TheEdiacaran (/ˌiːdiˈækərən/EE-dee-ACK-ər-ən; formerlyVendian)biota is ataxonomic period classification that consists of all life forms that were present on Earth during theEdiacaran Period (c. 635–538.8Mya). These were enigmatic tubular and frond-shaped, mostlysessile, organisms.[1][2]Trace fossils of these organisms have been found worldwide, and represent the earliest known complexmulticellular organisms. The term "Ediacara biota" has received criticism from some scientists due to its alleged inconsistency, arbitrary exclusion of certain fossils, and inability to be precisely defined.[3]
The Ediacaran biota may have undergoneevolutionary radiation in a proposed event called theAvalon explosion,575 million years ago.[4][5] This was after the Earth had thawed from theCryogenian period'sextensive glaciation. This biota largely disappeared with the rapid increase inbiodiversity known as theCambrian explosion. Most of the currently existingbody plans ofanimals first appeared in thefossil record of theCambrian rather than the Ediacaran. For macroorganisms, the Cambrian biota appears to have almost completely replaced the organisms that dominated the Ediacaran fossil record, although relationships are still a matter of debate.
The organisms of the Ediacaran Period first appeared around600 million years ago and flourished until the cusp of theCambrian538.8 million years ago, when the characteristic communities of fossils vanished. A diverse Ediacaran community was discovered in 1995 inSonora, Mexico, and is approximately 555 million years in age, roughly coeval with Ediacaran fossils of theEdiacara Hills inSouth Australia and theWhite Sea on the coast ofRussia.[6][7][8] While rare fossils that may represent survivors have been found as late as theMiddle Cambrian (510–500 Mya), the earlier fossil communitiesdisappear from the record at the end of the Ediacaran, leaving only curious fragments of once-thrivingecosystems.[9] Multiple hypotheses exist to explain the disappearance of this biota, includingpreservation bias, a changing environment, the advent ofpredators and competition from other life-forms. A sampling, reported in 2018, of late Ediacaran strata across the scattered remnants ofBaltica(< 560 Mya) suggests the flourishing of the organisms coincided with conditions of low overall productivity with a very high percentage produced by bacteria, which may have led to high concentrations of dissolved organic material in the oceans.[10]
Determining where Ediacaran organisms fit in thetree of life has proven challenging; it is not even established that most of them were animals, with suggestions that they werelichens (fungus-alga symbionts),algae,protists known asforaminifera,fungi ormicrobial colonies, or hypothetical intermediates between plants and animals.[11] The morphology and habit of some taxa (e.g.Funisia dorothea) suggest relationships toPorifera orCnidaria (e.g.Auroralumina).[12][13][14]Kimberella may show a similarity tomolluscs, and other organisms have been thought to possessbilateral symmetry, although this is controversial. Mostmacroscopic fossils aremorphologically distinct from later life-forms: they resemble discs, tubes, mud-filled bags or quilted mattresses. Due to the difficulty of deducing evolutionary relationships among these organisms, somepalaeontologists have suggested that these represent completely extinct lineages that do not resemble any living organism. PalaeontologistAdolf Seilacher proposed a separate subkingdom level categoryVendozoa (now renamedVendobionta)[15] in the Linnaean hierarchy for the Ediacaran biota. If these enigmatic organisms left no descendants, their strange forms might be seen as a "failed experiment" in multicellular life, with later multicellular life evolving independently from unrelated single-celled organisms.[16] A 2018 study confirmed that one of the period's most-prominent and iconic fossils,Dickinsonia, includedcholesterol,[17] suggesting affinities to animals, fungi, or red algae.[18]
The first Ediacaran fossils discovered were the disc-shapedAspidella terranovica in 1868. Their discoverer, Scottish geologistAlexander Murray, found them useful aids for correlating the age of rocks aroundNewfoundland.[21] However, since they lay below the "Primordial Strata" of theCambrian that was then thought to contain the very first signs of animal life, a proposal four years after their discovery byElkanah Billings that these simple forms represented fauna was dismissed by his peers. Instead, they were interpreted asgas escape structures or inorganicconcretions.[21] No similar structures elsewhere in the world were then known and the one-sided debate soon fell into obscurity.[21] In 1933,Georg Gürich discovered specimens inNamibia but assigned them to the Cambrian Period.[22] In 1946,Reg Sprigg noticed "jellyfishes" in theEdiacara Hills of Australia'sFlinders Ranges, which were at the time believed to be Early Cambrian.[23]
Palaeontologist Guy Narbonne examining Ediacaran fossils in Newfoundland
