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Ediacaran biota

January 31, 2023

For other uses, see Ediacara.

The Ediacaran (/ˌiːdiˈækərən/; formerly Vendian) biota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period (c.  635–538.8 Mya). These were composed of enigmatic tubular and frond-shaped, mostly sessile, organisms.[1][2] Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms.[a]

Life in the Ediacaran Period, as imagined by researchers in 1980.

The Ediacaran biota may have undergone evolutionary radiation in a proposed event called the Avalon explosion, 575 million years ago.[3][4] This was after the Earth had thawed from the Cryogenian period's extensive glaciation. This biota largely disappeared with the rapid increase in biodiversity known as the Cambrian explosion. Most of the currently existing body plans of animals first appeared in the fossil record of the Cambrian 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 around 600 million years ago and flourished until the cusp of the Cambrian 538.8 million years ago, when the characteristic communities of fossils vanished. A diverse Ediacaran community was discovered in 1995 in Sonora, Mexico, and is approximately 555 million years in age, roughly coeval with Ediacaran fossils of the Ediacara Hills in South Australia and the White Sea on the coast of Russia.[5][6][7] 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 curious fragments of once-thriving ecosystems.[8] Multiple hypotheses exist to explain the disappearance of this biota, including preservation bias, a changing environment, the advent of predators and competition from other life-forms. A sampling, reported in 2018, of late Ediacaran strata across Baltica (< 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.[9]

Determining where Ediacaran organisms fit in the tree of life has proven challenging; it is not even established that most of them were animals, with suggestions that they were lichens (fungus-alga symbionts), algae, protists known as foraminifera, fungi or microbial colonies, or hypothetical intermediates between plants and animals.[10] The morphology and habit of some taxa (e.g. Funisia dorothea) suggest relationships to Porifera or Cnidaria (e.g. Auroralumina).[11][12][13] Kimberella may show a similarity to molluscs, and other organisms have been thought to possess bilateral symmetry, although this is controversial. Most macroscopic fossils are morphologically 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, some palaeontologists have suggested that these represent completely extinct lineages that do not resemble any living organism. Palaeontologist Adolf Seilacher proposed a separate subkingdom level category Vendozoa (now renamed Vendobionta)[14] 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.[15] A 2018 study confirmed that one of the period's most-prominent and iconic fossils, Dickinsonia, included cholesterol,[16] suggesting affinities to animals, fungi, or red algae.[17]

History

The first Ediacaran fossils discovered were the disc-shaped Aspidella terranovica in 1868. Their discoverer, Scottish geologist Alexander Murray, found them useful aids for correlating the age of rocks around Newfoundland.[20] However, since they lay below the "Primordial Strata" of the Cambrian that was then thought to contain the very first signs of animal life, a proposal four years after their discovery by Elkanah Billings that these simple forms represented fauna was dismissed by his peers. Instead, they were interpreted as gas escape structures or inorganic concretions.[20] No similar structures elsewhere in the world were then known and the one-sided debate soon fell into obscurity.[20] In 1933, Georg Gürich discovered specimens in Namibia[21] but the firm belief that complex life originated in the Cambrian led to them being assigned to the Cambrian Period and no link to Aspidella was made. In 1946, Reg Sprigg noticed "jellyfishes" in the Ediacara Hills of Australia's Flinders Ranges[22] but these rocks were believed to be Early Cambrian so, while the discovery sparked some interest, little serious attention was garnered.

 
Palaeontologist Guy Narbonne examining Ediacaran fossils in Newfoundland

It was not until the British discovery of the iconic Charnia that the Precambrian was seriously considered as containing life. This frond-shaped fossil was found in England's Charnwood Forest first by a 15 year-old girl in 1956 (Tina Negus, who was not believed[23][b]) and then the next year by a group of three schoolboys including 15 year-old Roger Mason.[24][25][26] Due to the detailed geological mapping of the British Geological Survey, there was no doubt these fossils sat in Precambrian rocks. Palaeontologist Martin Glaessner finally, in 1959, made the connection between this and the earlier finds[27][28] and with a combination of improved dating of existing specimens and an injection of vigour into the search many more instances were recognised.[29]

All specimens discovered until 1967 were in coarse-grained sandstone that prevented preservation of fine details, making interpretation difficult. S.B. Misra's discovery of fossiliferous ash-beds at the Mistaken Point assemblage in Newfoundland changed all this as the delicate detail preserved by the fine ash allowed the description of features that were previously undiscernible.[30][31] It was also the first discovery of Ediacarans in deep water sediments.[32]

Poor communication, combined with the difficulty in correlating globally distinct formations, led to a plethora of different names for the biota. In 1960 the French name "Ediacarien" – after the Ediacara Hills – was added to the competing terms "Sinian" and "Vendian"[33] 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 of geological 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.[34]

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.[35][full citation needed]

Preservation

Main article: Ediacaran type preservation

Microbial mats

 
Modern cyanobacterial-algal mat, salty lake on the White Sea seaside

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.[36]

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, the evolution of grazing organisms in the Cambrian vastly reduced their numbers.[37] These communities are now limited to inhospitable refugia, such as the stromatolites found in Hamelin Pool Marine Nature Reserve in Shark Bay, Western Australia, where the salt levels can be twice those of the surrounding sea.[38]

Fossilization

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

The preservation of these fossils is one of their great fascinations to science. As soft-bodied organisms, they would normally not fossilize and, unlike later soft-bodied fossil biota such as the Burgess Shale or Solnhofen Limestone, the Ediacaran biota is not found in a restricted environment subject to unusual local conditions: They were a global phenomenon. The processes that were operating must have been systemic and worldwide. There was something very different about the Ediacaran Period that permitted these delicate creatures to be left behind and it is thought 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.[39] 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.[40] Ash beds provide more detail and can readily be dated to the nearest million years or better using radiometric dating.[41] 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.[39] 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.[42]

Scale of preservation

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 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 as the high-energy sedimentation did not destroy them as it would have the less-resistant discs. Further, in some cases, the bacterial precipitation of minerals formed a "death mask", ultimately leaving a positive, cast-like impression of the organism.[43][44]

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.  
Tateana inflata ('Cyclomedusa' radiata) were originally believed to have been Medusoids, although recent research suggests that they were holdfasts of Petalonamids.  
A cast of Charnia, the first accepted complex Precambrian organism. Charnia was once interpreted as a relative of the sea pens.  
Dickinsonia displays the characteristic quilted appearance of Ediacaran enigmata.  
Spriggina was originally interpreted as annelid or arthropod. However, lack of known limbs, and glide reflected isomers instead of true segments, rejects any such classification despite some superficial resemblance.  
Late Ediacaran Archaeonassa-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 of Yorgia waggoneri (right), which created these traces on microbial mat.  

