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Dickinsonia costata, an Ediacaran organism of unknown affinity, with a quilted appearance.

Fauna ediakarańska (IPA: [ˌiːdɪˈækəɹən] wcześniej fauna wendu (Vendian) biota – zespół wymarłych organizmów okresu ediakaru, reprezentujących najstarsze znane organizmy wielokomórkowe. Pojawiły się one wkrótce po "odwilży" epoki lodowcowej okresu kriogeńskiego, i w większości zniknęły tuż przed gwałtownym wzrostem bioróżnorodności znanym jako eksplozja kambryjska, z którego to okresu pochodzą najstarsze skamieniałości organizmów o planach budowy podobnych do tych, które cechują współcześnie żyjące organizmy zwierzęce. Różnorodność fauny ediakarańskiej w niewielkim stopniu przypomina nowe schematy budowy późniejszych organizmów: były to nowe, całkiem odrębne formy organizmów kambryjskich które zdominowały zapis kopalny ediakaru.

Organizmy ediakarańskie po raz pierwszy pojawiły się około 610 milionów lat temu i pozostawały w rozkwicie do początku kambru 542 milionów lat temu, kiedy z zapisu kopalnego zniknęły ich charakterystyczne (zespoły) skamieniałości. Najmłodsze skamieniałości nie są spotykane później niż u schyłku ediakaru, pozostawiając zaledwie fragmenty wcześniej kwitnących ekosystemów, o ile cokolwiek (if anything ;)) i tylko rzadkie skamieniałości mogące przynależeć do form które przeżyły znajdywano w warstwach tak młodych jak środkowy kambr (510-500 milionów lat temu) [1]. Sformułowano liczne hipotezy tłumaczące to zniknięcie, w tym hipotezę braku zapisu kopalnego (preservation bias), zmianę środowiska, pojawienie się drapieżnictwa i konkurencji z innymi organizmami.

Niektóre ediakarańskie organizmy mogły być blisko spokrewnione z grupami, które później osiągnęły znaczącą pozycję; przykładowo Kimberella wykazuje pewne podobieństwa do mięczaków, a u innych organizmów występuje symetria dwuboczna, cecha charakterystyczna dla podkrólestwa zwierząt dwubocznie symetrycznych — olbrzymiego taksonu obejmującego większość królestwa zwierząt. Skamieniałe ślady ryjących (?), przypominających robaki organizmów prawdopodobnie również zostawiły zwierzęta dwubocznie symetryczne. Jednak większość niemikroskopowych skamieniałości różni się morfologicznie od późniejszych form życia, i przypomina dyski, torby wypełnione mułem oraz pikowane materace. Klasyfikacja jest trudna, a przypisanie niektórych gatunków nawet na poziomie królestwa - zwierząt, grzybów, protistów lub innego - niepewne: pewien paleontolog uzyskał nawet poparcie dla idei oddzielnego królestwa Vendobionta (teraz zwanego Vendozoa).[2] Ich dziwne kształty oraz pozorny brak podobieństwa do późniejszych organizmów spowodowały, że niektórzy uznali je za "nieudany eksperyment" wielokomórkowego życia i uważali, że późniejsze wielokomórkowe formy życia niezależnie wyewoluowały ponownie z innych organizmów jednokomórkowych.[3] Ediacaran biota/Timeline2

Historia[edytuj | edytuj kod]