It was not until the British discovery of the iconicCharnia that thePrecambrian was seriously considered as containing life. Thisfrond-shaped fossil was found in England'sCharnwood Forest first by a 15 year-old girl in 1956 (Tina Negus, who was not believed[24][a]) and then the next year by a group of three schoolboys including 15 year-oldRoger Mason.[25][26][27] Due to the detailedgeological mapping of theBritish Geological Survey, there was no doubt these fossils sat in Precambrian rocks. PalaeontologistMartin Glaessner finally, in 1959, made the connection between this and the earlier finds[28][29] and with a combination of improved dating of existing specimens and an injection of vigour into the search, many more instances were recognised.[30]
All specimens discovered until 1967 were in coarse-grainedsandstone that prevented preservation of fine details, making interpretation difficult.S.B. Misra's discovery of fossiliferousash-beds at theMistaken Point assemblage in Newfoundland changed all this as the delicate detail preserved by the fine ash allowed the description of features that were previously undiscernible.[31][32] It was also the first discovery of Ediacarans in deep water sediments.[33]
Poor communication, combined with the difficulty in correlating globally distinctformations, led to a plethora of different names for the biota.In 1960 the French name "Ediacarien" – after the Ediacara Hills – was added to the competing terms "Sinian" and "Vendian"[34] for terminal-Precambrian rocks, and these names were also applied to the life-forms. "Ediacaran" and "Ediacarian" were subsequently applied to the epoch or period ofgeological time and its corresponding rocks. In March 2004, theInternational Union of Geological Sciences ended the inconsistency by formally naming the terminalperiod of theNeoproterozoic after the Australian locality.[35]
The term "Ediacaran biota" and similar ("Ediacara" / "Ediacaran" / "Ediacarian" / "Vendian" and "fauna" / "biota") has, at various times, been used in a geographic, stratigraphic, taphonomic, or biological sense, with the latter the most common in modern literature.[36]
Microbial mats are areas of sediment stabilised by the presence of colonies of microbes that 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") and tubercular texture.[37]
Some Ediacaran strata with the texture characteristics of microbial mats contain fossils, and Ediacaran fossils are almost always found in beds that contain these microbial mats. Although microbial mats were once widespread before theCambrian substrate revolution, the evolution of grazing organisms vastly reduced their numbers.[38] These communities are now limited to inhospitablerefugia, such as thestromatolites found inHamelin Pool Marine Nature Reserve inShark Bay,Western Australia, where the salt levels can be twice those of the surrounding sea.[39]
The fossilCharniodiscus is barely distinguishable from the "elephant skin" texture on this cast.
The preservation of Ediacaran fossils is of interest, since as soft-bodied organisms they would normally not fossilize. Further, unlike later soft-bodied fossil biota such as theBurgess Shale orSolnhofen Limestone, the Ediacaran biota is not found in a restricted environment subject to unusual local conditions: they are global. The processes that were operating must therefore have been systemic and worldwide. Something about the Ediacaran Period permitted these delicate creatures to be left behind; the fossils may have been preserved by virtue of rapid covering by ash or sand, trapping them against the mud or microbial mats on which they lived.[40] Their preservation was possibly enhanced by the high concentration of silica in the oceans before silica-secreting organisms such as sponges and diatoms became prevalent.[41] Ash beds provide more detail and can readily be dated to the nearest million years or better usingradiometric dating.[42] However, it is more common to find Ediacaran fossils under sandy beds deposited by storms or inturbidites formed by high-energy bottom-scraping ocean currents.[40] Soft-bodied organisms today rarely fossilize during such events, but the presence of widespread microbial mats probably aided preservation by stabilising their impressions in the sediment below.[43]
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, whereupon ash or sand slumped in to fill the void, leaving a cast of the organism's underside. Conversely,quilted fossils tended to decomposeafter 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 foundwithin storm beds as the high-energy sedimentation did not destroy them as it would have the less-resistant discs. Further, in some cases, thebacterialprecipitation of minerals formed a "death mask", ultimately leaving a positive, cast-like impression of the organism.[44][45]
The earliest discovered potential embryo, preserved within an acanthomorphicacritarch. The term 'acritarch' describes a range of unclassified cell-like fossils.
Tateana inflata ('Cyclomedusa' radiata) were originally believed to have been Medusoids, although recent research suggests that they were holdfasts ofPetalonamids.