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.[39] 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[45] generated intense media excitement[46] though some have claimed they are instead inorganic structures formed by the precipitation of minerals on the inside of a hole.[47] Other "embryos" have been interpreted as the remains of the giant sulfur-reducing bacteria akin to Thiomargarita,[48] a view that, while it had enjoyed a notable gain of supporters[49][50] as of 2007, has since suffered following further research comparing the potential Doushantuo embryos' morphologies with those of Thiomargarita specimens, both living and in various stages of decay.[51] A recent discovery of comparable Ediacaran fossil embryos from the Portfjeld Formation in Greenland has significantly expanded the paleogeograpical occurrence of Doushantuo-type fossil "embryos" with similar biotic forms now reported from differing paleolatitudes.[52]
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.[53] An alternative proposal is that these structures represent adult stages of the multicellular organisms of this period.[54] Microfossils of Caveasphaera are thought to foreshadow the evolutionary origin of animal-like embryology.[55]
Discs
Circular fossils, such as Ediacaria, Cyclomedusa and Rugoconites led to the initial identification of Ediacaran fossils as cnidaria, which include jellyfish and corals.[22] Further examination has provided alternative interpretations of all disc-shaped fossils: not one is now confidently recognised as a jellyfish. Alternate explanations include holdfasts and protists;[56] the patterns displayed where two meet have led to many 'individuals' being identified as microbial colonies,[57][58] and yet others may represent scratch marks formed as stalked organisms spun around their holdfasts.[59] Useful diagnostic characteristics are often lacking because only the underside of the organism is preserved by fossilisation.
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.[60]
Toroids
The fossil Vendoglossa 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 ridged ectoderm. 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.[61]
Quilted organisms
The organisms considered in Seilacher's revised definition of the Vendobionta[14] 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.[62]
These organisms appear to form two groups: the fractal rangeomorphs and the simpler erniettomorphs.[63] Including such fossils as the iconic Charnia and Swartpuntia, the group is both the most iconic of the Ediacaran biota and the most difficult to place within the existing tree 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[64] or osmosis.[65] However, others argue against this.[66]
Non-Vendobionts
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.[47][67] In 2020, Ikaria wariootia was claimed to represent one of the oldest organisms with anterior and posterior differentiation.[68] Later examples are almost universally accepted as bilaterians and include the mollusc-like Kimberella,[69] Spriggina (pictured)[70] and the shield-shaped Parvancorina[71] whose affinities are currently debated.[72]
A suite of fossils known as the small shelly fossils are represented in the Ediacaran, most famously by Cloudina[73] 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.
Organic microfossils known as small carbonaceous fossils are also found in Ediacaran sediments, including the spiral-shaped Cochleatina[74] which has been found to span the Ediacaran–Cambrian boundary.
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 nearly three billion years. Possible arthropods have also been described.[75]
Fossils of the hard-shelled foraminifera Platysolenites are known from the latest Ediacaran of western Siberia, coexisting with Cloudina and Namacalathus.[76]
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 be stolonic outgrowths.[77] A study of Brazilian Ediacaran fossils found filamentous microfossils, suggested to be eukaryotes or large sulfur-oxidizing-bacteria (SOBs).[78] 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.[79]
Trace fossils
With the exception of some very simple vertical burrows[80] 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 the bilateral clade of animals[81] but they could also have been made by simpler organisms feeding as they slowly rolled along the sea floor.[82] Putative "burrows" dating as far back as 1,100 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;[83] however their uneven width and tapering ends make a biological origin so difficult to defend[84] that even the original proponent no longer believes they are authentic.[85]
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;[86] these fossils are thought to be associated with ciliary feeding but the precise method of formation of these disconnected and overlapping fossils largely remains a mystery.[87] The potential mollusc Kimberella is associated with scratch marks, perhaps formed by a radula.[88]
 
A reconstruction of the Ediacaran biota at the Field Museum in Chicago.

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.

Martin Glaessner proposed in The Dawn of Animal Life (1984) that the Ediacaran biota were recognizable crown group members of modern phyla, but were unfamiliar because they had yet to evolve the characteristic features we use in modern classification.[89]

In 1998 Mark McMenamin claimed Ediacarans did not possess an embryonic stage, and thus could not be animals. He believed that they independently evolved a nervous system and brains, meaning that "the path toward intelligent life was embarked upon more than once on this planet".[56]

In 2018 analysis of ancient sterols was taken as evidence that one of the period's most-prominent and iconic fossils, Dickinsonia, was an early animal.[16]

Cnidarians

 
A sea pen, a modern cnidarian bearing a passing resemblance to Charnia

Since the most primitive eumetazoans—multi-cellular animals with tissues—are cnidarians, and the first recognized Ediacaran fossil Charnia looks very much like a sea pen, the first attempt to categorise these fossils designated them as jellyfish and sea pens.[90] 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 as Charniodiscus, a circular impression later found to be attached to the long 'stem' of a frond-like organism that now bears the name.[91][92]

The link between certain 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 Tertiary, and the apparent cohesion between segments in Ediacaran frond-like organisms.[93] Some researchers have suggested that an analysis of "growth poles" discredits the pennatulacean nature of Ediacaran fronds.[94][95]

This leaves the Canadian polyp Haootia, the British Auroralumina, the staurozoan Corumbella, and the early conulariid Paraconularia ediacara as the only recognized cnidarian body fossils from the Ediacaran, both being sessile forms showing a quadrilateral symmetry of the body. So while this shows that Cnidarians were already well established in the Ediacaran period, the question remains if there were only sessile polyps or that a medusal stage had also developed.

