The 'Ediacaran' (
IPA: , formerly 'Vendian') 'biota' are ancient lifeforms, of the
Ediacaran Period, that represent the earliest known complex
multicellular organisms. They appeared soon after the Earth thawed from the
Cryogenian period's
extensive glaciers, and largely disappeared soon before the rapid appearance of biodiversity known as the
Cambrian explosion, which saw the first appearance in the fossil record of the basic patterns and body-plans that would go on to form the basis of modern
animals. Little of the
diversity of the Ediacaran biota would be incorporated in this new scheme, with a distinct Cambrian biota arising and usurping the organisms that dominated the Ediacaran fossil record.
The organisms of the Ediacaran Period first appeared around and flourished until the cusp of the
Cambrian , when their characteristic fossil communities vanished. While rare fossils that may represent survivors have been found as late as the
Middle Cambrian (510 to 500 million years ago), the earlier fossil communities disappear from the record at the end of the
Ediacaran, leaving only fragments of once-thriving ecosystems, if anything.
Multiple hypotheses exist to explain this disappearance, including
preservation bias, a changing environment, the advent of
predators, and competition from other lifeforms.
Some Ediacaran organisms might have been closely related to groups that would rise to prominence later; for instance, ''Kimberella'' shows some similarity to
molluscs, and other organisms show
bilateral symmetry, a trait unique today to the
Bilateria — a huge grouping containing most of the
animal kingdom. Fossilised tracks of burrowing, worm-like organisms are also likely to have been made by bilaterians. However, most non-microscopic fossils are
morphologically distinct from later lifeforms and resemble discs, mud-filled bags, or quilted mattresses.
Classification is difficult, and the assignment of some
species even at the level of kingdom —
animal,
fungus,
protist or something else — is uncertain: one
paleontologist has even gained support for a separate kingdom 'Vendobionta' (now renamed 'Vendozoa').
Their strange form and apparent disconnectedness from later organisms have led some to consider them a "failed experiment" in multicellular life, with later multicellular life independently re-evolving from unrelated single-celled organisms.
[1]
History
The first Ediacaran fossils discovered were the disc-shaped ''Aspidella terranovica'', in 1868. Their discoverer, A. Murray, a geological surveyor, found them useful aids for correlating the age of rocks around
Newfoundland.
However, since they lay below the "Primordial Strata" (i.e., the
Cambrian strata), then thought to contain the very first signs of life, it took four years until someone, Elkanah Billings, dared to propose they could be fossils. Their simple form caused Billings' peers to dismiss his proposal, and they were instead declared gas escape structures, inorganic concretions, or even tricks played by a malicious God to promote unbelief.
No similar structures elsewhere in the world were then known, and the one-sided debate soon fell into obscurity.
[ The First Named Ediacaran Body Fossil, Aspidella terranovica, , James G., Gehling, Palaeontology, ] In 1933, Gürich discovered specimens in
Namibia,
[ Die Kuibis-Fossilien der Nama-Formation von Südwestafrika, Gürich, G., , , , 1933 ] but the belief that life originated in the
Cambrian led to them being assigned there, and no link to ''Aspidella'' was made. In 1946,
Reg Sprigg noticed "jellyfishes" in the
Ediacara Hills of Australia's
Flinders Ranges[ Early Cambrian "jellyfishes"Â of Ediacara, South Australia and Mount John, Kimberly District, Western Australia, Sprigg, R.C., , , Transactions of the Royal Society of South Australia, 1947 ] but these rocks were believed to be Early Cambrian, so while the discovery sparked some interest, little serious attention was garnered.
It was not until the British discovery of the iconic ''
Charnia'' in 1957 that the Ediacaran was seriously considered as containing life. This frond-shaped fossil was found in England's
Charnwood Forest,
[2] and due to the detailed
geologic mapping of the
British Geological Survey there was no doubt that these fossils sat in Precambrian rocks. Palæontologist
Martin Glaessner finally made the connection between this and the earlier finds,
[3][4] and with a combination of improved dating of existing specimens and an injection of vigour into the search, many more instances were recognised.
