Ediacara biota

The Ediacara (IPAEng|ˌiːdɪˈækərə, formerly Vendian) biota are ancient lifeforms of the Ediacaran Period, which 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 Ediacara 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 Ma|580 and flourished until the cusp of the Cambrian Ma|542, when the characteristic communities of fossils 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 controversial 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 have been thought to show bilateral symmetry, though this is controversial. Most microscopic fossils are morphologically distinct from later lifeforms: they 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 Vendozoa (now renamed Vendobionta). 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. [cite web
last = Narbonne
first = Guy
title = The Origin and Early Evolution of Animals
publisher = Department of Geological Sciences and Geological Engineering, Queen's University
month = June | year = 2006
url = http://geol.queensu.ca/people/narbonne/recent_pubs1.html
accessdate = 2007-03-10]

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", the Cambrian strata that were then thought to contain the very first signs of life, it took four years for anybody to dare propose they could be fossils. Elkanah Billings' proposal was dismissed by his peers on account of their simple form, 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.cite journal|title=The First Named Ediacaran Body Fossil, Aspidella terranovica|first=James G. |last=Gehling |coauthors=Guy M. Narbonne and Michael M. Anderson
journal=Palaeontology|volume=43|pages=429|year=2000|doi=10.1111/j.0031-0239.2000.00134.x] In 1933, Georg Gürich discovered specimens in Namibia,cite journal
author = Gürich, G.
year = 1933
title = Die Kuibis-Fossilien der Nama-Formation von Südwestafrika
volume = 15
pages = 137–155
language = German] 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 Rangescite journal
author = Sprigg, R.C.
year = 1947
title = Early Cambrian "jellyfishes" of Ediacara, South Australia and Mount John, Kimberly District, Western Australia
journal = Transactions of the Royal Society of South Australia
volume = 73
pages = 72–99] 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, [cite web
url=http://www.charnia.org.uk/saturday_school_2007.htm
title=Leicester’s fossil celebrity: Charnia and the evolution of early life
accessdate=2007-06-22] 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, [cite journal
author = Sprigg, R.C.
year = 1991
title = Martin F Glaessner: Palaeontologist extraordinaire
journal =Mem. Geol. Soc. India
volume =20
pages =13–20] [cite journal
title=The oldest fossil faunas of South Australia
author=Glaessner, M.F.
journal=International Journal of Earth Sciences
volume=47
issue=2
pages=522–531
year=1959
url=http://www.springerlink.com/content/th8t5154382750kq/
doi=10.1007/BF01800671
issn=1437-3254
publisher=Springer] and with a combination of improved dating of existing specimens and an injection of vigour into the search, many more instances were recognised. [cite journal
author =Glaessner, Martin F.
year=1961
title=Precambrian Animals
journal=Science. Am.
volume=204
pages=72–78]

However, 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 invisible. [cite journal
author=Misra, S.B.
year=1969
title=Late Precambrian(?) fossils from southeastern Newfoundland
journal=Geol. Soc. America Bull.
volume=80
url=http://bulletin.geoscienceworld.org/cgi/content/abstract/80/11/2133
doi=10.1130/0016-7606(1969)80 [2133:LPFFSN] 2.0.CO;2
pages=2133–2140
format=abstract] [cite web
last = Badham
first = Mark
title = The Mistaken Point Fossil Assemblage Newfoundland, Canada
publisher = The Miller Museum of Geology, Queen's University, Kingston, Ontario, Canada
date = 2003-01-30
url = http://geol.queensu.ca/people/narbonne/recent_pubs1.html
accessdate = 2007-03-10]

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 Southern Australia, which take their name from aborigine "Idiyakra", "water is present" — was added to the competing terms "Sinian" and "Vendian",cite journal
author = Termier, H.
coauthors = Termier, G.
year = 1960
title = L’Ediacarien, premier etage paleontologique
journal = Rev. Gen. Sci. Et Bull. Assoc. Francaise Avan. Sci.
issue=3-4
pages=175–192
volume = 67
language= French ] for terminal-Precambrian rocks, names that 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.cite journal
first = Andy H.
last = Knoll
coauthors = Walter, M.; Narbonne, G.; Christie-Blick, N.
title = The Ediacaran Period: a new addition to the geologic time scale
pages = 13–30
journal = Lethaia
volume = 39
year = 2006
url = http://geol.queensu.ca/people/narbonne/KnollWalterNarbonneChristieBlick_Lethaia_2006.pdf
format = PDF
doi = 10.1080/00241160500409223
accessdate = 2007-04-14Reprint, 2004 original available [http://www.stratigraphy.org/precambrian/Knoll_et_al_2004a.pdf here] (PDF).]