Protozoans

 
A single-celled xenophyophore in the Galapagos Rift

Adolf Seilacher has suggested the Ediacaran sees animals usurping giant protists as the dominant life form.[96] The modern xenophyophores are giant single-celled protozoans found throughout the world's oceans, largely on the abyssal plain. A recent genetic study suggested that the xenophyophores are a specialised group of Foraminifera. There are approximately 42 recognised species in 13 genera and 2 orders; one of which, Syringammina fragilissima, is among the largest known protozoans at up to 20 centimetres (7.9 in) in diameter.

New phylum

Seilacher has 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,[97][98] 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.[99] 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".[100]

Lichen hypothesis

 
Thin sections and substrates of a variety of Ediacaran fossils [101]

Greg Retallack has proposed that Ediacaran organisms were lichens.[102][103] He argues that thin sections of Ediacaran fossils show lichen-like compartments and hypha-like wisps of ferruginized clay,[101] and that Ediacaran fossils have been found in strata that he interprets as desert soils.[103][104]

The suggestion has been disputed by other scientists; some have described the evidence as ambiguous and unconvincing, for instance noting that Dickinsonia fossils have been found on rippled surfaces (suggesting a marine environment), while trace fossils like Radulichnus could not have been caused by needle ice as Retallack has proposed.[105][106][107] Ben Waggoner notes that the suggestion would place the root of the Cnidaria back from around 900 mya to between 1500 mya and 2000 mya, contradicting much other evidence.[108][109] Matthew Nelsen, examining phylogenies of ascomycete fungi and chlorophyte algae (components of lichens), calibrated for time, finds no support for the hypothesis that lichens predated the vascular plants.[110]

Other interpretations

Several classifications have been used to accommodate the Ediacaran biota at some point,[111] from algae,[112] to protozoans,[113] to fungi[114] to bacterial or microbial colonies,[57] to hypothetical intermediates between plants and animals.[10]

A new extant genus discovered in 2014, Dendrogramma, which at the time of discovery appeared to be a basal metazoan but of unknown taxonomic placement, had been noted to have similarities with the Ediacaran fauna.[115] It has since been found to be a siphonophore, possibly even sections of a more complex species,[116] though this in turn has raised suspicions for a similar status for at least some Ediacaran organisms.

Origin

It took almost 4 billion years from the formation of the Earth for the Ediacaran fossils to first appear, 655 million years ago. While putative fossils are reported from 3,460 million years ago,[117][118] the first uncontroversial evidence for life is found 2,700 million years ago,[119] and cells with nuclei certainly existed by 1,200 million years ago:[120] The reason why it took so long for forms with an Ediacaran grade of organisation to appear is uncertain.[citation needed]

It could be that no special explanation is required: the slow process of evolution simply required 4 billion years to accumulate the necessary adaptations. Indeed, there does seem to be a slow increase in the maximum level of complexity seen over this time, with more and more complex forms of life evolving as time progresses, with traces of earlier semi-complex life such as Nimbia, found in the 610 million year old Twitya formation,[121] and older rocks dating to 770 million years ago in Kazakhstan,[122] possibly displaying the most complex morphology of the time.[citation needed]

 
Global ice sheets might 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.[citation needed]

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.[citation needed]

On the early Earth, reactive elements, such as iron and uranium, existed in a reduced form that would react with any free oxygen produced by photosynthesising organisms. Oxygen would not be able to build up in the atmosphere until all the iron had rusted (producing banded iron formations), and 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[123] – and the presence of atmospheric oxygen was soon heralded as a possible trigger for the Ediacaran radiation.[124] 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.[125] 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.[126]

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.[127] 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.[citation needed]

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 to 565 million years ago; White Sea in Russia, 560 million years ago to 550 million years ago; and Nama in Namibia, 550 million years ago to 542 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 famous Cambrian explosion.[128]

Preservation bias

The paucity of Ediacaran fossils after the Cambrian could simply be due to conditions that no longer favoured the fossilisation of Ediacaran organisms, which may have continued to thrive unpreserved.[36] However, if they were common, more than the occasional specimen[8][129] might be expected in exceptionally preserved fossil assemblages (Konservat-Lagerstätten) such as the Burgess Shale and Chengjiang.[130] 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.[131]

Predation and grazing

 
Kimberella might have had a predatory or grazing lifestyle.

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

Alternatively, skeletonised animals could have fed directly on the relatively undefended Ediacaran biota.[56] However, if the interpretation of the Ediacaran age Kimberella as a grazer is correct then this suggests that the biota had already had limited exposure to "predation".[69]

Little evidence has been identified for any trace fossils in the Ediacaran Period, which could speak against the active grazing theory. Further, the onset of the Cambrian Period is defined by the appearance of a worldwide trace fossil assemblage, quite distinct from the activity-barren Ediacaran Period.[citation needed]

Competition

 
Cambrian animals such as Waptia might have competed with, or fed upon, Ediacaran life-forms.
 
Other animals, like the Radiodonts (Anomalocaris pictured) could also have competed against Ediacaran-type organisms.

