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Ediacara biota

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Ediacara biota

Dickinsonia costata, an iconic Ediacaran organism, displays the characteristic quilted appearance of Ediacaran enigmata.

The Ediacara (; formerly Vendian) biota consisted of enigmatic tubular and frond-shaped, mostly [note 1] The Ediacara biota radiated in an event called the Avalon explosion, ,[1][2] after the Earth had thawed from the Cryogenian period's extensive glaciation. The biota largely disappeared contemporaneously with the rapid increase in biodiversity known as the Cambrian explosion. Most of the currently existing body plans of animals first appeared only in the fossil record of the Cambrian rather than the Ediacaran. For macroorganisms, the Cambrian biota appears to have completely replaced the organisms that populated the Ediacaran fossil record, although relationships are still a matter of debate.

The organisms of the Ediacaran Period first appeared around and flourished until the cusp of the Cambrian when the characteristic communities of fossils vanished. The earliest reasonably diverse Ediacaran community was discovered in 1995 in Sonora, Mexico, and is approximately 600 million years in age, pre-dating the Gaskiers glaciation of about 580 million years ago.[3][4] 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.[5] 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.

Determining where Ediacaran organisms fit in the [8]

Breandán MacGabhann[11] presents the case that the designation/concept "Ediacara Biota" is artificial and arbitrary as it can not be defined geographically, stratigraphically, taphonomically nor biologically. He points out that 8 particular fossils or groups of fossils considered "Ediacaran" have 5 taphonomic modes (preservation styles), occur in 3 geological periods, and have no phylogenetic meaning as a whole.

History

Paleontologist Guy Narbonne examining Ediacaran fossils in Newfoundland

The first Ediacaran fossils discovered were the disc-shaped Namibia[13] but the firm belief that life originated in the Cambrian led to them being assigned to the Cambrian Period and no link to Aspidella was made. In 1946 Reg Sprigg noticed "jellyfishes" in the Ediacara Hills of Australia's Flinders Ranges[14] but these rocks were believed to be Early Cambrian so, while the discovery sparked some interest, little serious attention was garnered.

It was not until the British discovery of the iconic Charnia in 1957 that the pre-Cambrian was seriously considered as containing life. This frond-shaped fossil was found in England's Charnwood Forest,[15] and due to the detailed geological mapping of the British Geological Survey there was no doubt that these fossils sat in Precambrian rocks. Palæontologist Martin Glaessner finally, in 1959, made the connection between this and the earlier finds[16][17] and with a combination of improved dating of existing specimens and an injection of vigour into the search many more instances were recognised.[18]

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.[19][20]

Poor communication, combined with the difficulty in correlating globally distinct formations, led to a plethora of different names for the biota. In 1960 the French name "Ediacarien" – after the Ediacaran Hills in South Australia, which take their name from aborigine Idiyakra, "water is present" – was added to the competing terms "Sinian" and "Vendian"[21] 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.[22]

The term "Ediacara biota" and similar ("Ediacara"/"Ediacaran"/"Ediacarian"/"Vendian", "fauna"/"biota") has, at various times, been used in a geographic, stratigraphic, taphonomic, or biological sense, with the latter the most common in huge modern literature.[23]

Preservation

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

Microbial mats

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

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.[24]

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.[25] 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 that of the surrounding sea.[26]

Fossilisation

The preservation of these fossils is one of their great fascinations to science. As soft-bodied organisms they would normally not fossilise and, unlike later soft-bodied fossil biota such as the [29]

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, the ash or sand then slumped in to fill the void, leaving a cast of the underside of the organism.

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 [30]

Morphology

Forms of Ediacaran fossil
The earliest discovered potential embryo, preserved within an acanthomorphic acritarch. The term 'acritarch' describes a range of unclassified cell-like fossils.

The earliest discovered potential embryo, preserved within an acanthomorphic acritarch.