[5]
However, all specimens discovered until 1967 were in coarse-grained
sandstone that prevented preservation of fine details, making interpretation difficult. Mistra's discovery of fossiliferous ash-beds at the
Mistaken Point assemblage in Newfoundland changed all this, as the delicate detail preserved by the fine ash allowed the description of features that were previously invisible.
[6][7]
Poor communication, combined with the difficulty in correlating globally distinct
formations, has led to a plethora of different names for the biota.
In 1960, the French name "Ediacarien" — after the Ediacaran Hills in Southern Australia, which take their name from
aborigine ''Idiyakra'', "water is present" — was added to the competing "Sinian" and "Vendian",
[ L’Ediacarien, premier etage paleontologique, Termier, H., , , Rev. Gen. Sci. et Bull. Assoc. Francaise Avan. Sci., 1960 ] terms for terminal-Precambrian rocks which were also applied to the lifeforms. "Ediacaran" and "Ediacarian" were subsequently applied to the epoch or period of
geologic time and its corresponding rocks. In March 2004, the
International Union of Geological Sciences ended the inconsistency by formally naming the terminal
period of the
Neoproterozoic after the Australian locality.
[ The Ediacaran Period: a new addition to the geologic time scale, , Andy H., Knoll, Lethaia, Reprint, 2004 original available here (PDF).]
Preservation
The fossil ''Charniodiscus'' is barely distinguishable from the "elephant skin" texture on this cast.
All but the smallest fraction of the fossil record is comprised of the robust
skeletal matter of decayed corpses. Hence, since Ediacaran biota had soft bodies and no skeletons, their abundant preservation is surprising. The absence of burrowing creatures living in the sediments undoubtedly helped;
since after the evolution of these organisms in the Cambrian, soft-bodied impressions were usually disturbed before they could fossilize.
Microbial mats
Microbial mats are areas of sediment stabilised by the presence of colonies of microbes, which secrete sticky fluids or otherwise bind the sediment particles. They appear to migrate upwards when covered by a thin layer of sediment, but this is an illusion caused by the colony's growth; individuals do not, themselves, move. If too thick a layer of sediment is deposited before they can grow or reproduce through it, parts of the colony will die, leaving behind fossils with a characteristically wrinkled "elephant skin" texture.
[ The Proterozoic biosphere, Runnegar, B.N., , , , 1992, ]
Most Ediacaran strata with the "elephant skin" texture that signifies a microbial mat contain fossils, and Ediacaran fossils are almost never found in beds that do not contain these microbial mats.
Although microbial mats were once widespread, the evolution of grazing organisms in the Cambrian vastly reduced their numbers,
[ The Ecology of the Cambrian Radiation, Burzin, M.B., , , , 2001, ] and these communities are now limited to inhospitable
refugia where predators cannot survive long enough to eat them.
Fossilisation
The fossils were preserved by virtue of rapid covering by ash or sand, trapping them against the mud or microbial mats on which they lived.
[8] Ash beds provide more detail, and can readily be precisely dated to the nearest million years or better by means of
radiometric dating.
[9]
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.
Soft-bodied organisms today almost never fossilise during such events, but the presence of widespread microbial mats aided preservation by stabilising their impressions in the sediment below.
[10]
What is preserved?
The rate of cementation of the overlying substrate, relative to the rate of decomposition of the organism, determines whether the top or bottom surface of an organism is preserved. Most disc-shaped fossils decomposed before the overlying sediment was cemented, and the ash or sand slumped in to fill the void, leaving a cast of the underside of the organism.
Conversely, quilted fossils tend to decompose ''after'' the cementation of the overlying sediment; hence their upper surfaces are preserved. Their more resistant nature is reflected in the fact that in rare occasions, quilted fossils are found ''within'' storm beds, the high-energy sedimentation not destroying them as it would the less-resistant discs. Further, in some cases, the
bacterial
precipitation of minerals formed a "death mask", creating a mould of the organism.