It is possible that increased competition due to the evolution of key innovations among other groups, perhaps as a response to predation,[132] 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.[133]

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,[134] rising sea levels (creating shallow, "life-friendly" seas),[135] a nutrient crisis,[136] fluctuations in atmospheric composition, including oxygen and carbon dioxide levels,[137] and changes in ocean chemistry[138] (promoting biomineralisation)[139] could all have played a part.[citation needed]

Assemblages

Ediacaran-type fossils are recognised globally in 25 localities[34] and a variety of depositional conditions, and are commonly grouped into three main types, known as assemblages and named after typical localities. Each assemblage tends to occupy its own region of morphospace, and after an initial burst of diversification changes little for the rest of its existence.[140]

Avalon-type assemblage

The Avalon-type assemblage is defined at Mistaken Point in Canada, the oldest locality with a large quantity of Ediacaran fossils.[141] The assemblage is easily dated because it contains many fine ash-beds, which are a good source of zircons used in the uranium-lead method of radiometric dating. These fine-grained ash beds also preserve exquisite detail. Constituents of this biota appear to survive through until the extinction of all Ediacarans at the base of the Cambrian.[140]

One interpretation of the biota is as deep-sea-dwelling rangeomorphs[142] 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[143] – by most interpretations, the assemblage is found in water too deep for photosynthesis. The low diversity may reflect the depth of water – which would restrict speciation opportunities – or it may just be too young for a rich biota to have evolved. Opinion is currently divided between these conflicting hypotheses.[144]

Ediacara-type assemblage

The Ediacara-type assemblage is named after Australia's Ediacara Hills, and consists of fossils preserved in facies of coastal lagoons and rivers. They are typically found in red gypsiferous and calcareous paleosols formed on loess and flood deposits in an arid cool temperate paleoclimate.[103] Most fossils are preserved as imprints in microbial earths,[145] but a few are preserved within sandy units.[144][140]

Biota ranges[144]
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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 fossils represent burrowing organisms,[60] while Guy Narbonne maintains they were surface dwellers.[146] These beds are sandwiched between units comprising interbedded sandstones, siltstones and shales – with microbial mats, where present, usually containing the fossils. The environment is interpreted as sand bars formed at the mouth of a delta's distributaries.[144] Mattress-like vendobionts (Ernietta, Pteridinium, Rangea) in these sandstones form a very different assemblage from vermiform fossils (Cloudina, Namacalathus) of Ediacaran "wormworld" in marine dolomite of Namibia.[147]

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;[148] the same fossils are found at all palaeolatitudes (the latitude where the fossil was created, accounting for continental drift) and in separate sedimentary basins.[144]

It is most likely that the three assemblages mark organisms adapted to survival in different environments, and that any apparent patterns in diversity or age are in fact an artefact of the few samples that have been discovered – the timeline (right) demonstrates the paucity of Ediacaran fossil-bearing assemblages. An analysis of one of the White Sea fossil beds, where the layers cycle from continental 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.[144]

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.[149] 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.

See also

 
Wikimedia Commons has media related to Ediacaran biota.

Notes

  1. ^ Simple multicellular organisms such as red algae evolved at least 1,200 million years ago. The status of the Francevillian biota of 2,100 million years ago is unclear, but they may represent earlier multicellular forms of a more complex nature.
  2. ^ "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 the Journal 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."[23]

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Further reading

  • Derek Briggs; Peter Crowther, eds. (2001). Palæobiology II: A synthesis. Malden, MA: Blackwell Science. pp. Chapter 1. ISBN 978-0-632-05147-2. OCLC 43945263. — 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. ISBN 978-0-231-10558-3. OCLC 3758852. — A popular science account of these fossils, with a particular focus on the Namibian fossils.
  • Wood, R.A. (June 2019). "The rise of animals: New fossils and analyses of ancient ocean chemistry reveal the surprisingly deep roots of the Cambrian explosion". Scientific American. Vol. 320, no. 6. pp. 24–31.

