota. Subsequently, similar or identical fossils were found in England; Newfoundland; Russia, the Ukraine, Siberia; North Carolina; Canada; Nevada; and elsewhere. About 30 localities have been described, with the most diverse biotas found at Ediacara, Namibia, and on the coast of the White Sea in northern Russia. As some of these sites could not have been less than about 6000 mi (10,000 km) apart, no matter how the continents were arranged at the time, there is no doubt that the Ediacaran biota was a globally distributed marine biota.

Most known occurrences of Ediacaran organisms precede the earliest great radiation of skeletal fossils (archaeocyaths, trilobites, mollusks, brachiopods), or else they can be placed as latest Proterozoic on other evidence. Radiometric dates have confirmed that there was no significant time gap between the disappearance of the Ediacaran organisms and the Cambrian radiations. In fact, a few Ediacaran fossils have been found in Cambrian strata; the biota did not entirely die out before the Cambrian. See also Cambrian.

At almost all sites, the fossils are preserved as impressions in some kind of detrital sedimentary rock. Commonly, they are found on the bases of sandstone beds (South Australia), within sandstone beds (Namibia), or below volcanic ashes swept into deep water by wind and turbidity currents (Newfoundland). The organisms appear to have lived in continental shelf to slope environments and are normally preserved in sediments that were deposited under fairly quiet conditions below normal wave base.

At a conservative estimate there are probably 40–50 distinct genera or probable genera of Ediacaran organisms worldwide. Assigning these genera to higher taxa, however, has been controversial. A real problem is that few of these fossils can unequivocally be referred to living or extinct animal taxa. Because many of the fossils are simply circular structures with or without radial or concentric markings, they impart little information and are difficult to interpret. The recognition of many critical features is hampered by the nature of the preservation. In addition, many Ediacaran fossils, notably most forms from Namibia, are so unusual in shape that they cannot be placed firmly in any modern group. Many workers have placed the more unusual organisms in an extinct higher taxon of phylum grade, commonly named the Petalonamae. Others have used differences in symmetry and body organization to identify and characterize several major taxonomic groups regarded, in general, as extinct higher taxa of phylum or class grade. Some have proposed that all of the Ediacaran organisms were constructed on a single basic plan that was radically different from any other animal. According to this hypothesis, the Ediacarans were “quilted” organisms lacking heads, muscles, or digestive systems, and consisted of parallel sheetlike walls held together by regularly spaced internal partitions, and the whole organism was inflated by body fluids. Proponents of this hypothesis classify the Ediacaran organisms in an extinct kingdom of multicellular life, the Vendobionta. Still others have proposed that the Ediacaran organisms belonged to extant kingdoms other than the animals; they were lichens, algae, or single-celled or colonial protists. Controversy still reigns, but in all likelihood no hypothesis is entirely incorrect.

The marks left in soft sediments by otherwise unknown animals provide another source of knowledge of late Precambrian animal life. The figure-8-shaped trail in the illustration indicates that animals capable of directed, muscular, gliding motion (like that of a garden snail) coexisted with more typical members of the Ediacaran biota. In a similar fashion, strings of fecal pellets demonstrate the existence of animals with one-way guts, and closely meandering marks imply an ability for systematic grazing. There is little evidence for vertical burrowing in rocks of this age.

Ediacaran trace fossil <i>Gordia</i> found on the sole of a sandstone bed in South Australia gives clear evidence for the existence of mobile animals in the late Precambrian.
Ediacaran trace fossil Gordia found on the sole of a sandstone bed in South Australia gives clear evidence for the existence of mobile animals in the late Precambrian.

Although many Ediacaran fossils are enigmatic, there is sound evidence that sponges, cnidarians, bilaterian worms, and possibly arthropods and other phyla were present. This implies that the Animalia originated even farther back in time. The largest Ediacaran fossils, reaching up to 3 ft (1 m) in length, are flattened “fronds” that had large surfaces compared with their volumes. Proponents of the Vendobionta hypothesis have claimed that such large organisms, lacking guts or muscles, must have been photosynthetic or chemosynthetic and probably contained symbiotic, photosynthetic microorganisms. However, such a lifestyle is also found in modern reef corals and various other marine animals. Alternatively, the high ratio of surface to volume may represent adaptations to an atmosphere and hydrosphere relatively low in oxygen. See also Paleontology.