Preservation

All but the smallest fraction of the fossil record consists 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.cite book
author = Runnegar, B.N.
coauthors = Fedonkin, M.A.
editor = Schopf, W.J.; Klein, C.
title = The Proterozoic biosphere
publisher = Cambridge University Press
year = 1992
chapter = Proterozoic metazoan body fossils
journal = The Proterozoic Biosphere, a Multidisciplinary Study: Cambridge University Press, New York
pages = 369–388
isbn=9780521366151
oclc = 23583672 26310475 ] Most Ediacaran strata with the "elephant skin" texture characteristic of microbial mats 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,cite book
editor = Zhuravlev, A.Y.; Riding, R.
author = Burzin, M.B.
coauthors = Debrenne, F.; Zhuravlev, A.Y.
year = 2001
publisher = Columbia University Press, New York
chapter = Evolution of shallow-water level-bottom communities
pages = 216–237
title = The Ecology of the Cambrian Radiation
url = http://www.questia.com/PM.qst?a=o&docId=100738360
isbn=0231505167
accessdate = 2007-05-06
oclc = 51852000 ] and these communities are now limited to inhospitable refugia where predators cannot survive long enough to eat them.

Fossilisation

The preservation of these fossils is one of their great fascinations to science. As soft-bodied organisms they would normally not fossilise. Unlike later soft-bodied fossil biota (such as the Burgess Shale, or Solnhofen Limestone) the Ediacara 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. It is thought that 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.cite journal
author = Narbonne, Guy M.
year = 1998
title = The Ediacara biota: A terminal Neoproterozoic experiment in the evolution of life
journal = GSA
volume = 8
issue = 2
pages = 1–6
issn = 1052-5173
url = ftp://rock.geosociety.org/pub/GSAToday/gt9802.pdf
accessdate = 2007-03-08 ] Ash beds provide more detail, and can readily be precisely dated to the nearest million years or better by means of radiometric dating. [cite book
author = Bowring, S.A.
coauthors = Martin, M.W.
year = 2001
chapter=Calibration of the Fossil Record
title = Palæobiology II: A synthesis
editor = Briggs & Crowther
pages =
isbn = 9780632051496
url = http://www.blackwellpublishing.com/book.asp?ref=9780632051496
publisher= Blackwell publishing group
accessdate = 2007-06-21
oclc = 51481754 55536116 ] 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 probably aided preservation by stabilising their impressions in the sediment below. [cite journal
author = Gehling, J.G.
coauthors =
year = 1987
title = Earliest known echinoderm — A new Ediacaran fossil from the Pound Subgroup ofSouth Australia
journal = Alcheringa
volume = 11
pages = 337–345
issn = 0311-5518
accessdate = 2007-03-08
doi = 10.1080/03115518708619143 ]

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 having destroyed them as it would have the less-resistant discs. Further, in some cases, the bacterial precipitation of minerals formed a "death mask", creating a mould of the organism.cite journal
author = Gehling, J.G.
date = 1999-02-01
title = Microbial mats in terminal Proterozoic siliciclastics; Ediacaran death masks
journal = Palaios
volume = 14
issue = 1
pages = 40–57
issn = 0883-1351
url = http://palaios.geoscienceworld.org/cgi/content/abstract/14/1/40
accessdate = 2007-05-23
doi = 10.2307/3515360
format = abstract]

Morphology

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. 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 findscite journal
author = Chen, J-Y
year = 2004
title = Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian
journal = Science
volume = 305
issue = 5681
pages = 218–222
doi = 10.1126/science.1099213
url = http://www.sciencemag.org/cgi/rapidpdf/1099213v1?ijkey=TmRu98UQkv06Y&keytype=ref&siteid=sci
accessdate = 2007-04-27 ] generated intense media excitement [For example, cite web
url=http://www.msnbc.msn.com/id/5112628/
title=Fossil may be ancestor of most animals
accessdate=2007-06-22
publisher=msnbc, cite web
url=http://www.astrobio.net/news/article1005.html
title=Earliest Bilateral Fossil Discovered
accessdate=2007-06-22
author=Leslie Mullen
publisher=Astrobiology Magazine] 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 akin to "Thiomargarita",e.g. cite journal
author = Bailey, J.V.
coauthors = Joye, S.B., Kalanetra, K.M., Flood, B.E., Corsetti, F.A.
year = 2007
title = Evidence of giant sulphur bacteria in Neoproterozoic phosphorites
journal = Nature
volume = 445
issue = 7124
pages = 198–201
doi = 10.1038/nature05457
url = http://www.nature.com/nature/journal/v445/n7124/abs/nature05457.html
accessdate = 2007-04-28
format = abstract , summarised by cite journal
author = Donoghue, P.C.J.
year = 2007
title = Embryonic identity crisis
journal = Nature
volume = 445
pages = 155–156
issn = 0028–0836
doi = 10.1038/nature05520
url = http://www.nature.com/nature/journal/v445/n7124/full/nature05520.html
accessdate = 2007-06-21 ] a view which is highly contested yet gradually gaining supporters.