External links

  • Watson, Traci (28 October 2020). "These bizarre ancient species are rewriting animal evolution". News. Nature (review). Retrieved 18 December 2022. Early fossils with guts, segmented bodies and other sophisticated features reveal a revolution in animal life – before the Cambrian explosion.{{cite news}}: CS1 maint: url-status (link) — Ediacaran biota review article, with nice illustrations
  • . peripatus.gen.nz. 2010. Archived from the original on 10 October 2010. — Thorough, though slightly out-of-date, description
  • {{cite web |title=Database of Ediacaran biota |date=July 2010 |website=complex-life.org |url=http://www.complex-life.org/database |url-status=dead |archive-url= — compilation up to 7/2010
  • . Kingston, Ontario: Queen's University. 2007. Archived from the original on 28 May 2007.
  • . Kingston, Ontario: Queen's University. 2007. Archived from the original on 5 April 2007.
  • "Earth's oldest animal ecosystem held in fossils at Nilpena Station in SA outback". ABC News. 5 August 2013. Retrieved 6 August 2013.
  • Meet the fossils (TV program audio & transcript). Landline. Australian Broadcasting Corporation. 3 August 2013. Retrieved 28 December 2018. — ABC TV show on Ediacaran fossils at Nilpena
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January 31, 2023
ediacaran, biota, other, uses, ediacara, ediacaran, formerly, vendian, biota, taxonomic, period, classification, that, consists, life, forms, that, were, present, earth, during, ediacaran, period, these, were, composed, enigmatic, tubular, frond, shaped, mostl. For other uses see Ediacara The Ediacaran ˌ iː d i ˈ ae k er e n formerly Vendian biota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period c 635 538 8 Mya These were composed of enigmatic tubular and frond shaped mostly sessile organisms 1 2 Trace fossils of these organisms have been found worldwide and represent the earliest known complex multicellular organisms a Life in the Ediacaran Period as imagined by researchers in 1980 The Ediacaran biota may have undergone evolutionary radiation in a proposed event called the Avalon explosion 575 million years ago 3 4 This was after the Earth had thawed from the Cryogenian period s extensive glaciation This biota largely disappeared with the rapid increase in biodiversity known as the Cambrian explosion Most of the currently existing body plans of animals first appeared in the fossil record of the Cambrian 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 around 600 million years ago and flourished until the cusp of the Cambrian 538 8 million years ago when the characteristic communities of fossils vanished A diverse Ediacaran community was discovered in 1995 in Sonora Mexico and is approximately 555 million years in age roughly coeval with Ediacaran fossils of the Ediacara Hills in South Australia and the White Sea on the coast of Russia 5 6 7 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 curious fragments of once thriving ecosystems 8 Multiple hypotheses exist to explain the disappearance of this biota including preservation bias a changing environment the advent of predators and competition from other life forms A sampling reported in 2018 of late Ediacaran strata across Baltica lt 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 9 Determining where Ediacaran organisms fit in the tree of life has proven challenging it is not even established that most of them were animals with suggestions that they were lichens fungus alga symbionts algae protists known as foraminifera fungi or microbial colonies or hypothetical intermediates between plants and animals 10 The morphology and habit of some taxa e g Funisia dorothea suggest relationships to Porifera or Cnidaria e g Auroralumina 11 12 13 Kimberella may show a similarity to molluscs and other organisms have been thought to possess bilateral symmetry although this is controversial Most macroscopic fossils are morphologically 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 some palaeontologists have suggested that these represent completely extinct lineages that do not resemble any living organism Palaeontologist Adolf Seilacher proposed a separate subkingdom level category Vendozoa now renamed Vendobionta 14 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 15 A 2018 study confirmed that one of the period s most prominent and iconic fossils Dickinsonia included cholesterol 16 suggesting affinities to animals fungi or red algae 17 The Ediacara biota in context 650 640 630 620 610 600 590 580 570 560 550 540 530 520 510 500 490 CryogenianEdiacaranCambrian Last Ediacaran communities Last putative Ediacaran Embryos Last extensive glaciation First Ediacaran megafossil Gaskiersglaciation Avalon explosionAspidelladiscsCharnia Neoproterozoic last era of the Precambrian Palaeozoic first era of the Phanerozoic Axis scale million years References Waggoner 1998 18 Hofmann Narbonne amp Aitken 1990 19 Contents 1 History 2 Preservation 2 1 Microbial mats 2 2 Fossilization 2 3 Scale of preservation 3 Morphology 4 Classification and interpretation 4 1 Cnidarians 4 2 Protozoans 4 3 New phylum 4 4 Lichen hypothesis 4 5 Other interpretations 5 Origin 5 1 Preservation bias 5 2 Predation and grazing 5 3 Competition 5 4 Change in environmental conditions 6 Assemblages 6 1 Avalon type assemblage 6 2 Ediacara type assemblage 6 3 Nama type assemblage 6 4 Significance of assemblages 7 See also 8 Notes 9 References 10 Further reading 11 External linksHistory EditThe first Ediacaran fossils discovered were the disc shaped Aspidella terranovica in 1868 Their discoverer Scottish geologist Alexander Murray found them useful aids for correlating the age of rocks around Newfoundland 20 However since they lay below the Primordial Strata of the Cambrian that was then thought to contain the very first signs of animal life a proposal four years after their discovery by Elkanah Billings that these simple forms represented fauna was dismissed by his peers Instead they were interpreted as gas escape structures or inorganic concretions 20 No similar structures elsewhere in the world were then known and the one sided debate soon fell into obscurity 20 In 1933 Georg Gurich discovered specimens in Namibia 21 but the firm belief that complex life originated in the Cambrian led to them being assigned to the Cambrian Period and no link to Aspidella was made In 1946 Reg Sprigg noticed jellyfishes in the Ediacara Hills of Australia s Flinders Ranges 22 but these rocks were believed to be Early Cambrian so while the discovery sparked some interest little serious attention was garnered Palaeontologist Guy Narbonne examining Ediacaran fossils in Newfoundland It was not until the British discovery of the iconic Charnia that the Precambrian was seriously considered as containing life This frond shaped fossil was found in England s Charnwood Forest first by a 15 year old girl in 1956 Tina Negus who was not believed 23 b and then the next year by a group of three schoolboys including 15 year old Roger Mason 24 25 26 Due to the detailed geological mapping of the British Geological Survey there was no doubt these fossils sat in Precambrian rocks Palaeontologist Martin Glaessner finally in 1959 made the connection between this and the earlier finds 27 28 and with a combination of improved dating of existing specimens and an injection of vigour into the search many more instances were recognised 29 All specimens discovered until 1967 were in coarse grained sandstone that prevented preservation of fine details making interpretation difficult S B Misra s discovery of fossiliferous ash beds at the Mistaken Point assemblage in Newfoundland changed all this as the delicate detail preserved by the fine ash allowed the description of features that were previously undiscernible 30 31 It was also the first discovery of Ediacarans in deep water sediments 32 Poor communication combined with the difficulty in correlating globally distinct formations led to a plethora of different names for the biota In 1960 the French name Ediacarien after the Ediacara Hills was added to the competing terms Sinian and Vendian 33 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 