 
 
Wikipedia: Ediacaran biota
Dickinsonia costata, an Ediacaran organism of unknown affinity, with a quilted appearance.
Enlarge
Dickinsonia costata, an Ediacaran organism of unknown affinity, with a quilted appearance.

The Ediacaran (IPA: /ˌiːdɪˈækərən/, formerly Vendian) biota are ancient lifeforms, of the Ediacaran Period, that 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 Ediacaran 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 610 million years ago and flourished until the cusp of the Cambrian 542 million years ago, when their characteristic fossil communities 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 fragments of once-thriving ecosystems, if anything.[1] 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 show bilateral symmetry, a trait unique today to the Bilateria — a huge grouping containing most of the animal kingdom. Fossilised tracks of burrowing, worm-like organisms are also likely to have been made by bilaterians. However, most non-microscopic fossils are morphologically distinct from later lifeforms and 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 Vendobionta (now renamed Vendozoa).[2] 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.[3]


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.[4] However, since they lay below the "Primordial Strata" (i.e., the Cambrian strata), then thought to contain the very first signs of life, it took four years until someone, Elkanah Billings, dared to propose they could be fossils. Their simple form caused Billings' peers to dismiss his proposal, and they were instead declared gas escape structures, inorganic concretions, or even tricks played by a malicious God to promote unbelief.[4] No similar structures elsewhere in the world were then known, and the one-sided debate soon fell into obscurity.[4] In 1933, Gürich discovered specimens in Namibia,[5] but the belief that life originated in the Cambrian led to them being assigned there, and no link to Aspidella was made. In 1946, Reg Sprigg noticed "jellyfishes" in the Ediacara Hills of Australia's Flinders Ranges[6] 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 Ediacaran was seriously considered as containing life. This frond-shaped fossil was found in England's Charnwood Forest,[7] 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,[8][9] and with a combination of improved dating of existing specimens and an injection of vigour into the search, many more instances were recognised.[10]

However, all specimens discovered until 1967 were in coarse-grained sandstone that prevented preservation of fine details, making interpretation difficult. Mistra'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.[11][12]

Poor communication, combined with the difficulty in correlating globally distinct formations, has 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 "Sinian" and "Vendian",[13] terms for terminal-Precambrian rocks which 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.[14]

Preservation

The fossil Charniodiscus is barely distinguishable from the "elephant skin" texture on this cast.
Enlarge
The fossil Charniodiscus is barely distinguishable from the "elephant skin" texture on this cast.

All but the smallest fraction of the fossil record is comprised 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;[15] 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.[16] Most Ediacaran strata with the "elephant skin" texture that signifies a microbial mat 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,[17] and these communities are now limited to inhospitable refugia where predators cannot survive long enough to eat them.

Fossilisation

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.[18] Ash beds provide more detail, and can readily be precisely dated to the nearest million years or better by means of radiometric dating.[19] 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.[18] Soft-bodied organisms today almost never fossilise during such events, but the presence of widespread microbial mats aided preservation by stabilising their impressions in the sediment below.[20]

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 destroying them as it would the less-resistant discs. Further, in some cases, the bacterial precipitation of minerals formed a "death mask", creating a mould of the organism.[4]

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.