[ Microbial mats in terminal Proterozoic siliciclastics; Ediacaran death masks, Gehling, J.G., , , Palaios, ]
Morphology
| Forms of Ediacaran fossil |
|---|
| The earliest discovered potential embryo, preserved within an acanthomorphic acritarch. The term 'acritarch' describes a range of unclassified cell-like fossils. |  The earliest discovered potential embryo, preserved within an acanthomorphic acritarch. |
| ''Cyclomedusa'', a disc shaped fossil that has been interpreted as a microbial artefact. Metric scale. |  Cyclomedusa, a disc shaped fossil that has been interpreted as a microbial artefact |
| A cast of the quilted ''Charnia'', the first accepted complex Precambrian organism. ''Charnia'' was once interpreted as a relative of the sea-pens. |  A cast of Charnia |
| ''Spriggina'', a possible precursor to the Trilobites, may be one of the predators that led to the demise of the Ediacaran fauna[11] and subsequent diversification of animals.[12] |  Spriggina may be one of the predators that led to the demise of the Ediacaran fauna |
 A late Ediacaran trace fossil preserved on a bedding plane |
| A late Ediacaran trace fossil preserved on a bedding plane. |
The Ediacaran biota exhibited a vast range of
morphological characteristics. Size ranged from
millimetres to
metres; complexity from "blob-like" to intricate; rigidity from sturdy and resistant to jelly-soft. Almost all forms of
symmetry were present.
8 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
[13] generated intense media excitement
[14] though some have claimed they are instead inorganic structures formed by the
precipitation of minerals on the inside of a hole.
Other "embryos" have been interpreted as the remains of the giant
sulfur-reducing bacteria ''
Thiomargarita'',
[e.g. Evidence of giant sulphur bacteria in Neoproterozoic phosphorites, Bailey, J.V., , , Nature, 2007 , summarised by Embryonic identity crisis, Donoghue, P.C.J., , , Nature, 2007 ] a view which is highly contested.
Xiao ''et al''.'s response to Bailey ''et al''.'s original paper : Palaeontology: undressing and redressing Ediacaran embryos, Xiao, S., , , Nature, 2007
And Bailey ''et al''.'s reply: Palaeontology: Undressing and redressing Ediacaran embryos (Reply), Bailey, J.V., , , Nature, 2007 [ Eukaryotic organisms in Proterozoic oceans, Knoll, AH, , , Philosophical Transactions of the Royal Society B: Biological Sciences, 2006 ]
: 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.
[15]
; Discs : Circular fossils, such as ''
Ediacaria'', ''
Cyclomedusa'', and
Rugoconites led to the initial identification of Ediacaran fossils as
cnidaria, which include jellyfish and corals.
Further examination has provided alternative interpretations of all disc-shaped fossils: none is now confidently recognised as jellyfish. Alternate explanations include
holdfasts,
protists
and
anemones; the patterns displayed where two meet have led to many being recognised as microbial colonies.
[ Ediacaran microbial colonies, Grazhdankin, D., , , Lethaia, ] 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.
[(a) The only current description, far from universal acceptance, appears as: Underground Vendobionta From Namibia, Grazhdankin, D., , , Palaeontology, 2002 ]
; Quilted organisms : The organisms considered in Seilacher's revised definition of the Vendobionta
[16] 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.
[17]
:This "rangeomorph" class of organism, including the famous ''
Charnia'' and ''
Charniodiscus'', is both the most iconic of the Ediacaran biota, and the most difficult to place within the existing
tree of life. The quilted structure may be derived from a shared common ancestor (
synapomorphy), but if it represents the most ecologically sensible form for an organism to take, different lineages may have converged upon it (
plesiomorphy).
; Non-Ediacaran Ediacarans : Some Ediacaran organisms have more complex details preserved, which has allowed them to be interpreted as possible
early forms of
living phyla, excluding them from some definitions of the Ediacaran biota.