Xiao "et al".'s response to Bailey "et al".'s original paper : cite journal
author = Xiao, S.
coauthors = Zhou, C.; Yuan, X.
year = 2007
title = Palaeontology: undressing and redressing Ediacaran embryos
journal = Nature
volume = 446
issue = 7136
pages = E9–E10
doi = 10.1038/nature05753
url = http://www.nature.com/nature/journal/v446/n7136/abs/nature05753.html
accessdate = 2007-06-21
format = abstract

And Bailey "et al".'s reply: cite journal
author = Bailey, J.V.
coauthors = Joye, S.B.; Kalanetra, K.M.; Flood, B.E.; Corsetti, F.A.
year = 2007
title = Palaeontology: Undressing and redressing Ediacaran embryos (Reply)
journal = Nature
volume = 446
issue = 7136
pages = E10–E11
doi = 10.1038/nature05754
url = http://www.nature.com/nature/journal/v446/n7136/full/nature05754.html
accessdate = 2007-06-21 ] cite journal
author = Knoll, AH
coauthors = Javaux, EJ, Hewitt, D., Cohen, P.
year = 2006
title = Eukaryotic organisms in Proterozoic oceans
journal = Philosophical Transactions of the Royal Society B: Biological Sciences
volume = 361
issue = 1470
pages = 1023–1038
doi = 10.1098/rstb.2006.1843
url = http://www.journals.royalsoc.ac.uk/content/r33709390117w941/
accessdate = 2007-06-21 ] : Microfossils dating from Ma|632.5 — 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.cite journal
author = Leiming, Y.
coauthors = Zhu, M; Knoll, A; Yuan, X; Zhang, J; Hu, J
year = 2007
date = 2007-04-05
title = Doushantuo embryos preserved inside diapause egg cysts
journal = Nature
volume = 446
issue = 7136
pages = 661–663
doi=10.1038/nature05682
url = http://www.nature.com/nature/journal/v446/n7136/abs/nature05682.html
accessdate = 2007-04-27
format = abstract ] : An alternative proposal is that these structures represent adult stages of the animals of this period. cite journal
author = Newman, S.A.
coauthors = Forgacs, G.; Müller, G.B.
year = 2006
title = Before programs: The physical origination of multicellular forms
journal = Int. J. Dev. Biol.
volume = 50
pages = 289–299
doi = 10.1387/ijdb.052049sn
url = http://www.ijdb.ehu.es/web/contents.php?vol=50&issue=2–3
accessdate = 2007-11-02 ]

; 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 sea anemones; the patterns displayed where two meet have led to many apparent individuals being recognised as microbial colonies.cite journal
author=Grazhdankin, D.
coauthor=Gerdes, G.
title=Ediacaran microbial colonies
journal=Lethaia
volume=40
pages=201–210
doi=10.1111/j.1502-3931.2007.00025.x
url=http://www.blackwell-synergy.com/doi/abs/10.1111/j.1502-3931.2007.00025.x
format=abstract] 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: cite journal
title=Underground Vendobionta From Namibia
doi=10.1111/1475-4983.00227
url=http://www.blackwell-synergy.com/links/doi/10.1111/1475-4983.00227/abs/
author=Grazhdankin, D.
coauthors=Seilacher, A.
journal=Palaeontology
volume=45
issue=1
pages=57–78
year=2002
format=abstract]

; Quilted organisms : The organisms considered in A. Seilacher's revised definition of the Vendobiontacite journal
author = Seilacher, A.
year = 1992
title = Vendobionta and Psammocorallia: lost constructions of Precambrian evolution
journal = Journal of the Geological Society, London
volume = 149
issue = 4
pages = 607–613
url = http://jgs.lyellcollection.org/cgi/content/abstract/149/4/607
doi = 10.1144/gsjgs.149.4.0607
issn = 0016–7649
accessdate = 2007-06-21
format = abstract ] 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. [cite journal
author = Narbonne, G.M.
coauthors = Saylor, B.Z. & Grotzinger, J.P.
year = 1997
title = The Youngest Ediacaran Fossils from Southern Africa
journal = Journal of Paleontology
volume = 71
issue = 6
pages = 953–967
url = http://links.jstor.org/sici?sici=0022–3360%28199711%2971%3A6%3C953%3ATYEFFS%3E2.0.CO%3B2-T
issn = 0022–3360
accessdate = 2007-06-21 ]

: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 in their morphology (convergent evolution).