The earliest discovered potential embryo, preserved within an acanthomorphic acritarch.
Cyclomedusa, a disc shaped fossil that has been interpreted as a microbial artefact. Metric scale. Cyclomedusa, a disc shaped fossil that has been interpreted as a microbial artefact
A cast of the quilted Charnia, the first accepted complex Precambrian organism. Charnia was once interpreted as a relative of the sea-pens. A cast of Charnia
Spriggina, a possible precursor to the Trilobites, may be one of the predators that led to the demise of the Ediacaran fauna[21] and subsequent diversification of animals.[22] Spriggina may be one of the predators that led to the demise of the Ediacaran fauna
A late Ediacaran trace fossil preserved on a bedding plane
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.[18] 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[23] generated intense media excitement[24] though some have claimed they are instead inorganic structures formed by the precipitation of minerals on the inside of a hole.[25] Other "embryos" have been interpreted as the remains of the giant sulfur-reducing bacteria Thiomargarita,[26] a view which is highly contested.[27][28]
Microfossils dating from 632.5 million years ago — 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.[29]
Discs 
Circular fossils, such as Ediacaria, Cyclomedusa, and Rugoconites led to the initial identification of Ediacaran fossils as cnidaria, which include jellyfish and corals.[6] Further examination has provided alternative interpretations of all disc-shaped fossils: none is now confidently recognised as jellyfish. Alternate explanations include holdfasts, protists[30] and anemones; the patterns displayed where two meet have led to many being recognised as microbial colonies.[31][32] 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.[33]
Quilted organisms 
The organisms considered in Seilacher's revised definition of the Vendobionta[2] 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.[34]
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 upon it (plesiomorphy).
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.[25][35] Later examples, almost universally accepted as bilaterians, include the mollusc-like Kimberella,[36] Spriggina (pictured),[21] and the shield-shaped Parvancorina,[37] whose affinities are currently debated.[38]
A suite of fossils known as the Small Shelly Fossils are represented in the Ediacaran, most famously by Cloudina,[39] 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, 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.[40] Putative "burrows" dating as far back as 1100 million years may have been made by animals which fed on the undersides of microbial mats, which would have shielded them from a chemically unpleasant ocean;[41] however their uneven width and tapering ends make a biological origin difficult to defend.[42] 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;[43] the method of formation of these disconnected and overlapping fossils largely remains a mystery. The potential mollusc Kimberella is associated with scratch marks thought to have been formed by its radula,[44] further traces from 555 million years ago appear to imply active crawling or burrowing activity.[44]

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.

A sea-pen, a cnidarian bearing a passing resemblance to Charnia
Enlarge
A sea-pen, a cnidarian bearing a passing resemblance to Charnia

Cnidarians

Since the most primitive metazoans — multi-cellular animals in possession of a nervous system — are recognised as cnidarians, the first attempt to categorise these fossils designated them as jellyfish and sea-pens.[45] However, detailed study of their growth pattern has discounted this hypothesis.[46]

"The dawn of animal life"

Martin Glaessner proposed in his 1985 book "The dawn of animal life" that the Ediacaran biota were early stem group members of all modern phyla, and were unrecognisable because they had yet to evolve the characteristic features we use in modern classification.[47] Adolf Seilacher responded by suggesting that the Ediacaran sees animals usurping giant protists as the dominant life form.[48]

Mark McMenamin goes one step further: he claims that Ediacarans did not possess an embryonic stage, and thus could not be animals. He believes 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."[30]

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,[49][50] 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.[51]

Lichen with a 3D structure may be preserved in a similar fashion to wood.
Enlarge
Lichen with a 3D structure may be preserved in a similar fashion to wood.

Lichens

Gregory Retallack's hypothesis that Ediacaran organisms were lichens[52] 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,[53] from algæ,[54] to protists known as foraminifera,[55] to fungi[56] to bacterial or microbial colonies,[31] to hypothetical intermediates between plants and animals.[57] Since representatives of almost all modern phyla were in existence by the Middle Cambrian, it is probable that the precursors of many phyla would be represented in the Ediacaran. The accumulation of random changes in sequences of DNA — assumed to accumulate at a constant rate — can be used to estimate the time that two lineages shared a common ancestor, and applying this technique to modern phyla produces estimated divergence dates long before the Cambrian.[58] If this is indeed the case, attempts to group everything alive in the Ediacaran into one phylum are doomed to failure.