:The earliest such fossil is the reputed bilaterian ''
Vernanimalcula'', claimed by some, however, to represent the infilling of an egg-sac or
acritarch.
[ Comment on "Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian", , , , Science, ][ Response to Comment on "Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian", , , , Science, ] Later examples, almost universally accepted as bilaterians, include the mollusc-like ''
Kimberella'',
[ The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism, Fedonkin, M.A., , , Nature, 1997 ]
''
Spriggina'' (pictured),
and the shield-shaped ''
Parvancorina'',
[18] whose affinities are currently debated.
[19]
:A suite of fossils known as the
Small Shelly Fossils are represented in the Ediacaran, most famously by ''
Cloudina'',
[ New shelly fossils from Nama Group, South West Africa, Germs, G.J.B., , , American Journal of Science, ] a shelly tube-like fossil that often shows evidence of predatory boring, suggesting that whilst predation may not have been common in the Ediacaran Period, it was at least present.
: Representatives of modern taxa existed in the Ediacaran, some of which are recognisable today.
Sponges, red and green
algæ,
protists and
bacteria are all easily recognisable, with some pre-dating the Ediacaran by thousands of millions of years.
;
Trace fossils : The only Ediacaran burrows are horizontal, or just below the surface. Such burrows imply the presence of motile organisms with heads, which would probably have had a bilateral symmetry. This could place them in the
bilateral clade of
animals.
[20] Putative "burrows" dating as far back as million years may have been made by animals which fed on the undersides of microbial mats, which would have shielded them from a chemically unpleasant ocean;
[ Triploblastic Animals More Than 1 Billion Years Ago: Trace Fossil Evidence from India, Seilacher, A., , , Science, ] however their uneven width and tapering ends make a biological origin difficult to defend.
[ A critical reappraisal of the fossil record of the bilaterian phyla, Budd, G.E., , , Biological Reviews, 2000 ] 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;
[ Giant Traces of Vendian Animals, Ivantsov, A.Y., , , Doklady Earth Sciences (Doklady Akademii Nauk), 2002 ] the method of formation of these disconnected and overlapping fossils largely remains a mystery. The potential
mollusc ''Kimberella'' is associated with scratch marks thought to have been formed by its
radula,
[According to Age of Neoproterozoic Bilatarian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution, Martin, M.W., , , Science, For a more cynical perspective see Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale, Butterfield, N.J., , , Bioessays, 2006 ] further traces from appear to imply active crawling or burrowing activity.
Classification and interpretation
Classification of the Ediacarans is difficult, and hence a variety of theories exist as to their placement on the tree of life.
A sea-pen, a cnidarian bearing a passing resemblance to ''Charnia''
Cnidarians
Since the most primitive
metazoans — multi-cellular animals in possession of a
nervous system — are recognised as
cnidarians, the first attempt to categorise these fossils designated them as
jellyfish and
sea-pens.
[21] However, detailed study of their growth pattern has discounted this hypothesis.
[22]
"The dawn of animal life"
Martin Glaessner proposed in his 1985 book "The dawn of animal life" that the Ediacaran biota were early
stem group members of all modern phyla, and were unrecognisable because they had yet to evolve the characteristic features we use in modern classification.
[ The Dawn of Animal Life: A Biohistorical Study, Glaessner, M.F., , , Cambridge University Press, 1984, ] Adolf Seilacher responded by suggesting that the Ediacaran sees animals usurping
giant protists as the dominant life form.
[23]
Mark McMenamin goes one step further: he claims that Ediacarans did not possess an
embryonic stage, and thus could not be animals. He believes that they independently evolved a nervous system and brains, meaning that "the path toward intelligent life was embarked upon more than once on this planet."
New phylum
Seilacher most famously suggested that the Ediacaran organisms represented a unique and extinct grouping of related forms descended from a common ancestor (
clade) and created the
kingdom Vendozoa,
[24][ Vendozoa: organismic construction in the Proterozoic biosphere, Seilacher, A., , , Lethaia, 1989 ] 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.