of geological 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 34 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 35 full citation needed Preservation EditMain article Ediacaran type preservation Microbial mats Edit Modern cyanobacterial algal mat salty lake on the White Sea seaside 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 36 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 the evolution of grazing organisms in the Cambrian vastly reduced their numbers 37 These communities are now limited to inhospitable refugia such as the stromatolites found in Hamelin Pool Marine Nature Reserve in Shark Bay Western Australia where the salt levels can be twice those of the surrounding sea 38 Fossilization Edit The fossil Charniodiscus is barely distinguishable from the elephant skin texture on this cast The preservation of these fossils is one of their great fascinations to science As soft bodied organisms they would normally not fossilize and unlike later soft bodied fossil biota such as the Burgess Shale or Solnhofen Limestone the Ediacaran biota is not found in a restricted environment subject to unusual local conditions They were a global phenomenon The processes that were operating must have been systemic and worldwide There was something very different about the Ediacaran Period that permitted these delicate creatures to be left behind and it is thought 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 39 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 40 Ash beds provide more detail and can readily be dated to the nearest million years or better using radiometric dating 41 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 39 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 42 Scale of preservation Edit 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 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 as the high energy sedimentation did not destroy them as it would have the less resistant discs Further in some cases the bacterial precipitation of minerals formed a death mask ultimately leaving a positive cast like impression of the organism 43 44 Morphology EditForms of Ediacaran fossilThe earliest discovered potential embryo preserved within an acanthomorphic acritarch 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 of Petalonamids A cast of Charnia the first accepted complex Precambrian organism Charnia was once interpreted as a relative of the sea pens Dickinsonia displays the characteristic quilted appearance of Ediacaran enigmata Spriggina was originally interpreted as annelid or arthropod However lack of known limbs and glide reflected isomers instead of true segments rejects any such classification despite some superficial resemblance Late Ediacaran Archaeonassa 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 of Yorgia waggoneri right which created these traces on microbial mat 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 39 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 45 generated intense media excitement 46 though some have claimed they are instead inorganic structures formed by the precipitation of minerals on the inside of a hole 47 Other embryos have been interpreted as the remains of the giant sulfur reducing bacteria akin to Thiomargarita 48 a view that while it had enjoyed a notable gain of supporters 49 50 as of 2007 has since suffered following further research comparing the potential Doushantuo embryos morphologies with those of Thiomargarita specimens both living and in various stages of decay 51 A recent discovery of comparable Ediacaran fossil embryos from the Portfjeld Formation in Greenland has significantly expanded the paleogeograpical occurrence of Doushantuo type fossil embryos with similar biotic forms now reported from differing paleolatitudes 52 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 53 An alternative proposal is that these structures represent adult stages of the multicellular organisms of this period 54 Microfossils of Caveasphaera are thought to foreshadow the evolutionary origin of animal like embryology 55 Discs Circular fossils such as Ediacaria Cyclomedusa and Rugoconites led to the initial identification of Ediacaran fossils as cnidaria which include jellyfish and corals 22 Further examination has provided alternative interpretations of all disc shaped fossils not one is now confidently recognised as a jellyfish Alternate explanations include holdfasts and protists 56 the patterns displayed where two meet have led to many individuals being identified as microbial colonies 57 58 and yet others may represent scratch marks formed as stalked organisms spun around their holdfasts 59 Useful diagnostic characteristics are often lacking because only the underside of the organism is preserved by fossilisation 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 60 Toroids The fossil Vendoglossa 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 ridged ectoderm 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 61 Quilted organisms The organisms considered in Seilacher s revised definition of the Vendobionta 14 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 62 These organisms appear to form two groups the fractal rangeomorphs and the simpler erniettomorphs 63 Including such fossils as the iconic Charnia and Swartpuntia the group is both the most iconic of the Ediacaran biota and the most difficult to place within the existing tree 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 64 or osmosis 65 However others argue against this 66 Non Vendobionts 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 47 67 In 2020 Ikaria wariootia was claimed to represent one of the oldest organisms with anterior and posterior differentiation 68 Later examples are almost universally accepted as bilaterians and include the mollusc like Kimberella 69 Spriggina pictured 70 and the shield shaped Parvancorina 71 whose affinities are currently debated 72 A suite of fossils known as the small shelly fossils are represented in the Ediacaran most famously by Cloudina 73 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 Organic microfossils known as small carbonaceous fossils are also found in Ediacaran sediments including the spiral shaped Cochleatina 74 which has been found to span the Ediacaran Cambrian boundary Representatives of modern taxa existed in the Ediacaran some of which are recognisable today Sponges red and green algae protists and bacteria are all easily recognisable with some pre dating the Ediacaran by nearly three billion years Possible arthropods have also been described 75 Fossils of the hard shelled foraminifera Platysolenites are known from the latest Ediacaran of western Siberia coexisting with Cloudina and Namacalathus 76 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 be stolonic outgrowths 77 A study of Brazilian Ediacaran fossils found filamentous microfossils suggested to be eukaryotes or large sulfur oxidizing bacteria SOBs 78 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 79 Trace fossils With the exception of some very simple vertical burrows 80 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 the bilateral clade of animals 81 but they could also have been made by simpler organisms feeding as they slowly rolled along the sea floor 82 Putative burrows dating as far back as 1 100 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 83 however their uneven width and tapering ends make a biological origin so difficult to defend 84 that even the original proponent no longer believes they are authentic 85 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 86 these fossils are thought to be associated with ciliary feeding but the precise method of formation of these disconnected and overlapping fossils largely remains a mystery 87 The potential mollusc Kimberella is associated with scratch marks perhaps formed by a radula 88 A reconstruction of the Ediacaran biota at the Field Museum in Chicago Classification and interpretation EditClassification of the Ediacarans is difficult and hence a variety of theories exist as to their placement on the tree of life Martin Glaessner proposed in The Dawn of Animal