Origin

It took 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 3,460 million years ago,[59][60] the first uncontroversial evidence for life is found 2,700 million years ago,[61] and cells with nuclei certainly existed by 1,200 million years ago:[62] 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 appearing as time progresses, with traces of earlier semi-complex life such as Nimbia, found in the 610 million-year-old Twitya formation,[63] possibly displaying the most complex morphology of the time.

Global ice sheets may have delayed or prevented the establishment of multicellular life.
Enlarge
Global ice sheets may have delayed or prevented the establishment of multicellular life.

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, and other reactive elements had been oxidised. Donald Canfield detected records of the first significant quantities of atmospheric oxygen just before the first Ediacaran fossils appeared[64] — and the presence of atmospheric oxygen was soon heralded as a possible trigger for the Ediacaran radiation.[65] 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.[66] 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.[67]

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.[68] 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.

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.[16] However, if they were common, more than the occasional specimen[1] might be expected in exceptionally preserved fossil assemblages (Konservat-Lagerstätten) such as the Burgess Shale and Chengjiang[69] — unless such assemblages represent an environment never occupied by the Ediacaran biota, or unsuitable conditions for their preservation.

Kimberella may have had a predatory or grazing lifestyle.
Enlarge
Kimberella may have had a predatory or grazing lifestyle.

Predation and grazing

By the Early Cambrian, organisms higher in the food chain caused the microbial mats to largely disappear. These grazers first appeared as the Ediacaran biota started to decline, which 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.

Alternatively, skeletonised animals could have fed directly on the relatively undefended Ediacaran biota.[30] However, the existence in the Ediacaran of the recognized predator Kimberella suggests that the biota had already had limited exposure to predation.[36]

Cambrian animals such as Waptia may have competed with, or fed upon, Ediacaran lifeforms.
Enlarge
Cambrian animals such as Waptia may have competed with, or fed upon, Ediacaran lifeforms.

Competition

It is possible that increased competition due to the evolution of key innovations amongst other groups, perhaps as a response to predation,[15] 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.[70]

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,[71] rising sea levels (creating shallow, "life-friendly" seas),[72] a nutrient crisis,[73] fluctuations in atmospheric composition, including oxygen and carbon dioxide levels,[74] and changes in ocean chemistry[75] (promoting biomineralisation)[76] could all have played a part.

Assemblages

Ediacaran-type fossils are recognised globally in 25 localities[14] and a variety of depositional conditions, and are commonly grouped into three main types, named after typical localities.

Ediacara-type assemblage

The Ediacara-type assemblage is named after Australia's Ediacara Hills, and consist of fossils preserved in prodeltaic facies (areas near the mouths of rivers). 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.[77]

Biota ranges[77]
view • talk • 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,[33] while Guy Narbonne maintains they were surface dwellers.[78] 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.[77]

Avalon-type assemblage

The Avalon-type assemblage is defined at Mistaken Point in Canada, the oldest locality with a large quantity of Ediacaran fossils.[79] 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.

The biota comprises deep sea dwelling rangeomorphs[80] 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[81] — 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 just be too young for evolution to rich biota. Opinion is currently divided between these conflicting hypotheses.[77]

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;[82] the same fossils are found at all palæolatitudes (the latitude where the fossil was created, accounting for continental drift) and in separate sedimentary basins.[77]

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.

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.[83] 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.

Further reading

  • Simon Conway Morris (7 October 1999). The Crucible of Creation: The Burgess Shale and the Rise of Animals. ISBN 978-0-19-286202-0. 
  • Mark McMenamin (1998). The Garden of Ediacara: Discovering the First Complex Life, 368pp. ISBN 0231105584. 
  • Derek Briggs & Peter Crowther (Editors) (2001). Palæobiology II: A synthesis, Chapter 1. ISBN 0-632-05147-7.  Good further reading for the keen - includes many interesting chapters with macroevolutionary theme.