A cast ofCharnia, the first accepted complex Precambrian organism.Charnia was once interpreted as a relative of thesea pens.
Dickinsonia displays the characteristic quilted appearance of Ediacaran enigmata.
Spriggina was originally interpreted asannelid orarthropod. However, lack of known limbs, and glide reflectedisomers instead of truesegments, rejects any such classification despite some superficial resemblance.
Late EdiacaranArchaeonassa-type trace fossils are commonly preserved on the top surfaces of sandstone strata.
Epibaion waggoneris, chain of trace platforms and the imprint of the body ofYorgia waggoneri (right), which created these traces on microbial mat.
The Ediacaran biota exhibited a vast range ofmorphological characteristics. Size ranged from millimetres to metres; complexity from "blob-like" to intricate; rigidity from sturdy and resistant to jelly-soft. Almost all forms ofsymmetry were present. These organisms differed from earlier, mainly microbial, fossils in having an organised, differentiated multicellular construction and centimetre-plus sizes.[40]
These disparate morphologies can be broadly grouped intoform taxa:
"Embryos"
Recent discoveries of Precambrian multicellular life have been dominated by reports of embryos, particularly from theDoushantuo Formation in China. Some finds[46] generated intense media excitement[47] though some have claimed they are instead inorganic structures formed by the precipitation of minerals on the inside of a hole.[48] Other "embryos" have been interpreted as the remains of the giantsulfur-reducing bacteria akin toThiomargarita,[49] a view that, while it had enjoyed a notable gain of supporters[50][51] as of 2007, has since suffered following further research comparing the potential Doushantuo embryos' morphologies with those ofThiomargarita specimens, both living and in various stages of decay.[52] A recent discovery of comparable Ediacaran fossil embryos from the Portfjeld Formation in Greenland has significantly expanded the paleogeographical occurrence of Doushantuo-type fossil "embryos" with similar biotic forms now reported from differing paleolatitudes.[53]
Microfossils dating from632.5 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.[54] An alternative proposal is that these structures represent adult stages of the multicellular organisms of this period.[55] Microfossils ofCaveasphaera are thought to foreshadow the evolutionary origin of animal-like embryology.[56]
Discs
Circular fossils, such asEdiacaria,Cyclomedusa, andRugoconites led to the initial identification of Ediacaran fossils ascnidaria, which include jellyfish and corals.[23] Further examination has provided alternative interpretations of all disc-shaped fossils: not one is now confidently recognised as a jellyfish. Alternate explanations includeholdfasts andprotists;[57] the patterns displayed where two meet have led to many 'individuals' being identified as microbial colonies,[58][59] and yet others may represent scratch marks formed as stalked organisms spun around their holdfasts.[60]
Bags
Fossils such asPteridinium preserved within sediment layers resemble "mud-filled bags". The scientific community is a long way from reaching a consensus on their interpretation.[61]
Toroids
The fossilVendoglossa tuberculata from the Nama Group, Namibia, has been interpreted as a dorso-ventrally compressed stem-group metazoan, with a large gut cavity and a transversely ridgedectoderm. The organism is in the shape of a flattened torus, with the long axis of its toroidal body running through the approximate center of the presumed gut cavity.[62]
Quilted organisms
The organisms considered in Seilacher's revised definition of the Vendobionta[15] share a "quilted" appearance and resembled an inflatablemattress. 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 ofSwartpuntia germsi could only be recognised in a posthumously damaged specimen – usually multiple fronds were hidden as burial squashed the organisms flat.[63] These organisms appear to form two groups: thefractalrangeomorphs and the simplererniettomorphs.[64] Including such fossils as the iconicCharnia andSwartpuntia, the group is both the most iconic of the Ediacaran biota and the most difficult to place within the existingtree of life. Lacking any mouth, gut, reproductive organs, or indeed any evidence of internal anatomy, their lifestyle was somewhat peculiar by modern standards; the most widely accepted hypothesis holds that they sucked nutrients out of the surrounding seawater by osmotrophy[65] orosmosis.[66] However, others argue against this.[67]
Non-Vendobionts