Life 1984 that the Ediacaran biota were recognizable crown group members of modern phyla but were unfamiliar because they had yet to evolve the characteristic features we use in modern classification 89 In 1998 Mark McMenamin claimed Ediacarans did not possess an embryonic stage and thus could not be animals He believed that they independently evolved a nervous system and brains meaning that the path toward intelligent life was embarked upon more than once on this planet 56 In 2018 analysis of ancient sterols was taken as evidence that one of the period s most prominent and iconic fossils Dickinsonia was an early animal 16 Cnidarians Edit A sea pen a modern cnidarian bearing a passing resemblance to Charnia Since the most primitive eumetazoans multi cellular animals with tissues are cnidarians and the first recognized Ediacaran fossil Charnia looks very much like a sea pen the first attempt to categorise these fossils designated them as jellyfish and sea pens 90 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 as Charniodiscus a circular impression later found to be attached to the long stem of a frond like organism that now bears the name 91 92 The link between certain 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 Tertiary and the apparent cohesion between segments in Ediacaran frond like organisms 93 Some researchers have suggested that an analysis of growth poles discredits the pennatulacean nature of Ediacaran fronds 94 95 This leaves the Canadian polyp Haootia the British Auroralumina the staurozoan Corumbella and the early conulariid Paraconularia ediacara as the only recognized cnidarian body fossils from the Ediacaran both being sessile forms showing a quadrilateral symmetry of the body So while this shows that Cnidarians were already well established in the Ediacaran period the question remains if there were only sessile polyps or that a medusal stage had also developed Protozoans Edit A single celled xenophyophore in the Galapagos Rift Adolf Seilacher has suggested the Ediacaran sees animals usurping giant protists as the dominant life form 96 The modern xenophyophores are giant single celled protozoans found throughout the world s oceans largely on the abyssal plain A recent genetic study suggested that the xenophyophores are a specialised group of Foraminifera There are approximately 42 recognised species in 13 genera and 2 orders one of which Syringammina fragilissima is among the largest known protozoans at up to 20 centimetres 7 9 in in diameter New phylum Edit Seilacher has 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 97 98 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 99 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 100 Lichen hypothesis Edit Thin sections and substrates of a variety of Ediacaran fossils 101 Greg Retallack has proposed that Ediacaran organisms were lichens 102 103 He argues that thin sections of Ediacaran fossils show lichen like compartments and hypha like wisps of ferruginized clay 101 and that Ediacaran fossils have been found in strata that he interprets as desert soils 103 104 The suggestion has been disputed by other scientists some have described the evidence as ambiguous and unconvincing for instance noting that Dickinsonia fossils have been found on rippled surfaces suggesting a marine environment while trace fossils like Radulichnus could not have been caused by needle ice as Retallack has proposed 105 106 107 Ben Waggoner notes that the suggestion would place the root of the Cnidaria back from around 900 mya to between 1500 mya and 2000 mya contradicting much other evidence 108 109 Matthew Nelsen examining phylogenies of ascomycete fungi and chlorophyte algae components of lichens calibrated for time finds no support for the hypothesis that lichens predated the vascular plants 110 Other interpretations Edit Several classifications have been used to accommodate the Ediacaran biota at some point 111 from algae 112 to protozoans 113 to fungi 114 to bacterial or microbial colonies 57 to hypothetical intermediates between plants and animals 10 A new extant genus discovered in 2014 Dendrogramma which at the time of discovery appeared to be a basal metazoan but of unknown taxonomic placement had been noted to have similarities with the Ediacaran fauna 115 It has since been found to be a siphonophore possibly even sections of a more complex species 116 though this in turn has raised suspicions for a similar status for at least some Ediacaran organisms Origin EditIt took almost 4 billion years from the formation of the Earth for the Ediacaran fossils to first appear 655 million years ago While putative fossils are reported from 3 460 million years ago 117 118 the first uncontroversial evidence for life is found 2 700 million years ago 119 and cells with nuclei certainly existed by 1 200 million years ago 120 The reason why it took so long for forms with an Ediacaran grade of organisation to appear is uncertain citation needed It could be that no special explanation is required the slow process of evolution simply required 4 billion years to accumulate the necessary adaptations Indeed there does seem to be a slow increase in the maximum level of complexity seen over this time with more and more complex forms of life evolving as time progresses with traces of earlier semi complex life such as Nimbia found in the 610 million year old Twitya formation 121 and older rocks dating to 770 million years ago in Kazakhstan 122 possibly displaying the most complex morphology of the time citation needed Global ice sheets might 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 citation needed 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 citation needed On the early Earth reactive elements such as iron and uranium existed in a reduced form that would react with any free oxygen produced by photosynthesising organisms Oxygen would not be able to build up in the atmosphere until all the iron had rusted producing banded iron formations and 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 123 and the presence of atmospheric oxygen was soon heralded as a possible trigger for the Ediacaran radiation 124 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 125 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 126 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 127 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 citation needed 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 to 565 million years ago White Sea in Russia 560 million years ago to 550 million years ago and Nama in Namibia 550 million years ago to 542 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 famous Cambrian explosion 128 Preservation bias Edit The paucity of Ediacaran fossils after the Cambrian could simply be due to conditions that no longer favoured the fossilisation of Ediacaran organisms which may have continued to thrive unpreserved 36 However if they were common more than the occasional specimen 8 129 might be expected in exceptionally preserved fossil assemblages Konservat Lagerstatten such as the Burgess Shale and Chengjiang 130 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 131 Predation and grazing Edit Kimberella might have had a predatory or grazing lifestyle It is suggested that by the Early Cambrian organisms higher in the food chain caused the microbial mats to largely disappear If these grazers first appeared as the Ediacaran biota started to decline then it may suggest that they destabilised the microbial substrate leading to displacement or detachment of the biota or that the destruction of the mat destabilised the ecosystem causing extinctions citation needed Alternatively skeletonised animals could have fed directly on the relatively undefended Ediacaran biota 56 However if the interpretation of the Ediacaran age Kimberella as a grazer is correct then this suggests that the biota had already had limited exposure to predation 69 Little evidence has been identified for any trace fossils in the Ediacaran Period which could speak against the active grazing theory Further the onset of the Cambrian Period is defined by the appearance of a worldwide trace fossil assemblage quite distinct