; 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.cite journal
journal=Science
date=19 November 2004
volume=306
number=5700
pages=1291
doi=10.1126/science.1101338
url=http://www.sciencemag.org/cgi/content/full/306/5700/1291a
title=Comment on "Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian"
author=Bengtson, S.] cite journal |journal=Science
date=19 November 2004
volume=306
number=5700
pages=1291
doi=10.1126/science.1102328
url=http://www.sciencemag.org/cgi/content/full/sci;306/5700/1291b
title=Response to Comment on "Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian"
author=Chen, J.-Y.] Later examples, almost universally accepted as bilaterians, include the mollusc-like "Kimberella",cite journal
author = Fedonkin, M.A.
coauthors = Waggoner, B.M.
year = 1997
title = The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism
journal = Nature
volume = 388
issue = 6645
pages = 868–871
doi = 10.1038/42242
url = http://www.nature.com/nature/journal/v388/n6645/full/388868a0.html
issn = 0028–0836
accessdate = 2007-03-08 ] "Spriggina" (pictured), and the shield-shaped "Parvancorina", [cite journal
author = Glaessner, M.F.
coauthors =
year = 1980
title = Parvancorina — an arthropod from the late Precambrian of South Australia
journal = Ann. Nat. Hist. Mus. Wien.
volume = 83
pages = 83–90
accessdate = 2007-03-08 ] whose affinities are currently debated. [For a reinterpretation, see cite journal
author = Ivantsov, A.Y.
coauthors = Malakhovskaya, Y.E., Serezhnikova, E.A.
year = 2004
title = Some Problematic Fossils from the Vendian of the Southeastern White Sea Region
journal = Paleontological Journal
volume = 38
issue = 1
pages = 1–9
issn = 0031–0301
url = http://www.maik.rssi.ru/abstract/paleng/4/paleng1_4p1abs.htm Abstract] [http://www.vend.paleo.ru/pub/Ivantsov_et_al_2004_eng.pdf]
language = Russian; English translation available
accessdate = 2007-06-21
format = abstract ] :A suite of fossils known as the Small shelly fossils are represented in the Ediacaran, most famously by "Cloudina",cite journal
author=Germs, G.J.B.
title=New shelly fossils from Nama Group, South West Africa
journal=American Journal of Science
issn=0002–9599
month=October | year=1972
volume=272
pages=752–761
url=http://www.ajsonline.org/cgi/reprint/272/8/752] 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, on 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. [cite journal
author = Fedonkin, M.A.
year = 1992
title = Vendian faunas and the early evolution of Metazoa
journal = In Lipps, J., and Signor, P. W., eds., Origin and early evolution of the Metazoa: New York, Plenum Press.
pages = 87–129
url = http://books.google.co.uk/books?id=gUQMKiJOj64C&pg=PP1&ots=BkpdtmDml1&sig=ap0OD3JXuSkTVhJTSqQbT5uC2P8
isbn = 0306440679
publisher = Springer
accessdate = 2007-03-08
oclc = 231467647 25873335 ] Putative "burrows" dating as far back as Ma|1100|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; cite journal
author = Seilacher, A.
authorlink = Adolf Seilacher
coauthors = Bose, P.K.; Pflüger, F.
date = 1998-10-02
title = Triploblastic Animals More Than 1 Billion Years Ago: Trace Fossil Evidence from India
journal = Science
volume = 282
issue = 5386
pages = 80–83
doi = 10.1126/science.282.5386.80
accessdate = 2007-05-21
format = abstract ] however their uneven width and tapering ends make a biological origin difficult to defend.cite journal
author = Budd, G.E.
coauthors = Jensen, S.
year = 2000
title = A critical reappraisal of the fossil record of the bilaterian phyla
journal = Biological Reviews
volume = 75
issue = 02
pages = 253–295
doi = 10.1017/S000632310000548X
url = http://www.journals.cambridge.org/abstract_S000632310000548X
accessdate = 2007-06-27
format = abstract ] 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;cite journal
author = Ivantsov, A.Y.
coauthors = Malakhovskaya, Y.E.
year = 2002
title = Giant Traces of Vendian Animals
journal = Doklady Earth Sciences (Doklady Akademii Nauk)
volume = 385
issue = 6
pages = 618–622
issn = 1028–334X
language = Russian; English translation available
url = http://vend.paleo.ru/pub/Ivantsov_et_Malakhovskaya_2002-e.pdf
accessdate = 2007-05-10 ] the method of formation of these disconnected and overlapping fossils largely remains a mystery. The potential mollusc "Kimberella" is associated with scratch marks, perhaps formed by a radula,According to cite journal
author = Martin, M.W.
coauthors = Grazhdankin, D.V.; Bowring, S.A.; Evans, D.A.D.; Fedonkin, M.A.; Kirschvink, J.L.
date = 2000-05-05
title = Age of Neoproterozoic Bilatarian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution
journal = Science
volume = 288
issue = 5467
pages = 841
doi = 10.1126/science.288.5467.841
url = http://www.scienceonline.org/cgi/content/abstract/288/5467/841
format = abstract For a more cynical perspective see cite journal
author = Butterfield, N.J.
year = 2006
title = Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale
journal = Bioessays
volume = 28
issue = 12
pages = 1161–6
doi = 10.1002/bies.20507
url = http://doi.wiley.com/10.1002/bies.20507 ] further traces from Ma|555 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.