External links

See also

References

  1. ^ a b
  2. ^ a b
  3. ^ Narbonne, Guy (June 2006). The Origin and Early Evolution of Animals. Department of Geological Sciences and Geological Engineering, Queen's University. Retrieved on 2007-03-10.
  4. ^ a b c d
  5. ^ Gürich, G. (1933). "Die Kuibis-Fossilien der Nama-Formation von Südwestafrika" (in German) 15: 137-155. 
  6. ^ a b Sprigg, R.C. (1947). "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. 
  7. ^ Leicester’s fossil celebrity: Charnia and the evolution of early life. Retrieved on 2007-06-22.
  8. ^ Sprigg, R.C. (1991). "Martin F Glaessner: Palaeontologist extraordinaire". Mem. Geol. Soc. India 20: 13-20. 
  9. ^ Glaessner, M.F. (1959). "The oldest fossil faunas of South Australia". International Journal of Earth Sciences 47 (2): 522-531. Springer. DOI:10.1007/BF01800671. ISSN 1437-3254. 
  10. ^ Glaessner, Martin F. (1961). "Precambrian Animals". Science. Am. 204: 72-78. 
  11. ^ Misra, S.B. (1969). "Late Precambrian(?) fossils from southeastern Newfoundland". Geol. Soc. America Bull. 80: 2133-2140. DOI:10.1130/0016-7606(1969)80%5B2133:LPFFSN%5D2.0.CO;2. 
  12. ^ Badham, Mark (30 January 2003). The Mistaken Point Fossil Assemblage Newfoundland, Canada. The Miller Museum of Geology, Queen's University, Kingston, Ontario, Canada. Retrieved on 2007-03-10.
  13. ^ Termier, H.; Termier, G. (1960). "L’Ediacarien, premier etage paleontologique" (in French). Rev. Gen. Sci. et Bull. Assoc. Francaise Avan. Sci. 67 (3-4): 175–192. 
  14. ^ a b Knoll, Andy H.; Walter, M.; Narbonne, G.; Christie-Blick, N. (2006). "The Ediacaran Period: a new addition to the geologic time scale" (PDF). Lethaia 39: 13-30. DOI:10.1080/00241160500409223. Retrieved on 2007-04-14. Reprint, 2004 original available here (PDF).
  15. ^ a b
  16. ^ a b Runnegar, B.N.; Fedonkin, M.A. (1992). "Proterozoic metazoan body fossils", in Schopf, W.J.; Klein, C.: The Proterozoic biosphere, 369-388. ISBN 9780521366151. 
  17. ^ Burzin, M.B.; Debrenne, F.; Zhuravlev, A.Y. (2001). "Evolution of shallow-water level-bottom communities", in Zhuravlev, A.Y.; Riding, R.: The Ecology of the Cambrian Radiation, 216—237. ISBN 0231505167. Retrieved on 2007-05-06. 
  18. ^ a b c Narbonne, Guy M. (1998). "The Ediacara biota: A terminal Neoproterozoic experiment in the evolution of life". GSA 8 (2): 1-6. ISSN 1052-5173. Retrieved on 2007-03-08. 
  19. ^ Bowring, S.A.; Martin, M.W. (2001). "Calibration of the Fossil Record", in Briggs & Crowther: Palæobiology II: A synthesis. Blackwell publishing group. ISBN 9780632051496. Retrieved on 2007-06-21. 
  20. ^ Gehling, J.G. (1987). "Earliest known echinoderm — A new Ediacaran fossil from the Pound Subgroup of South Australia". Alcheringa 11: 337-345. ISSN 0311-5518. Retrieved on 2007-03-08. 
  21. ^ a b McMenamin, Mark A.S. (2003). "Spriggina is a Trilobitoid Ecdysozoan" in Seattle Annual Meeting of the GSA.  . Retrieved on 2007-06-21. 
  22. ^ e.g. Butterfield, N.J. (2007). "Macroevolution and microecology through deep time". Palaeontology 51 (1): 41-55. DOI:10.1111/j.1475-4983.2006.00613.x. 
  23. ^ Chen, J-Y (2004). "Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian". Science 305 (5681): 218-222. DOI:10.1126/science.1099213. Retrieved on 2007-04-27. 