Pierwszymi odkrytymi skamieniałościami fauny ediakarańskiej były dyskokształtne Aspidella terranovica, odkryte w 1868 roku. Ich odkrywca, A. Murray, geodeta i geolog (?), uznał te skamieniałości za pomocne w ustaleniu wieku skał okolic Nowej Fundlandii.[4] Jednak, jako że warstwa, w której się znajdowały, leżała poniżej "Primordial Strata" (czyli kambryjskich)), uważanych wówczas za zawierające pierwsze ślady życia, zajęło cztery lata zanim ktoś (Elkanah Billings) odważył się wysunąć hipotezę, że mogą to być skamieniałości organizmów żywych. Ich prosty kształt był powodem, dla którego współcześni Billingsowi odrzucili jego propozycję i uznali skamieniałości za struktury powstałe skutek ucieczki gazu, nieorganiczne konkrecje lub nawet sztuczki, spłatane przez złośliwego Boga, aby odwieść ludzi od wiary.[4] Nie znano wówczas podobnych struktur z żadnego innego miejsca na świecie, a jednostronna debata wkrótce została zapomniana.[4] W 1933 roku Gürich odkrył skamieniałości w Namibii [5], ale pogląd o kambryjskich początkach życia sprawił, ze przypisano je do kambru i nie powiązano ich z Aspidella. W 1946 Reg Sprigg odnotował obecność skamieniałosci "meduz" w skałach australijskich Wzgórz Ediacara w Górach Flindersa [6], ale w owym czasie skały te datowano na wczesny kambr, i mimo pewnego zainteresowania odkryciem nie zyskało ono należytej uwagi.

Dopiero w wyniku brytyjskiego odkrycia ikony fauny ediakarańskiej - Charnii w 1957 zaczęto poważnie rozważać możliwość, że skamieniałości ediakarańskie zawierają ślady życia. Tę przypoinającą kształtem liść paproci skamieniałość odkryto w angielskim lesie Charnwood Forest,[7] a dzięki szczegółowym mapom geologicznym British Geological Survey nie było wątpliwości, że te skamieniałości znajdują się w skałach prekambryjskich. W końcu paleontolog Martin Glaessner zauważył związek między tym i wcześniejszymi odkryciami[8][9], a połączenie poprawionego datowania istniejących okazów i nowa energia w poszukiwaniach spowodowały, że odkryto wiele innych przypadków.[10]

Jednak wszytkie okazy odkryte przed 1967 znajdowały się w gruboziarnistym piaskowcu, w którym nie zachowały się drobne szczegóły, co utrudniało interpretację znalezisk. 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]

Słabe możliwości komunikacyjne oraz trudności w porównaniu oddalonych od siebie formacji geologicznych, doprowadziły do powstania wielu różnych nazw dla fauny. 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[edytuj | edytuj kod]

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.

Maty mikrobiologiczne[edytuj | edytuj kod]

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.

Fosylizacja[edytuj | edytuj kod]

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 roks 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]

Co się zachowało?[edytuj | edytuj kod]

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 fakt 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[edytuj | edytuj kod]

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 artefakt. Metric scale. Cyclomedusa, a disc shaped fossil that has been interpreted as a microbial artefakt
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 Szablon:Ma — just 3 million roks 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 and Cyclomedusa, 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]Błąd w przypisach: Nieprawidłowe nazwy parametrów elementu <ref>. Later examples, almost universally accepted as bilaterians, include the mollusc-like Kimberella,[35] Spriggina (pictured),[21] and the shield-shaped Parvancorina,[36] whose affinities are currently debated.[37]
A suite of fossils known as the Small Shelly Fossils are represented in the Ediacaran, most famously by Cloudina,[38] 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 roks.
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.[39] Putative "burrows" dating as far back as 1100 million roks may have been made by animals which fed on the undersides of microbial mats, which would have shielded them from a chemically unpleasant ocean;[40] however their uneven width and tapering ends make a biological origin difficult to defend.[41] 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;[42] 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,[43] further traces from Szablon:Ma appear to imply active crawling or burrowing activity.[43]

Classification and interpretation[edytuj | edytuj kod]

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

Cnidarians[edytuj | edytuj kod]

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.[44] However, detailed study of their growth pattern has discounted this hypothesis.[45]

"The dawn of animal life"[edytuj | edytuj kod]

Martin Glaessner proposed in his 1985 paper "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.[46] Adolf Seilacher responded by suggesting that the Ediacaran sees animals usurping giant protists as the dominant life form.[47]

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[edytuj | edytuj kod]

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,[48][49] 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.[50]

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

Lichens[edytuj | edytuj kod]

Gregory Retallack's hypothesis that Ediacaran organisms were lichens[51] 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[edytuj | edytuj kod]

Almost every possible phylum has been used to accommodate the Ediacaran biota,[52] from algæ,[53] to protists known as foraminifera,[54] to fungi[55] to bacterial or microbial colonies,[31] to hypothetical intermediates between plants and animals.[56] 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.[57] If this is indeed the case, attempts to group everything alive in the Ediacaran into one phylum are doomed to failure.