Possibleearly forms oflivingphyla, excluding them from some definitions of the Ediacaran biota. The earliest such fossil is the reputed bilaterianVernanimalcula claimed by some, however, to represent the infilling of an egg-sac oracritarch.[48][68] In 2020,Ikaria wariootia was claimed to represent one of the oldest organisms with anterior and posterior differentiation.[69] Later examples are almost universally accepted as bilaterians and include the mollusc-likeKimberella,[70]Spriggina (pictured)[71] and the shield-shapedParvancorina,[72] whose affinities are currently debated.[73] A suite of fossils known as thesmall shelly fossils are represented in the Ediacaran, most famously byCloudina,[74] a shelly tube-like fossil that often shows evidence of predatory boring, suggesting that, while predation may not have been common in the Ediacaran Period, it was at least present.[75][76] Organic microfossils known assmall carbonaceous fossils are found in Ediacaran sediments, including the spiral-shapedCochleatina which spans the Ediacaran–Cambrian boundary.[77]Ediacaria also survived well into the Cambrian. Representatives of modern taxa existed in the Ediacaran, some of which are recognisable today.Sponges, red and greenalgæ,protists andbacteria are all easily recognisable with some pre-dating the Ediacaran by nearly three billion years. Possible arthropods have also been described.[78] Surface trails left byTreptichnus bear similarities to modernpriapulids. Fossils of the hard-shelledforaminiferaPlatysolenites are known from the latest Ediacaran of westernSiberia, coexisting withCloudina andNamacalathus.[79]
Filaments
Filament-shaped structures in Precambrian fossils have been observed on many occasions.Frondose fossils in Newfoundland have been observed to have developed filamentous bedding planes, inferred to bestolonic outgrowths.[80] A study of Brazilian Ediacaran fossils found filamentous microfossils, suggested to be eukaryotes or largesulfur-oxidizing-bacteria (SOBs).[81] Fungus-like filaments found in the Doushantuo Formation have been interpreted as eukaryotes and possibly fungi, providing possible evidence for the evolution and terrestrialization of fungi ~635 Ma.[82]
With the exception of some very simplevertical burrows[83] the only Ediacaran burrows are horizontal, lying on or just below the surface of the seafloor. Such burrows have been taken to imply the presence of motile organisms with heads, which would probably have had a bilateral symmetry. This could place them in thebilateralclade ofanimals[84] but they could also have been made by simpler organisms feeding as they slowly rolled along the sea floor.[85] Putative "burrows" dating as far back as1,100 million years may have been made by animals that fed on the undersides of microbial mats, which would have shielded them from a chemically unpleasant ocean;[86] however their uneven width and tapering ends make a biological origin so difficult to defend[87] that even the original proponent no longer believes they are authentic.[88]
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 andDickinsonia are often found at the end oflong pathways of trace fossils matching their shape;[89] these fossils are thought to be associated withciliary feeding but the precise method of formation of these disconnected and overlapping fossils largely remains a mystery.[90] The potentialmolluscKimberella is associated with scratch marks, perhaps formed by aradula.[91]
A reconstruction of the Ediacaran biota at theField Museum in Chicago
Classification of the Ediacarans is inevitably difficult, hence a variety of theories exist as to their placement on the tree of life.
Martin Glaessner proposed inThe Dawn of Animal Life (1984) that the Ediacaran biota were recognizablecrown group members of modern phyla, but were unfamiliar because they had yet to evolve the characteristic features we use in modern classification.[92]
In 1998Mark McMenamin claimed Ediacarans did not possess anembryonic stage, and thus could not be animals. He believed that they independently evolved anervous system andbrains, meaning that "the path toward intelligent life was embarked upon more than once on this planet".[57]
In 2018 analysis of ancientsterols was taken as evidence that one of the period's most-prominent and iconic fossils,Dickinsonia, was an early animal.[17]
Asea pen, a modern cnidarian bearing a passing resemblance toCharnia
Since the most primitiveeumetazoans—multi-cellular animals with tissues—arecnidarians, and the first recognized Ediacaran fossilCharnia looks very much like asea pen, the first attempt to categorise these fossils designated them asjellyfish andsea pens.[93] However, more recent discoveries have established that many of the circular forms formerly considered "cnidarian medusa" are actually holdfasts – sand-filled vesicles occurring at the base of the stem of upright frond-like Ediacarans. A notable example is the form known asCharniodiscus, a circular impression later found to be attached to the long 'stem' of a frond-like organism that now bears the name.[94][95]
The link between frond-like Ediacarans and sea pens has been thrown into doubt by multiple lines of evidence; chiefly the derived nature of the most frond-like pennatulacean octocorals, their absence from the fossil record before the Cenozoic, and the apparent cohesion between segments in Ediacaran frond-like organisms.[96] Some researchers have suggested that an analysis of "growth poles" discredits the pennatulacean nature of Ediacaran fronds.[97][98]
Adolf Seilacher has suggested that in the Ediacaran, animals take over from giantprotists as the dominant life form.[99] The modernxenophyophores are giant single-celled protozoans found throughout the world's oceans, largely on theabyssal plain. Genomic evidence suggests that the xenophyophores are a specialised group ofForaminifera.[100]