  24. ^ For example, Fossil may be ancestor of most animals. msnbc. Retrieved on 2007-06-22., Leslie Mullen. Earliest Bilateral Fossil Discovered. Astrobiology Magazine. Retrieved on 2007-06-22.
  25. ^ a b
  26. ^ e.g. Bailey, J.V.; Joye, S.B., Kalanetra, K.M., Flood, B.E., Corsetti, F.A. (2007). "Evidence of giant sulphur bacteria in Neoproterozoic phosphorites". Nature 445 (7124): 198-201. DOI:10.1038/nature05457. Retrieved on 2007-04-28. , summarised by Donoghue, P.C.J. (2007). "Embryonic identity crisis". Nature 445: 155-156. DOI:10.1038/nature05520. ISSN 0028-0836. Retrieved on 2007-06-21. 
  27. ^ Xiao et al.'s response to Bailey et al.'s original paper : Xiao, S.; Zhou, C.; Yuan, X. (2007). "Palaeontology: undressing and redressing Ediacaran embryos". Nature 446 (7136): E9-E10. DOI:10.1038/nature05753. Retrieved on 2007-06-21.  And Bailey et al.'s reply: Bailey, J.V.; Joye, S.B.; Kalanetra, K.M.; Flood, B.E.; Corsetti, F.A. (2007). "Palaeontology: Undressing and redressing Ediacaran embryos (Reply)". Nature 446 (7136): E10-E11. DOI:10.1038/nature05754. Retrieved on 2007-06-21. 
  28. ^ Knoll, AH; Javaux, EJ, Hewitt, D., Cohen, P. (2006). "Eukaryotic organisms in Proterozoic oceans". Philosophical Transactions of the Royal Society B: Biological Sciences 361 (1470): 1023-1038. DOI:10.1098/rstb.2006.1843. Retrieved on 2007-06-21. 
  29. ^ Leiming, Y.; Zhu, M; Knoll, A; Yuan, X; Zhang, J; Hu, J (2007-04-05). "Doushantuo embryos preserved inside diapause egg cysts". Nature 446 (7136): 661-663. DOI:10.1038/nature05682. Retrieved on 2007-04-27. 
  30. ^ a b c
  31. ^ a b
  32. ^ Grazhdankin, D. (in press). "Ediacaran microbial colonies". Lethaia. DOI:10.1111/j.1502-3931.2007.00025.x. 
  33. ^ a b (a) The only current description, far from universal acceptance, appears as: Grazhdankin, D.; Seilacher, A. (2002). "Underground Vendobionta From Namibia". Palaeontology 45 (1): 57-78. DOI:10.1111/1475-4983.00227. 
  34. ^ Narbonne, G.M.; Saylor, B.Z. & Grotzinger, J.P. (1997). "The Youngest Ediacaran Fossils from Southern Africa". Journal of Paleontology 71 (6): 953-967. ISSN 0022-3360. Retrieved on 2007-06-21. 
  35. ^ (19 November 2004) "Response to Comment on "Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian"". Science 306: 1291. DOI:10.1126/science.1102328. 
  36. ^ a b Fedonkin, M.A.; Waggoner, B.M. (1997). "The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism". Nature 388 (6645): 868-871. DOI:10.1038/42242. ISSN 0028-0836. Retrieved on 2007-03-08. 
  37. ^ Glaessner, M.F. (1980). "Parvancorina — an arthropod from the late Precambrian of South Australia". Ann. Nat. Hist. Mus. Wien. 83: 83-90. Retrieved on 2007-03-08. 
  38. ^ For a reinterpretation, see Ivantsov, A.Y.; Malakhovskaya, Y.E., Serezhnikova, E.A. (2004). "Abstract [1] Some Problematic Fossils from the Vendian of the Southeastern White Sea Region]" (in Russian; English translation available). Paleontological Journal 38 (1): 1-9. ISSN 0031-0301. Retrieved on 2007-06-21. 
  39. ^ Germs, G.J.B. (October 1972). "New shelly fossils from Nama Group, South West Africa". American Journal of Science 272: 752-761. ISSN 0002-9599. 
  40. ^<