Tateana inflata (= 'Cyclomedusa' radiata) is the attachment disk of an unknown organism. Metric scale.
A cast of the quilted Charnia, the first accepted complex Precambrian organism. Charnia was once interpreted as a relative of the sea pens. A cast of Charnia
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. Spriggina may be one of the predators that led to the demise of the Ediacaran fauna
A late Ediacaran Archaeonassa-type trace fossils are commonly preserved on the top surfaces of sandstone strata. A late Ediacaran Archaeonassa-type trace fossils are commonly preserved on the top surfaces of sandstone strata
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.[27] 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 [33] Other "embryos" have been interpreted as the remains of the giant sulfur-reducing bacteria akin to Thiomargarita,[34] a view that is highly contested yet gradually gaining supporters.[35][36]
Microfossils dating from  – just 3 million years after the end of the Cryogenian glaciations – may represent embryonic 'resting stages' in the life cycle of the earliest known animals.[37] An alternative proposal is that these structures represent adult stages of the multicellular organisms of this period.[38]
Discs
Circular fossils, such as [42] Useful diagnostic characters are often lacking because only the underside of the organism is preserved by fossilization.
Bags
Fossils such as Pteridinium preserved within sediment layers resemble "mud-filled bags". The scientific community is a long way from reaching a consensus on their interpretation.[43]
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.[44]
Quilted organisms
The organisms considered in Seilacher's revised definition of the Vendobionta[7] share a "quilted" appearance and resembled an inflatable [45]
These organisms appear to form two groups: the [47] or osmosis.[48]
Non-Ediacaran Ediacarans
Some Ediacaran organisms have more complex details preserved, which has allowed them to be interpreted as possible early forms of living phyla excluding them from some definitions of the Ediacaran biota.
The earliest such fossil is the reputed bilaterian Vernanimalcula claimed by some, however, to represent the infilling of an egg-sac or acritarch.[33][49] Later examples are almost universally accepted as bilaterians and include the mollusc-like Kimberella,[50] Spriggina (pictured)[51] and the shield-shaped Parvancorina[52] whose affinities are currently debated.[53]
A suite of fossils known as the Small shelly fossils are represented in the Ediacaran, most famously by Cloudina[54] 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.
Representatives of modern taxa existed in the Ediacaran, some of which are recognisable today. Sponges, red and green algæ, protists and bacteria are all easily recognisable with some pre-dating the Ediacaran by thousands of millions of years . Possible arthropods have also been described.[55]
Trace fossils
With the exception of some very simple [58] Putative "burrows" dating as far back as may have been made by animals that fed on the undersides of microbial mats, which would have shielded them from a chemically unpleasant ocean;[59] however their uneven width and tapering ends make a biological origin so difficult to defend[60] that even the original proponent no longer believes they are authentic.[61]
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. Dickinsonia are often found at the end of long pathways of trace fossils matching their shape;[62] 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.[63] The potential mollusc Kimberella is associated with scratch marks, perhaps formed by a radula.[64]

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 Ediacara 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.[65]

In 1998 Mark McMenamin claimed that Ediacarans did not possess an embryonic stage, and thus could not be animals. He believed that they independently evolved a nervous system and brains, meaning that "the path toward intelligent life was embarked upon more than once on this planet".[39]

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

Cnidarians

Since the most primitive [67][68]

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.[69] Some researchers have suggested that an analysis of "growth poles" discredits the pennatulacean nature of Ediacaran fronds.[70][71]

Protozoans

A single-celled xenophyophore in the Galapagos Rift

Adolf Seilacher has suggested that the Ediacaran sees animals usurping giant protists as the dominant life form.[72] 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 specialized group of Foraminifera. There are approximately 42 recognized species in 13 genera and 2 orders; one of which, Syringammina fragilissima, is among the largest known protozoans at up to 20 centimeters 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,[73][74] 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 [75] 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".[76]

Lichens

paleosols, interpreted as soils of well drained temperate desert soils.[78][81] Such habitats limit interpretive options for fractal Ediacaran fossils such as Dickinsonia to lichenized or unlichenized fungi, but other Ediacaran fossils could have been slime molds or microbial colonies.

Other interpretations

Several classifications have been used to accommodate the Ediacaran biota at some point,[82] from algae,[83] to protozoans,[84] to fungi[85] to bacterial or microbial colonies,[40] to hypothetical intermediates between plants and animals.[86]

A new extant genus discovered in 2014, Dendrogramma, which appears to be a basal metazoan but of unknown taxonomic placement, has been noted to have similarities with the Ediacaran fauna.[87]

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 ,[88][89] the first uncontroversial evidence for life is found ,[90] and cells with nuclei certainly existed by :[91] The reason why it took so long for forms with an Ediacaran grade of organisation to appear is uncertain.