Seilacher has suggested that the Ediacaran organisms represented a unique and extinct grouping of related forms descended from a common ancestor (clade) and created thekingdom Vendozoa,[101][102] named after the now-obsoleteVendian era. He later excluded fossils identified asmetazoans and relaunched thephylum "Vendobionta", which he described as "quilted"cnidarians lackingstinging cells. This absence precludes the current cnidarian method of feeding, so Seilacher suggested that the organisms may have survived bysymbiosis withphotosynthetic orchemoautotrophic organisms.[103]Mark McMenamin saw such feeding strategies as characteristic for the entire biota, and referred to the marine biota of this period as a "Garden of Ediacara".[104]
Greg Retallack's analysis of thin sections and substrates of a variety of Ediacaran fossils.[105] His findings have been disputed by other scientists.[106][107][108]
Greg Retallack has proposed that Ediacaran organisms werelichens.[109][110] He argues that thin sections of Ediacaran fossils show lichen-like compartments andhypha-like wisps of ferruginized clay,[105] and that Ediacaran fossils have been found in strata that he interprets as desert soils.[110][111]
The suggestion has been disputed by other scientists; some have described the evidence as ambiguous and unconvincing, for instance noting thatDickinsonia fossils have been found on rippled surfaces (suggesting a marine environment), while trace fossils likeRadulichnus could not have been caused by needle ice as Retallack has proposed.[106][107][108] Ben Waggoner notes that the suggestion would place the root of theCnidaria back from around 900 mya to between 1500 mya and 2000 mya, contradicting much other evidence.[112][113] Matthew Nelsen, examining phylogenies ofascomycete fungi andchlorophyte algae (components of lichens), calibrated for time, finds no support for the hypothesis that lichens predated thevascular plants.[114]
A new extant genus discovered in 2014,Dendrogramma, which at the time of discovery appeared to be abasal metazoan but of unknown taxonomic placement, had been noted to have similarities with the Ediacaran fauna.[119] It has since been found to be asiphonophore, possibly even sections of a more complex species.[120]
It took almost 4 billion years from the formation of the Earth for Ediacaran fossils to first appear, 655 million years ago. While putative fossils are reported from3,460 million years ago,[121][122] the first uncontroversial evidence for life is found2,700 million years ago,[123] and cells with nuclei certainly existed by1,200 million years ago.[124]
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 morecomplex forms of life evolving as time progresses, with traces of earlier semi-complex life such asNimbia, found in the610 million year old Twitya formation,[125] and older rocks dating to770 million years ago in Kazakhstan.[126]
Globalice sheets might have delayed or prevented the establishment of multicellular life.
On the early Earth, reactive elements, such asiron anduranium, existed in areduced form that would react with any free oxygen produced byphotosynthesising organisms. Oxygen would not be able to build up in theatmosphere until all the iron had rusted (producingbanded iron formations), and all the 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[127] – and the presence of atmospheric oxygen was soon heralded as a possible trigger for the Ediacaranradiation.[128] 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.[129] However, the assumptions underlying the reconstruction of atmospheric composition have attracted some criticism, with widespread anoxia having little effect on life where it occurs in the Early Cambrian and the Cretaceous.[130]
Periods of intense cold have also been suggested as a barrier to the evolution of multicellular life.The earliest known embryos, from China'sDoushantuo Formation, appear just a million years after the Earth emerged from aglobal glaciation, suggesting that ice cover and cold oceans may have prevented the emergence of multicellular life.[131]
In early 2008, a team analysed the range of basic body structures ("disparity") of Ediacaran organisms from three different fossil beds: Avalon in Canada,575 million years ago to565 million years ago; White Sea in Russia,560 million years ago to550 million years ago; and Nama in Namibia,550 million years ago to542 million years ago, 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 famousCambrian explosion.[132]
The paucity of Ediacaran fossils after the Cambrian could simply be due to conditions no longer favoring the fossilization of Ediacaran organisms, which may have continued to thrive unpreserved for a considerable time.[37] However, if they were common, more than the occasional specimen[9][133] might be expected in exceptionallypreserved fossil assemblages (Konservat-Lagerstätten) such as theBurgess Shale andChengjiang.[134] Although no reports of Ediacara-type organisms in the Cambrian period are widely accepted at present, a few disputed reports have been made, as well as unpublished observations of 'vendobiont' fossils from 535 Ma Orsten-type deposits in China.[135]
Kimberella might have had a predatory or grazing lifestyle.