Cnidarians

Since the most primitive eumetazoans — multi-cellular animals with tissues — are recognised as cnidarians, the first attempt to categorise these fossils designated them as jellyfish and sea-pens. [cite journal
author =Donovan, Stephen K., Lewis, David N.
year = 2001
title = Fossils explained 35. The Ediacaran biota
journal = Geology Today
volume =17
issue =3
pages =115–120
doi =10.1046/j.0266-6979.2001.00285.x
url = http://www.blackwell-synergy.com/links/doi/10.1046/j.0266-6979.2001.00285.x/abs/
accessdate = 2007-03-08
format = abstract ] However, detailed study of their growth pattern has discounted this hypothesis. [cite journal
author = Antcliffe, J.B.
coauthors = Brasier, M.D.
year = 2007
title = Charnia and sea pens are poles apart
journal = Journal of the Geological Society
volume = 164
issue = 1
pages = 49–51
doi = 10.1144/0016-76492006-080
url = http://jgs.geoscienceworld.org/cgi/reprint/164/1/49
accessdate = 2007-03-08 ] cite journal
author = Antcliffe, J.B.
coauthors = Brasier, M.D.
year = 2007
title = Charnia At 50: Developmental Models For Ediacaran Fronds
journal = Palaeontology
volume = 51
issue = 1
pages = 11–26
url=http://www.blackwell-synergy.com/doi/pdf/10.1111/j.1475-4983.2007.00738.x
doi = 10.1111/j.1475-4983.2007.00738.x
doi-label = Not yet active.]

"The dawn of animal life"

Martin Glaessner proposed in "The dawn of animal life" (1984) that the Ediacara biota were recognisable stem group members of modern phyla, but were unfamiliar because they had yet to evolve the characteristic features we use in modern classification.cite book
title=The Dawn of Animal Life: A Biohistorical Study
author=Glaessner, M.F.
year=1984
isbn=0521312167
publisher=Cambridge University Press
oclc=9394425] Adolf Seilacher responded by suggesting that the Ediacaran sees animals usurping giant protists as the dominant life form. [cite journal
author = Seilacher, A.
coauthors = Grazhdankin, D., Legouta, A.
year = 2003
title = Ediacaran biota: The dawn of animal life in the shadow of giant protists
journal = Paleontological research
volume = 7
issue = 1
pages = 43–54
issn = 13428144
url = http://ci.nii.ac.jp/naid/110002695304/
accessdate = 2007-03-08
doi = 10.2517/prpsj.7.43]

In 1986 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", though this idea has not been widely accepted.

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, [cite book
author = Seilacher, A.
year = 1984
chapter = Late Precambrian and Early Cambrian Metazoa: preservational or real extinctions?
title = Patterns of Change in Earth Evolution
editors = Holland, H.D.; Trendall, A.F.
pages = 159–168
isbn = 0387127496
publisher = Heidelberg: Springer-Verlag
accessdate = 2007-03-08
oclc = 11202424 ] cite journal
author = Seilacher, A.
year = 1989
title = Vendozoa: organismic construction in the Proterozoic biosphere
journal = Lethaia
volume = 17
pages = 229–239
issn = 0024-1164
doi = 10.1111/j.1502-3931.1989.tb01332.x ] 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. [cite journal
author = Buss, L.W. and Seilacher, A.
year = 1994
title = The Phylum Vendobionta: A Sister Group of the Eumetazoa?
journal = Paleobiology
volume = 20
issue = 1
pages = 1–4
issn = 0094-8373
url = http://links.jstor.org/sici?sici=0094-8373%28199424%2920%3A1%3C1%3ATPVASG%3E2.0.CO%3B2-F
accessdate = 2007-06-21 ]