from the activity barren Ediacaran Period citation needed Competition Edit Cambrian animals such as Waptia might have competed with or fed upon Ediacaran life forms Other animals like the Radiodonts Anomalocaris pictured could also have competed against Ediacaran type organisms It is possible that increased competition due to the evolution of key innovations among other groups perhaps as a response to predation 132 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 133 Change in environmental conditions Edit 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 134 rising sea levels creating shallow life friendly seas 135 a nutrient crisis 136 fluctuations in atmospheric composition including oxygen and carbon dioxide levels 137 and changes in ocean chemistry 138 promoting biomineralisation 139 could all have played a part citation needed Assemblages EditEdiacaran type fossils are recognised globally in 25 localities 34 and a variety of depositional conditions and are commonly grouped into three main types known as assemblages and named after typical localities Each assemblage tends to occupy its own region of morphospace and after an initial burst of diversification changes little for the rest of its existence 140 Avalon type assemblage Edit The Avalon type assemblage is defined at Mistaken Point in Canada the oldest locality with a large quantity of Ediacaran fossils 141 The assemblage is easily dated because it contains many fine ash beds which are a good source of zircons used in the uranium lead method of radiometric dating These fine grained ash beds also preserve exquisite detail Constituents of this biota appear to survive through until the extinction of all Ediacarans at the base of the Cambrian 140 One interpretation of the biota is as deep sea dwelling rangeomorphs 142 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 143 by most interpretations the assemblage is found in water too deep for photosynthesis The low diversity may reflect the depth of water which would restrict speciation opportunities or it may just be too young for a rich biota to have evolved Opinion is currently divided between these conflicting hypotheses 144 Ediacara type assemblage Edit The Ediacara type assemblage is named after Australia s Ediacara Hills and consists of fossils preserved in facies of coastal lagoons and rivers They are typically found in red gypsiferous and calcareous paleosols formed on loess and flood deposits in an arid cool temperate paleoclimate 103 Most fossils are preserved as imprints in microbial earths 145 but a few are preserved within sandy units 144 140 Biota ranges 144 view title Discussion about this template gt talk editAxis scale millions of years ago dated with U Pb of zirconsNama type assemblage Edit 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 fossils represent burrowing organisms 60 while Guy Narbonne maintains they were surface dwellers 146 These beds are sandwiched between units comprising interbedded sandstones siltstones and shales with microbial mats where present usually containing the fossils The environment is interpreted as sand bars formed at the mouth of a delta s distributaries 144 Mattress like vendobionts Ernietta Pteridinium Rangea in these sandstones form a very different assemblage from vermiform fossils Cloudina Namacalathus of Ediacaran wormworld in marine dolomite of Namibia 147 Significance of assemblages Edit 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 148 the same fossils are found at all palaeolatitudes the latitude where the fossil was created accounting for continental drift and in separate sedimentary basins 144 It is most likely that the three assemblages mark organisms adapted to survival in different environments and that any apparent patterns in diversity or age are in fact an artefact of the few samples that have been discovered the timeline right demonstrates the paucity of Ediacaran fossil bearing assemblages An analysis of one of the White Sea fossil beds where the layers cycle from continental 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 144 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 149 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 See also Edit Wikimedia Commons has media related to Ediacaran biota Cambrian explosion Large ornamented Ediacaran microfossil List of Ediacaran genera Origin of life Francevillian biotaNotes Edit Simple multicellular organisms such as red algae evolved at least 1 200 million years ago The status of the Francevillian biota of 2 100 million years ago is unclear but they may represent earlier multicellular forms of a more complex nature 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 the Journal 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 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for distinguishing microbial mats and earths Report Special Paper Vol 101 Tulsa Society of Economic Paleontologists and Mineralogists pp 136 152 Narbonne Guy M 2005 The Ediacara biota Neoproterozoic origin of animals and their ecosystems PDF Annual Review of Earth and Planetary Sciences 33 421 442 Bibcode 2005AREPS 33 421N doi 10 1146 annurev earth 33 092203 122519 Archived from the original PDF on 21 February 2007 Retrieved 3 January 2009 via Queen s University Canada Schiffbauer J F 2016 The Latest Ediacaran Wormworld Fauna Setting the Ecological Stage for the Cambrian Explosion GSA Today Vol 26 no 11 pp 4 11 doi 10 1130 GSATG265A 1 hdl 1807 77824 S2CID 52204161 Waggoner B 1999 Biogeographic analyses of the Ediacara biota A conflict with paleotectonic reconstructions Paleobiology 25 4 440 458 doi 10 1017 S0094837300020315 JSTOR 2666048 S2CID 130817983 Bambach R K Bush A M Erwin D H 2007 Autecology and the filling of Ecospace Key metazoan radiations Palaeontology 50 1 1 22 doi 10 1111 j 1475 4983 2006 00611 x Further reading EditDerek Briggs Peter Crowther eds 2001 Palaeobiology II A synthesis Malden MA Blackwell Science pp Chapter 1 ISBN 978 0 632 05147 2 OCLC 43945263 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 ISBN 978 0 231 10558 3 OCLC 3758852 A popular science account of these fossils with a particular focus on the Namibian fossils Wood R A June 2019 The rise of animals New fossils and analyses of ancient ocean chemistry reveal the surprisingly deep roots of the Cambrian explosion Scientific American Vol 320 no 6 pp 24 31 External links EditWatson Traci 28 October 2020 These bizarre ancient species are rewriting animal evolution News Nature review Retrieved 18 December 2022 Early fossils with guts segmented bodies and other sophisticated features reveal a revolution in animal life before the Cambrian explosion a href Template Cite news html title Template Cite news cite news a CS1 maint url status link Ediacaran biota review article with nice illustrations The Ediacaran assemblage peripatus gen nz 2010 Archived from the original on 10 October 2010 Thorough though slightly out of date description cite web title Database of Ediacaran biota date July 2010 website complex life org url http www complex life org database url status dead archive url https web archive org web 20110725191154 http www complex life org database compilation up to 7 2010 The oldest complex animal fossils Kingston Ontario Queen s University 2007 Archived from the original on 28 May 2007 Ediacaran fossils of Canada Kingston Ontario Queen s University 2007 Archived from the original on 5 April 2007 Earth s oldest animal ecosystem held in fossils at Nilpena Station in SA outback ABC News 5 August 2013 Retrieved 6 August 2013 Meet the fossils TV program audio amp transcript Landline Australian Broadcasting Corporation 3 August 2013 Retrieved 28 December 2018 ABC TV show on Ediacaran fossils at NilpenaListen to this article 35 minutes source source This audio file was created from a revision of this article dated 29 August 2009 2009 08 29 and does not reflect subsequent edits Audio help More spoken articles Ediacara Biota on In Our Time at the BBC 2009 radio program Retrieved from https en wikipedia org w index php title Ediacaran biota amp oldid 1136325211, wikipedia, wiki, book, books, library,

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