[25]
Lichen with a 3D structure may be preserved in a similar fashion to wood.
Lichens
Gregory Retallack's hypothesis that Ediacaran organisms were
lichens
[26] has failed to gain wide-spread acceptance. He argues that the fossils are not as squashed as jellyfish fossilised in similar situations, and their relief is closer to
petrified wood. He points out the
chitinous walls of lichen colonies would provide a similar resistance to compaction, and claims the large size of the organisms — sometimes over a metre across, far larger than any of the preserved burrows — also hints against a classification with the animals.
Other interpretations
Almost every possible phylum has been used to accommodate the Ediacaran biota,
[27] from
algæ,
[28] to
protists known as
foraminifera,
[29] to
fungi[30] to
bacterial or
microbial colonies,
[ ]
to hypothetical intermediates between plants and animals.
[31]
Since representatives of almost all modern phyla were in existence by the Middle Cambrian, it is probable that the precursors of many phyla would be represented in the Ediacaran. The accumulation of random changes in sequences of DNA — assumed to accumulate at a constant rate — can be used to estimate the time that two lineages shared a common ancestor, and applying
this technique to modern phyla produces estimated divergence dates long before the Cambrian.
[ Hox genes in brachiopods and priapulids and protostome evolution, De Rosa, R., , , Nature, 1999 ] If this is indeed the case, attempts to group ''everything'' alive in the Ediacaran into one phylum are doomed to failure.
Origin
It took 4 billion years from the formation of the Earth for the Ediacaran fossils to first appear, 655 million years ago. Whilst putative fossils are reported from ,
[ Early Archean (3.3-billion to 3.5-billion-year-old) microfossils from Warrawoona Group, Australia, Schopf, J.W., , , Science, ][ Origin of 3.45 Ga coniform stromatolites in Warrawoona Group, Western Australia, Hofmann, H.J., , , Bulletin of the Geological Society of America, ] the first uncontroversial evidence for life is found ,
[ Coupled Fe and S isotope evidence for Archean microbial Fe (III) and sulfate reduction, Archer, C., , , Geology, ] and cells with nuclei certainly existed by :
[ ''Bangiomorpha pubescens n. gen., n. sp.'': implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes, Butterfield, N.J., , , Paleobiology, ] why did it take so long for forms with an Ediacaran grade of organisation to appear?
It could be that no special explanation is required: the slow process of evolution simply required 4 billion years to accumulate the necessary adaptations. Indeed, there does seem to be a slow increase in the maximum level of complexity seen over this time, with more and more
complex forms of life appearing as time progresses, with traces of earlier semi-complex life such as ''
Nimbia'', found in the million-year-old Twitya formation,
[ New representatives of the Precambrian coelenterates in the northern Russian platform, Fedonkin, M.A., , , Paleontologicheskij Zhurnal, 1980 ] 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 iron and uranium existed in a
reduced form; these would react with any free oxygen produced by
photosynthesising organisms. Oxygen would not be able to build up in the atmosphere until all the iron had rusted, and other reactive elements had been oxidised.
Donald Canfield detected records of the first significant quantities of atmospheric oxygen just before the first Ediacaran fossils appeared
[ Late Proterozoic rise in atmospheric oxygen concentration inferred from phylogenetic and sulphur-isotope studies, Canfield, D.E., , , Nature, 1996 ] — and the presence of atmospheric oxygen was soon heralded as a possible trigger for the Ediacaran
radiation.
[ Late-Neoproterozoic Deep-Ocean Oxygenation and the Rise of Animal Life, Canfield, D.E., , , Science, ] 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.
[ Oxidation of the Ediacaran ocean, Fike, DA, , , Nature, 2006 ] The resolution of the fossil record is too low to make this assertion definite, and current research seeks to accurately determine the role that oxygen may have played.
[32]
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.