It has been suggested that by the Early Cambrian, organisms higher in thefood 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 mats in a "Cambrian substrate revolution", leading to displacement or detachment of the biota; or that the destruction of the microbialsubstrate destabilized the ecosystem, causing extinctions.[136][137]
Alternatively, skeletonized animals could have fed directly on the relatively undefended Ediacaran biota.[57]However, if the interpretation of the Ediacaran ageKimberella as a grazer is correct then this suggests that the biota had already had limited exposure to "predation".[70]
Cambrian animals such asWaptia might have competed with, or fed upon, Ediacaran life-forms.
Increased competition due to the evolution of key innovations among other groups, perhaps as a response to predation, may have driven the Ediacaran biota from their niches.[138] However, the supposed "competitive exclusion" ofbrachiopods bybivalve molluscs was eventually deemed to be a coincidental result of two unrelated trends.[139]
Great changes were happening at the end of the Precambrian and the start of the Early Cambrian. The breakup of thesupercontinents,[140] rising sea levels (creating shallow, "life-friendly" seas),[141] a nutrient crisis,[142] fluctuations in atmospheric composition, including oxygen and carbon dioxide levels,[143] and changes inocean chemistry[144] (promotingbiomineralisation) could all have played a part.[145]
Late Ediacaran macrofossils are recognized globally in at least 52 formations and a variety ofdepositional conditions.[146] Each formation is commonly grouped into three main types, known asassemblages and named after typical localities. Each assemblage tends to occupy its own time period and region of morphospace, and after an initial burst of diversification (or extinction) changes little for the rest of its existence.[147]
The Avalon assemblage is defined atMistaken Point one theAvalon Peninsula of Canada, the oldest locality with a large quantity of Ediacaran fossils.[149]The assemblage is easily dated because it contains many fine ash-beds, which are a good source ofzircons used in the uranium-lead method ofradiometric 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.[147]
One interpretation of the biota is as deep-sea-dwellingrangeomorphs[150] such asCharnia, all of which share afractal growth pattern. They were probably preservedin situ (without post-mortem transportation), although this point is not universally accepted. The assemblage, while less diverse than the White Sea or Nama assemblages, resemblesCarboniferous suspension-feeding communities, which may suggestfilter feeding as the assemblage is often found in water too deep for photosynthesis.[151]
The White Sea or Ediacaran assemblage is named after Russia'sWhite Sea or Australia'sEdiacara Hills and is marked by much higher diversity than the Avalon or Nama assemblages.[146] Most fossils are preserved as imprints in microbial beds,[152] but a few are preservedwithin sandy units.[153][147]
In Australia, they are typically found in red gypsiferous and calcareous paleosols formed onloess and flood deposits in an arid cool temperate paleoclimate.[110] Since the early 2000s, around 40 fossil surfaces preserving organisms from the White Sea Assemblage have been excavated from the Ediacara Member of the Rawnsley Quartzite in the Ediacara Hills, which lie within theNilpena Ediacara National Park, west of theFlinders Ranges inSouth Australia. The fossil bed known as 1T-F has the highest diversity of Ediacaran fossils found so far, which also show significant ecological complexity. The bed includes more than 400 fossils across 16 genera.[154]
The Nama assemblage is best represented inNamibia. It is marked by extreme biotic turnover, with rates of extinction exceeding rates of origination for the whole period.[146] Three-dimensional preservation is most common, with organisms preserved in sandy beds containing internal bedding. Dima Grazhdankin believes that these fossils represent burrowing organisms,[61] while Guy Narbonne maintains they were surface dwellers.[155] These beds are sandwiched between units comprising interbedded sandstones, siltstones andshales—with microbial mats, where present, usually containing the fossils. The environment is interpreted assand bars formed at the mouth of adelta'sdistributaries.[153] Mattress-like vendobionts (Ernietta,Pteridinium,Rangea) in these sandstones form a very different assemblage from vermiform fossils (Cloudina,Namacalathus) of Ediacaran "wormworld" in marinedolomite of Namibia.[156]
Since they are globally distributed – described on all continents exceptAntarctica – geographical boundaries do not appear to be a factor;[157] the same fossils are found at all palaeolatitudes (the latitude where the fossil was created, accounting forcontinental drift - an application ofpaleomagnetism) and in separatesedimentary basins.[153] An analysis of one of the White Sea fossil beds, where the layers cycle from continental seabed to inter-tidal to estuarine and back again a few times, found that a specific set of Ediacaran organisms was associated with each environment.[153] However, while there is some delineation in organisms adapted to different environments, the three assemblages are more distinct temporally than paleoenvironmentally.[158] Because of this, the three assemblages are often separated by temporal boundaries rather than environmental ones (timeline at right).