It could be that no special explanation is required: the slow process of evolution simply required 4 billion years to accumulate the necessary adaptations. Indeed, there does seem to be a slow increase in the maximum level of complexity seen over this time, with more and more complex forms of life evolving as time progresses, with traces of earlier semi-complex life such as Nimbia, found in the Twitya formation,[92] (and possibly older rocks dating to [93]) possibly displaying the most complex morphology of the time.

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

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

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

On the early Earth, reactive elements such as [96] 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.[97]

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.[98] Potentially, complex life may have evolved before these glaciations, and been wiped out. However, the diversity of life in modern Antarctica has sparked disagreement over whether cold temperatures increase or decrease the rate of evolution.

In early 2008 a team analysed the range of basic body structures ("disparity") of Ediacaran organisms from three different fossil beds: Avalon in Canada, to ; White Sea in Russia, to ; and Nama in Namibia, to , immediately before the start of the Cambrian. They found that, while the White Sea assemblage had the most species, there was no significant difference in disparity between the three groups, and concluded that before the beginning of the Avalon timespan these organisms must have gone through their own evolutionary "explosion", which may have been similar to the famous Cambrian explosion .[99]

Preservation bias

The paucity of Ediacaran fossils after Cambrian could simply be because conditions no longer favoured the fossilisation of Ediacaran organisms, which may have continued to thrive unpreserved.[24] However, if they were common, more than the occasional specimen[5][100] might be expected in exceptionally preserved fossil assemblages (Konservat-Lagerstätten) such as the Burgess Shale and Chengjiang.[101] However, Vendobionta remain locally common in paleosols throughout the Cambrian[102] and into the Ordovician,[103] and may always have been rare in marine environments.[78]

A general discussion of preservation bias may be found under Taphonomy.

Kimberella may have had a predatory or grazing lifestyle.

Predation and grazing

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

Alternatively, skeletonised animals could have fed directly on the relatively undefended Ediacaran biota.[39] 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".[50]

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

Cambrian animals such as Waptia may have competed with, or fed upon, Ediacaran life-forms.

Competition

It is possible that increased competition due to the evolution of key innovations among other groups, perhaps as a response to predation,[104] 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.[105]

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,[106] rising sea levels (creating shallow, "life-friendly" seas),[107] a nutrient crisis,[108] fluctuations in atmospheric composition, including oxygen and carbon dioxide levels,[109] and changes in ocean chemistry[110] (promoting biomineralisation)[111] could all have played a part.

Assemblages

Ediacaran-type fossils are recognised globally in 25 localities[22] and a variety of depositional conditions, and are commonly grouped into three main types, named after typical localities. Each assemblage tends to occupy its own region of morphospace, and after an initial burst of diversification changes little for the rest of its existence.[112]

Avalon-type assemblage

The Avalon-type assemblage is defined at Mistaken Point in Canada, the oldest locality with a large quantity of Ediacaran fossils.[113] 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.[112]

The biota comprises deep sea dwelling rangeomorphs[114] 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[115] – 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.[116]

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.[78] Most fossils are preserved as imprints in microbial earths,[117] but a few are preserved within sandy units.[116][112]
Biota ranges[116]
Axis scale: millions of years ago, dated with U/Pb of zircons

Nama-type assemblage

The Nama assemblage is best represented in [43] while Guy Narbonne maintains they were surface dwellers.[118] 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.[116]

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;[119] the same fossils are found at all palæolatitudes (the latitude where the fossil was created, accounting for continental drift) and in separate sedimentary basins.[116]

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.[116]

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.[120] 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

Notes

  1. ^ Simple multicellular organisms such as red algae evolved at least .

References

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Further reading

External links

  • Ediacara Biota on In Our Time at the BBC. (listen now)
  • "The oldest complex animal fossils" – Queen's University, Canada
  • "Ediacaran fossils of Canada" – Queen's University, Canada
  • "The Ediacaran Assemblage" – Thorough, though slightly out-of-date, description
  • "Database of Ediacaran Biota" Advent of Complex Life
  • Earth's oldest animal ecosystem held in fossils at Nilpena Station in SA outback ABC News, 5 August 2013. Accessed 6 August 2013.
  • Meet the fossils ABC Landline TV program on Ediacaran fossils at Nilpena (audio + transcript). First broadcast 3 August 2013. Accessed 11 August 2013.
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