[33] Potentially, complex life may have evolved before these glaciations, and been wiped out. However, the diversity of life in modern Antarctica has sparked disagreement over whether cold temperatures increase or decrease the rate of evolution.
Disappearance
The low resolution of the fossil record means that the disappearance of the Ediacarans remains something of a mystery. There appears to have been a relatively abrupt disappearance at the end of the Ediacaran period; reports of Cambrian "Ediacarans" are not universally accepted. The cause — and reality — of this disappearance is open to debate.
Preservation bias
The sudden vanishing of Ediacaran fossils at the Cambrian boundary could simply be because conditions no longer favoured the fossilisation of Ediacaran organisms, which may have continued to thrive unpreserved.
However, if they were common, more than the occasional specimen[ Ediacaran-like fossils in Cambrian Burgess Shale–type faunas of North America, Conway Morris, S., , , Palaeontology, 1993 ] might be expected in exceptionally preserved fossil assemblages (Konservat-Lagerstätten) such as the Burgess Shale and Chengjiang[ Lower Cambrian Vendobionts from China and Early Diploblast Evolution, Shu, D.-G.; Morris, S. Conway; Han, J.; Li, Y.; Zhang, X.-L.; Hua, H.; Zhang, Z.-F.; Liu, J.-N.; Guo, J.-F.; Yao, Y.; Yasui, K., , , Science, ] — unless such assemblages represent an environment never occupied by the Ediacaran biota, or unsuitable conditions for their preservation.
''Kimberella'' may have had a predatory or grazing lifestyle.
Predation and grazing
By the Early Cambrian, organisms higher in the food chain caused the microbial mats to largely disappear. These grazers first appeared as the Ediacaran biota started to decline, which may suggest that they destabilised the microbial substrate, leading to displacement or detachment of the biota; or that the destruction of the mat destabilised the ecosystem.
Alternatively, skeletonised animals could have fed directly on the relatively undefended Ediacaran biota.[ The Garden of Ediacara, McMenamin M., , , , 1986, ]
However, the existence in the Ediacaran of the recognized predator ''Kimberella'' suggests that the biota had already had limited exposure to predation.
Cambrian animals such as ''
Waptia'' may have competed with, or fed upon, Ediacaran lifeforms.
Competition
It is possible that increased competition due to the evolution of key innovations amongst other groups, perhaps as a response to predation,[34] 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.[35]
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,[36] rising sea levels (creating shallow, "life-friendly" seas),[37] a nutrient crisis,[ Background to the Cambrian explosion, Brasier, M.D., , , Journal of the Geological Society, London, 1992 ] fluctuations in atmospheric composition, including oxygen and carbon dioxide levels,[ Global ocean-atmosphere change across the Precambrian-Cambrian transition, Brasier, M.D., , , Geological Magazine, 1992 ] and changes in ocean chemistry[ Oscillations in Phanerozoic Seawater Chemistry: Evidence from Fluid Inclusions, Lowenstein, T.K., , , Science, 2001 ]
(promoting biomineralisation)[38] could all have played a part.
Assemblages
Ediacaran-type fossils are recognised globally in 25 localities[ and a variety of depositional conditions, and are commonly grouped into three main types, named after typical localities.]
Ediacara-type assemblage
The Ediacara-type assemblage is named after Australia's Ediacara Hills, and consist of fossils preserved in prodeltaic facies (areas near the mouths of rivers). They are typically found in interbedded sandy and silty layers formed below the normal base of wave-related water motion, but in waters shallow enough to be affected by wave motion during storms. Most fossils are preserved as imprints in microbial mats, but a few are preserved ''within'' sandy units.[39]
{| class=toccolours width="150" align="right" style="margin:0 0 0.5em 1em;"
|-
!bgcolor=#77bb77| Biota ranges
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|align=right style="font-size:smaller;"| • • [ edit]
|-
|
|-
|align=center style="font-size:smaller;"|Axis scale: millions of years ago, dated with U/Pb of zircons
|}
Nama-type assemblage
The Nama assemblage is best represented in Namibia. Three-dimensional preservation is most common, with organisms preserved in sandy beds containing internal bedding. Dima Grazhdankin believes that these organisms represent burrowing organisms,
Avalon-type assemblage
The Avalon-type assemblage is defined at Mistaken Point in Canada, the oldest locality with a large quantity of Ediacaran fossils.[29]
The assemblage is easily dated because it contains many fine ash-beds, which are a good source of zircons used in the uranium-lead method of radiometric dating. These fine-grained ash beds also preserve exquisite detail.