As the Ediacaran biota represent an early stage in multicellular life's history, it is unsurprising that not all possiblemodes 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.[159]
^"In April 1957, I went rock-climbing in Charnwood Forest with two friends, Richard Allen and Richard Blachford ('Blach'), fellow students at Wyggeston Grammar School, Leicester. I was already interested in geology and knew that the rocks of the Charnian Supergroup were Precambrian although I had not heard of the Australian fossils.
Richard Allen and I agree that Blach (who died in the early 1960s) drew my attention to the leaf-like fossil holotype now on display in Leicester City Museum. I took a rubbing and showed it to my father, who was Minister of the Great Meeting Unitarian Chapel in East Bond Street, taught part-time at University College (soon to be Leicester University) and thus knew Trevor Ford. We took Trevor to visit the fossil site and convinced him that it was a genuine fossil. His publication of the discovery in theJournal of the Yorkshire Geological Society established the genus Charnia and aroused worldwide interest. ... I was able to report the discovery because of my father's encouragement and the enquiring approach fostered by my science teachers. Tina Negus saw the frond before I did but no one took her seriously."[24]
^abPflug (1973). "Zur fauna der Nama-Schichten in Südwest-Afrika. IV. Mikroscopische anatomie der petalo-organisme".Palaeontographica (in German) (B144):166–202.
^Narbonne, Guy (June 2006).The Origin and Early Evolution of Animals. Department of Geological Sciences and Geological Engineering. Queen's University. Archived fromthe original on 24 July 2015. Retrieved8 September 2016.
^abSprigg, 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.
^Narbonne, G.M. (2007).The Rise of Animals. Johns Hopkins University Press. p. 55.ISBN978-0-8018-8679-9.
^Termier, H.; Termier, G. (1960). "L'Édiacarien, premier étage paléontologique".Revue générale des sciences pures et appliquées (in French).67 (3–4):175–192.
^abRunnegar, B.N.; Fedonkin, M.A. (1992). "Proterozoic metazoan body fossils". In Schopf, W.J.; Klein; C. (eds.).The Proterozoic Biosphere. Cambridge University Press. pp. 369–388.ISBN978-0-521-36615-1.OCLC23583672.
^Xiaoet al..'s response to Baileyet al..'s original paper : Xiao, S.; Zhou, C.; Yuan, X. (April 2007). "Palaeontology: undressing and redressing Ediacaran embryos".Nature.446 (7136):E9–E10.Bibcode:2007Natur.446....9X.doi:10.1038/nature05753.PMID17410133.S2CID4406666.And Baileyet 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.Bibcode:2007Natur.446...10B.doi:10.1038/nature05754.S2CID25500052.
^Fedonkin, M.A. (1985). "Paleoichnology of Vendian Metazoa". In Sokolov, B.S.; Iwanowski, A.B. (eds.).Vendian System: Historical–Geological and Paleontological Foundation (in Russian). Vol. 1 Paleontology. Moscow, RU: Nauka. pp. 112–116.
^Ivantsov, A. Y.; Malakhovskaya, Y. E. (2002)."Giant Traces of Vendian Animals"(PDF).Doklady Earth Sciences (in Russian).385 (6):618–622. Archived fromthe original(PDF) on 4 July 2007.
^Ivantsov, A.Yu. (2008).Feeding traces of the Ediacaran animals. International Geological Congress. HPF-17 Trace fossils : Ichnological concepts and methods. Oslo. Archived fromthe original on 18 January 2020. Retrieved7 July 2009.
^According to Fedonkin, M.A.; Simonetta, A; Ivantsov, A.Y. (2007), "New data onKimberella, the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications", in Vickers-Rich, Patricia; Komarower, Patricia (eds.),The Rise and Fall of the Ediacaran Biota, Special publications, vol. 286, London: Geological Society, pp. 157–179,doi:10.1144/SP286.12,ISBN978-1-86239-233-5,OCLC156823511
For a more cynical perspective see Butterfield, N.J. (December 2006). "Hooking some stem-group "worms": Fossil lophotrochozoans in the Burgess Shale".BioEssays.28 (12):1161–1166.doi:10.1002/bies.20507.ISSN0265-9247.PMID17120226.S2CID29130876.