Origin[edytuj | edytuj kod]

It took 4 billion roks from the formation of the Earth for the Ediacaran fossils to first appear, 655 million roks ago. Whilst putative fossils are reported from Szablon:Ma,[58][59] the first uncontroversial evidence for life is found Szablon:Ma,[60] and cells with nuclei certainly existed by Szablon:Ma:[61] 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 roks 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-rok-old Twitya formation,[62] possibly displaying the most complex morphology of the time.

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 faktor 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[63] — and the presence of atmospheric oxygen was soon heralded as a possible trigger for the Ediacaran radiation.[64] 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.[65] 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.[66]

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 roks after the Earth emerged from a global glaciation, suggesting that ice cover and cold oceans may have prevented the emergence of multicellular life.[67] 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[edytuj | edytuj kod]

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[edytuj | edytuj kod]

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[68] — 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

Predation and grazing[edytuj | edytuj kod]

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.[potrzebny przypis]

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

Competition[edytuj | edytuj kod]

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.[69]

Change in environmental conditions[edytuj | edytuj kod]

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

Assemblages[edytuj | edytuj kod]

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[edytuj | edytuj kod]

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.[76]

Biota ranges[76]
view • talk • edit
Szablon:Include timeline
Axis scale: millions of roks ago, dated with U/Pb of zircons

Nama-type assemblage[edytuj | edytuj kod]

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.[77] 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.[76]

Avalon-type assemblage[edytuj | edytuj kod]

The Avalon-type assemblage is defined at Mistaken Point in Canada, the oldest locality with a large quantity of Ediacaran fossils.[78] 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[79] 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[80] — 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.[76]

Significance of assemblages[edytuj | edytuj kod]

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 faktor;[81] the same fossils are found at all palæolatitudes (the latitude where the fossil was created, accounting for continental drift) and in separate sedimentary basins.[76]

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 fakt an artefakt 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.[82] The lack of large-scale predation and vertical burrowing are perhaps the most significant faktors limiting the ecological diversity; the emergence of these during the Early Cambrian allowed the number of lifestyles occupied to rise to 30.

Polecane lektury[edytuj | edytuj kod]

  • Derek Briggs & Peter Crowther (Wydawcas): Palæobiology II: A synthesis. s. Rozdział 1. ISBN 0-632-05147-7. Dobra lektura dla keen(?) - zawiera wiele interesujących rozdziałów z macroevolutionary theme.

Linki zewnętrzne[edytuj | edytuj kod]

Zobacz też[edytuj | edytuj kod]

Przypisy[edytuj | edytuj kod]

  1. ↑ a b Conway Morris, S.. Ediacaran-like fossils in Cambrian Burgess Shale–type faunas of North America. „Palaeontology”. 36, s. 593–635, 1993. 
  2. ↑ a b Seilacher, A.. Vendobionta and Psammocorallia: lost constructions of Precambrian evolution. „Czasopismo of the Geological Society, London”. 149, s. 607-613, 1992. DOI: 10.1144/gsjgs.149.4.0607. ISSN 0016-7649. [dostęp 2007-06-21]. 
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[[ca:Fauna d'Ediacara]] [[da:Ediacara-faunaen]] [[de:Vendobionten]] [[en:Ediacaran biota]] [[es:Fauna de Ediacara]] [[fr:Faune d'Édiacara]] [[ja:エディアカラ生物群]]

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