The biota comprises deep sea dwelling rangeomorphs[42]
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[43] — by most interpretations, the assemblage is found in water too deep for photosynthesis. The low diversity may reflect the depth of water — which would restrict speciation opportunities — or it just be too young for evolution to rich biota. Opinion is currently divided between these conflicting hypotheses.
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;[44] the same fossils are found at all palæolatitudes (the latitude where the fossil was created, accounting for continental drift) and in separate sedimentary basins.
It is most likely that the three assemblages mark organisms adapted to survival in different environments, and that any apparent patterns in diversity or age are in fact an artefact of the few samples that have been discovered — the timeline (right) demonstrates the paucity of Ediacaran fossil-bearing assemblages.
As the Ediacaran biota represent an early stage in multicellular life's history, it is unsurprising that not all possible modes of life are occupied.
It has been estimated that of 92 potentially possible modes of life — combinations of feeding style, tiering and motility — no more than a dozen are occupied by the end of the Ediacaran. Just four are represented in the Avalon assemblage.[45] The lack of large-scale predation and vertical burrowing are perhaps the most significant factors limiting the ecological diversity; the emergence of these during the Early Cambrian allowed the number of lifestyles occupied to rise to 30.
Further reading
★ The Crucible of Creation: The Burgess Shale and the Rise of Animals, Simon Conway Morris, , , , ,
★ The Garden of Ediacara: Discovering the First Complex Life, Mark McMenamin, , , , ,
★ Palæobiology II: A synthesis, Derek Briggs & Peter Crowther (Editors), , , , , Good further reading for the keen - includes many interesting chapters with macroevolutionary theme.
External links
★ "The oldest complex animal fossils" - Queens' University, Canada
★ "Ediacaran fossils of Canada" - Queens' University, Canada
★ "The Ediacaran Assemblage" - Thorough, though slightly out-of-date, description
See also
★ List of Ediacaran genera
★ Origin of life
★ Cambrian explosion
References
1. The Origin and Early Evolution of Animals
2. Leicester’s fossil celebrity: Charnia and the evolution of early life
3. Martin F Glaessner: Palaeontologist extraordinaire, Sprigg, R.C., , , Mem. Geol. Soc. India, 1991
4. The oldest fossil faunas of South Australia, Glaessner, M.F., , , International Journal of Earth Sciences, 1959
5. Precambrian Animals, Glaessner, Martin F., , , Science. Am., 1961
6. Late Precambrian(?) fossils from southeastern Newfoundland, Misra, S.B., , , Geol. Soc. America Bull., 1969
7. The Mistaken Point Fossil Assemblage Newfoundland, Canada
8. The Ediacara biota: A terminal Neoproterozoic experiment in the evolution of life, Narbonne, Guy M., , , GSA, 1998
9. Palæobiology II: A synthesis, Bowring, S.A., , , Blackwell publishing group, 2001,
10. Earliest known echinoderm — A new Ediacaran fossil from the Pound Subgroup of
South Australia, Gehling, J.G., , , Alcheringa, 1987
11.