^Seilacher, Adolf (1984). "Late Precambrian and early Cambrian metazoa: Preservational or real extinctions?". In Holland, H.D.; Trendall, A.F. (eds.).Patterns of Change in Earth Evolution. Heidelberg, Germany: Springer-Verlag. pp. 159–168.ISBN978-0-387-12749-1.OCLC11202424.
^Archer, C.; Vance, D. (1 March 2006). "Coupled Fe and S isotope evidence for Archean microbial Fe (III) and sulfate reduction".Geology.34 (3):153–156.Bibcode:2006Geo....34..153A.doi:10.1130/G22067.1.
^Fedonkin, M. A. (1980). "New representatives of the Precambrian coelenterates in the northern Russian platform".Paleontologicheskii Zhurnal (in Russian):7–15.ISSN0031-031X.
^Meert, J.G.; Gibsher, A.S.; Levashova, N.M.; Grice, W.C.; Kamenov, G.D.; Rybanin, A. (2010). "Glaciation and ~770 Ma Ediacara (?) Fossils from the Lesser Karatau Microcontinent, Kazakhstan".Gondwana Research.19 (4):867–880.Bibcode:2011GondR..19..867M.doi:10.1016/j.gr.2010.11.008.
^Bottjer, D.J.; Hagadorn, J.W.; Dornbos, S.Q. (September 2000)."The Cambrian substrate revolution"(PDF).GSA Today. Vol. 10, no. 9. pp. 1–9.Archived(PDF) from the original on 7 October 2022. Retrieved28 June 2008.
^Seilacher, Adolf; Pflüger, F. (1994). "From biomats to benthic agriculture: A biohistoric revolution". In Krumbein, W. E.; Peterson, D. M.; Stal, L. J. (eds.).Biostabilization of Sediments. Bibliotheks-und Informationssystem der Carl von Ossietzky Universität Oldenburg. pp. 97–105.ISBN3-8142-0483-2.
^Bartley, J.K.; Pope, M.; Knoll, A.H.; Semikhatov, M. A.; Petrov, P.Y.U. (1998). "A Vendian-Cambrian boundary succession from the northwestern margin of the Siberian Platform: stratigraphy, palaeontology, chemostratigraphy and correlation".Geological Magazine.135 (4):473–494.Bibcode:1998JGSoc.155..957P.doi:10.1144/gsjgs.155.6.0957.PMID11542817.S2CID129884125.
^Benus (May 1988).Trace fossils, small shelly fossils and the Precambrian-Cambrian boundary. Vol. 463. University of the State of New York. p. 81.ISBN978-1-55557-178-8.
^Retallack G. J. (2012). Criteria for distinguishing microbial mats and earths (Report). Special Paper. Vol. 101. Tulsa: Society of Economic Paleontologists and Mineralogists. pp. 136–152.
^Boag, Thomas H.; Darroch, Simon A.F.; Laflamme, Marc (2016). "Ediacaran distributions in space and time: testing assemblage concepts of earliest macroscopic body fossils".Paleobiology.42 (4):574–594.Bibcode:2016Pbio...42..574B.doi:10.1017/pab.2016.20.
Derek Briggs; Peter Crowther, eds. (2001).Palæobiology II: A synthesis. Malden, MA: Blackwell Science. pp. Chapter 1.ISBN978-0-632-05147-2.OCLC43945263. — Excellent further reading for the keen – includes many interesting chapters with macroevolutionary theme.
McMenamin, M.A.S. (1998).The Garden of Ediacara: Discovering the first complex life. New York: Columbia University Press.ISBN978-0-231-10558-3.OCLC3758852. — A popular science account of these fossils, with a particular focus on theNamibian fossils.
Wood, R.A. (June 2019). "The rise of animals: New fossils and analyses of ancient ocean chemistry reveal the surprisingly deep roots of theCambrian explosion".Scientific American. Vol. 320, no. 6. pp. 24–31.
Watson, Traci (28 October 2020)."These bizarre ancient species are rewriting animal evolution". News.Nature (review). Retrieved18 December 2022.Early fossils with guts, segmented bodies and other sophisticated features reveal a revolution in animal life – before the Cambrian explosion. — Ediacaran biota review article, with nice illustrations
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