Lichens

Gregory Retallack's hypothesis that Ediacaran organisms were lichens [cite journal
author = Retallack, G.J.
year = 1994
title = Were the Ediacaran fossils lichens?
journal = Paleobiology
volume = 17
pages = 523–544
issn = 0094-8373
accessdate = 2007-03-08 ] has failed to gain wide-spread acceptance.cite journal
author = Waggoner, B.M.
year = 1995
title = Ediacaran Lichens: A Critique
journal = Paleobiology
volume = 21
issue = 3
pages = 393–397
url = http://links.jstor.org/sici?sici=0094-8373(199522)21%3A3%3C393%3AELAC%3E2.0.CO%3B2-R
accessdate = 2008-02-11] cite journal
author = Waggoner, B.
coauthors = Collins, A.G.
year = 2004
title = "Reductio Ad Absurdum": Testing The Evolutionary Relationships Of Ediacaran And Paleozoic Problematic Fossils Using Molecular Divergence Dates
journal = Journal of Paleontology
volume = 78
issue = 1
pages = 51–61
doi = 10.1666/0022-3360(2004)078] 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 at some point,cite journal
author = Waggoner, Ben
year = 1998
title = Interpreting the Earliest Metazoan Fossils: What Can We Learn?
journal = Integrative and Comparative Biology
volume = 38
issue = 6
pages = 975–982
issn = 1540-7063
doi = 10.1093/icb/38.6.975
url = http://intl-icb.oxfordjournals.org/cgi/content/abstract/38/6/975
accessdate = 2007-03-08
format = abstract ] from algæ, [cite journal
author = Ford, T.D.
year = 1958
title = Pre-Cambrian fossils from Charnwood Forest
journal = Proceedings of the Yorkshire Geological Society
volume = 31
pages = 211–217
doi = 10.1046/j.1365-2451.1999.00007.x
accessdate = 2007-03-08] to protists known as foraminifera, [cite conference
author = Zhuralev
year = 1992
title = Were Vend-Ediacaran multicellulars metazoa?
conference = International Geological Congress, Kyoto, Japan
volume = 2
booktitle =
conferenceurl = http://www.iugs.org/index.htm
url = http://www.jstor.org/view/00948373/di995962/99p0929r/0
pages = 339
accessdate = 2007-06-21 ] to fungi [cite journal
author = Peterson, K.J. and Waggoner, B. and Hagadorn, J.W.
year = 2003
title = A Fungal Analog for Newfoundland Ediacaran Fossils?
journal = Integrative and Comparative Biology
volume = 43
issue = 1
pages = 127–136
doi = 10.1668/1540-7063(2003)043 [0127:AFAFNE] 2.0.CO;2 ] to bacterial or microbial colonies,cite conference
author = Grazhdankin, D
year = 2001
title = Microbial origin of some of the Ediacaran fossils
booktitle =
conference = GSA Annual Meeting, November 5-8, 2001
pages = 177
url = http://gsa.confex.com/gsa/2001AM/finalprogram/abstract_21602.htm
accessdate = 2007-03-08 ] to hypothetical intermediates between plants and animals. [cite journal
author = Pflug
year = 1973
title = Zur fauna der Nama-Schichten in Südwest-Afrika. IV. Mikroscopische anatomie der petalo-organisme
journal = Paleontographica
issue = B144
pages = 166–202
issn = 0375-0299
language = German
accessdate = 2007-03-08 ]

Origin

It took almost 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 Ma|3460,cite journal
author = Schopf, J.W.
coauthors = Packer, B.M.
date = 1987-07-03
title = Early Archean (3.3-billion to 3.5-billion-year-old) microfossils from Warrawoona Group, Australia
journal = Science
volume = 237
issue = 4810
pages = 70
doi = 10.1126/science.11539686
url = http://www.sciencemag.org/cgi/content/abstract/237/4810/70
accessdate = 2007-05-21
format = abstract ] cite journal
author = Hofmann, H.J.
coauthors = Grey, K.; Hickman, A.H.; Thorpe, R.I.
date = 1999-08-01
title = Origin of 3.45 Ga coniform stromatolites in Warrawoona Group, Western Australia
journal = Bulletin of the Geological Society of America
volume = 111
issue = 8
pages = 1256–1262
issn = 0016-7606
url = http://bulletin.geoscienceworld.org/cgi/content/abstract/111/8/1256
accessdate = 2007-05-21
doi = 10.1130/0016-7606(1999)111<1256:OOGCSI>2.3.CO;2
format = abstract] the first uncontroversial evidence for life is found Ma|2700,cite journal
author = Archer, C.
coauthors = Vance, D.
date = 2006-03-01
title = Coupled Fe and S isotope evidence for Archean microbial Fe (III) and sulfate reduction
journal = Geology
volume = 34
issue = 3
pages = 153–156
doi = 10.1130/G22067.1
url = http://geology.geoscienceworld.org/cgi/content/abstract/34/3/153
accessdate = 2007-05-24
format = abstract ] and cells with nuclei certainly existed by Ma|1200:cite journal
issn = 0094-8373
year = 2000
volume = 26
pages = 386
doi = 10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2
title = Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes
author = Butterfield, Nicholas J.
journal = 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 evolving as time progresses, with traces of earlier semi-complex life such as "Nimbia", found in the Ma|610|million year old Twitya formation,cite journal
author = Fedonkin, M.A.
year = 1980
title = New representatives of the Precambrian coelenterates in the northern Russian platform
journal = Paleontologicheskij Zhurnal
issn = 0031-031X
pages = 7–15
language = Russian ] possibly displaying the most complex morphology of the time.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 (producing banded iron formations), and other reactive elements had also been oxidised.
Donald Canfield detected records of the first significant quantities of atmospheric oxygen just before the first Ediacaran fossils appearedcite journal
author = Canfield, D.E.
coauthors = Teske, A.
year = 1996
title = Late Proterozoic rise in atmospheric oxygen concentration inferred from phylogenetic and sulphur-isotope studies
journal = Nature
volume = 382
pages = 127–132
doi = 10.1038/382127a0
url = http://www.nature.com/nature/journal/v382/n6587/abs/382127a0.html
accessdate = 2007-06-22
format = abstract ] — and the presence of atmospheric oxygen was soon heralded as a possible trigger for the Ediacaran radiation.cite journal
author = Canfield, D.E.
coauthors = Poulton, S.W.; Narbonne, G.M.
date = 2007-01-05
title = Late-Neoproterozoic Deep-Ocean Oxygenation and the Rise of Animal Life
journal = Science
volume = 315
issue = 5808
pages = 92
doi = 10.1126/science.1135013
url = http://www.sciencemag.org/cgi/content/abstract/315/5808/92
accessdate = 2007-06-22
format = abstract ] 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.cite journal
author = Fike, DA
coauthors = Grotzinger, JP, Pratt, LM, Summons, RE
year = 2006
title = Oxidation of the Ediacaran ocean
journal = Nature
volume = 444
issue = 7120
pages = 744–7
doi = 10.1038/nature05345
url = http://www.nature.com/nature/journal/v444/n7120/abs/nature05345.html
accessdate = 2007-04-28
format = abstract ] 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. [cite web
url=http://www.earth.ox.ac.uk/research/2006/Oxyen.htm
title=PhD Project description
accessdate=2007-06-22
last=Brasier
first=Martin
authorlink=
coauthors=McIlroy, Duncan
year=2005
publisher=University of Oxford]