12. e.g. Macroevolution and microecology through deep time, Butterfield, N.J., , , Palaeontology, 2007
13. Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian, Chen, J-Y, , , Science, 2004
14. For example, Fossil may be ancestor of most animals , Earliest Bilateral Fossil Discovered Leslie Mullen
15. Doushantuo embryos preserved inside diapause egg cysts, Leiming, Y., , , Nature, 2007
16. Vendobionta and Psammocorallia: lost constructions of Precambrian evolution, Seilacher, A., , , Journal of the Geological Society, London, 1992
17. The Youngest Ediacaran Fossils from Southern Africa, Narbonne, G.M., , , Journal of Paleontology, 1997
18. Parvancorina — an arthropod from the late Precambrian of South Australia, Glaessner, M.F., , , Ann. Nat. Hist. Mus. Wien., 1980
19. For a reinterpretation, see Some Problematic Fossils from the Vendian of the Southeastern White Sea Region, Ivantsov, A.Y., , , Paleontological Journal, 2004
20. Vendian faunas and the early evolution of Metazoa, Fedonkin, M.A., , , in Lipps, J., and Signor, P. W., eds., Origin and early evolution of the Metazoa: New York, Plenum Press., 1992
21. Fossils explained 35. The Ediacaran biota, Donovan, Stephen K., Lewis, David N., , , Geology Today, 2001
22. Charnia and sea pens are poles apart, Antcliffe, J.B., , , Journal of the Geological Society, 2007
23.
Ediacaran biota: The dawn of animal life in the shadow of giant protists, Seilacher, A., , , Paleontological research, 2003
24. Patterns of Change in Earth Evolution, Seilacher, A., , , Heidelberg: Springer-Verlag, 1984,
25. The Phylum Vendobionta: A Sister Group of the Eumetazoa?, Buss, L.W. and Seilacher, A., , , Paleobiology, 1994
26. Were the Ediacaran fossils lichens?, Retallack, G.J., , , Paleobiology, 1994
27. Interpreting the Earliest Metazoan Fossils: What Can We Learn?, Waggoner, Ben, , , Integrative and Comparative Biology, 1998
28. Pre-Cambrian fossils from Charnwood Forest, Ford, T.D., , , Proceedings of the Yorkshire Geological Society, 1958
29.
30. A Fungal Analog for Newfoundland Ediacaran Fossils?, Peterson, K.J. and Waggoner, B. and Hagadorn, J.W., , , Integrative and Comparative Biology, 2003
31. Zur fauna der Nama-Schichten in Südwest-Afrika. IV. Mikroscopische anatomie der petalo-organisme., Pflug, , , Paleontographica, 1973
32. PhD Project description
33.
34. An ecological theory for the sudden origin of multicellular life in the Late Precambrian, Stanley, S.M., , , Proc. Nat. Acad. Sci. U.S.A., 1973
35. Clams and Brachiopods-Ships that Pass in the Night, Gould, S.J., , , Paleobiology, 1980
36. Early Cambrian continental reconstructions, McKerrow, W.S., , , Journal of the Geological Society, London, 1992
37. Pre-Quaternary sea-level changes, Hallam, A., , , Annual Reviews, 1984
38. A Vendian-Cambrian boundary succession from the northwestern margin of the Siberian Platform: stratigraphy, palaeontology, chemostratigraphy and correlation, Bartley, J.K., , , Geological Magazine, 1998
39. Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution, Grazhdankin, Dima, , , Palæobiology, 2004 (Source of data for Timeline synthesis, p218. Further citations available in caption to Fig. 8.)
40. The Ediacara Biota: Neoproterozoic Origin of Animals and Their Ecosystems, , Guy M., Narbonne, , 2005
41.
42. Paleoecology of the oldest known animal communities: Ediacaran assemblages at Mistaken Point, Newfoundland, Clapham, Matthew E., , , Paleobiology, 2003
43. Ediacaran epifaunal tiering, Clapham, M.E., , , Geology, 2002
44. Biogeographic Analyses of the Ediacara Biota: A Conflict with Paleotectonic Reconstructions, Waggoner, B., , , Paleobiology, 1999
45. Autecology and the filling of Ecospace: Key metazoan radiations, Bambach, R.K., , , Palæontology, 2007
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