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. [cite conference
author = Narbonne, Guy M.
year = 2003
month = September
title = Life after Snowball: The Mistaken Point biota and the origin of animal ecosystems
conference = Seattle Annual Meeting of the GSA
booktitle = Geological Society of America Abstracts with Programs
volume = 35
issue = 6
pages = 516
url = http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_57850.htm
accessdate = 2007-06-22 ] 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, ma|575| to ma|565; White Sea in Russia, ma|560| to ma|550; and Nama in Namibia, ma|550| to ma|542, 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 . [ cite journal | title=The Avalon Explosion: Evolution of Ediacara Morphospace | journa=Science | month=January | volume=319| issue=5859 |pages=81–84 | doi=10.1126/science.1150279 | url=http://www.sciencemag.org/cgi/content/abstract/319/5859/81 | author=Shen, B., Dong, L., Xiao, S. and Kowalewski, M. | year=2008 | journal=Science | format=abstract ]

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 specimencite journal
author = Conway Morris, S.
year = 1993
title = Ediacaran-like fossils in Cambrian Burgess Shale–type faunas of North America
journal = Palaeontology
volume = 36
issue = 0031-0239
pages = 593–635
accessdate = 2007-03-08 ] might be expected in exceptionally preserved fossil assemblages (Konservat-Lagerstätten) such as the Burgess Shale and Chengjiangcite journal
author = 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.
date = 2006-05-05
title =Lower Cambrian Vendobionts from China and Early Diploblast Evolution
journal = Science
volume = 312
issue = 5774
pages = 731
doi = 10.1126/science.1124565
url = http://science-mag.aaas.org/cgi/content/abstract/312/5774/731
accessdate = 2007-04-28
format =abstract ] — unless such assemblages represent an environment never occupied by the Ediacaran biota, or unsuitable conditions for their preservation.

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.cite book
author = McMenamin M.
year = 1986
title = The Garden of Ediacara
isbn=9780231105590
accessdate = 2007-03-08
publisher = Columbia Univ Press
location = New York
oclc = 228271905 ] 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".

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.

Competition

It is possible that increased competition due to the evolution of key innovations amongst other groups, perhaps as a response to predation,cite journal
author = Stanley, S.M.
year = 1973
title = An ecological theory for the sudden origin of multicellular life in the Late Precambrian
journal = Proc. Nat. Acad. Sci. U.S.A.
volume = 70
issue = 5
pages = 1486–1489
pmid=16592084
url =http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=16592084
accessdate = 2007-06-21
doi = 10.1073/pnas.70.5.1486 ] 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. [cite journal
author = Gould, S.J.
coauthors = Calloway, C.B.
year = 1980
title = Clams and Brachiopods-Ships that Pass in the Night
journal = Paleobiology
volume = 6
issue = 4
pages = 383–396
issn = 0094-8373
accessdate = 2007-03-08 ]

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, [cite journal
author = McKerrow, W.S.
coauthors = Scotese, C.R., Brasier, M.D.
year = 1992
title = Early Cambrian continental reconstructions
journal = Journal of the Geological Society, London
volume = 149
issue = 4
pages = 599–606
issn = 0016-7649
url = http://www.ingentaconnect.com/content/geol/jgs/1992/00000149/00000004/14940599
accessdate = 2007-06-22

doi = 10.1144/gsjgs.149.4.0599] rising sea levels (creating shallow, "life-friendly" seas), [cite journal
author = Hallam, A.
year = 1984
title = Pre-Quaternary sea-level changes
journal = Annual Reviews
volume = 12
pages = 205–243
doi = 10.1146/annurev.ea.12.050184.001225 ] a nutrient crisis,cite journal
author = Brasier, M.D.
year = 1992
title = Background to the Cambrian explosion
journal = Journal of the Geological Society, London
volume = 149
pages = 585–587
doi = 10.1144/gsjgs.149.4.0585 ] fluctuations in atmospheric composition, including oxygen and carbon dioxide levels,cite journal
author = Brasier, M.D.
year = 1992
title = Global ocean-atmosphere change across the Precambrian-Cambrian transition
journal = Geological Magazine
volume = 129
issue = 2
pages = 161–168
issn = 0016-7568 ] and changes in ocean chemistrycite journal
author = Lowenstein, T.K.
coauthors = Timofeeff, M.N.; Brennan, S.T.; Hardie, L.A.; Demicco, R.V.
year = 2001
title = Oscillations in Phanerozoic Seawater Chemistry: Evidence from Fluid Inclusions
journal = Science
volume = 294
issue = 5544
pages = 1086–1089
pmid = 11691988
doi = 10.1126/science.1064280
url = http://adsabs.harvard.edu/abs/2001Sci...294.1086L
accessdate = 2007-06-22 ] (promoting biomineralisation) [cite journal
author = Bartley, J.K.
coauthors = Pope, M., Knoll, A.H., Semikhatov, M.A., Petrov, P.Y.U.
year = 1998
title = A Vendian-Cambrian boundary succession from the northwestern margin of the Siberian Platform: stratigraphy, palaeontology, chemostratigraphy and correlation
journal = Geological Magazine
volume = 135
issue = 4
pages = 473–494
doi = 10.1144/gsjgs.155.6.0957
accessdate = 2007-03-08] 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. 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.

Avalon-type assemblage

The Avalon-type assemblage is defined at Mistaken Point in Canada, the oldest locality with a large quantity of Ediacaran fossils. [cite conference
author = Benus
year = 1988
title = Trace fossils, small shelly fossils and the Precambrian-Cambrian boundary
booktitle = New York State Museum Bulletin
volume = 463
pages = 81
isbn = 978-1555571788
publisher = University of the State of New York (May 1988) ] 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.cite journal
author = Erwin, Douglas H
year = 2008
title = Wonderful Ediacarans, Wonderful Cnidarians?
journal = Evolution & Development
volume = 10
issue = 3
pages = 263–264
doi = 10.1111/j.1525-142X.2008.00234.x]

The biota comprises deep sea dwelling rangeomorphs [cite journal
author = Clapham, Matthew E.
coauthors = Narbonne, Guy M., Gehling, James G.
year = 2003
title = Paleoecology of the oldest known animal communities: Ediacaran assemblages at Mistaken Point, Newfoundland
journal = Paleobiology
issn = 0094-8373
volume = 29
issue = 4
pages = 527–544
doi = 10.1666/0094-8373(2003)029<0527:POTOKA>2.0.CO;2
url= = http://paleobiol.geoscienceworld.org/cgi/content/abstract/29/4/527
accessdate=2007-06-22
format = abstract] 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 [cite journal
author=Clapham, M.E.
coauthors=Narbonne, G.M.
year=2002
volume=30
issue=7
pages=627–630
url=http://geology.geoscienceworld.org/cgi/content/abstract/30/7/627
doi=10.1130/0091-7613(2002)030<0627:EET>2.0.CO;2
journal=Geology
title=Ediacaran epifaunal tiering
format=abstract] — 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 evolution to rich biota. Opinion is currently divided between these conflicting hypotheses.

Ediacara-type assemblage

The Ediacara-type assemblage is named after Australia's Ediacara Hills, and consist of fossils preserved in areas near the mouths of rivers (prodeltaic facies). 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.cite journal
author = Grazhdankin, Dima
year = 2004
title = Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution
journal = Palæobiology
volume = 30
issue = 2
pages = 203–221
doi=10.1666/0094-8373(2004)030<0203:PODITE>2.0.CO;2
url = http://paleobiol.geoscienceworld.org/cgi/reprint/30/2/203.pdf
accessdate = 2007-03-08 (Source of data for Timeline synthesis, p218. Further citations available in caption to Fig. 8.)]

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, while Guy Narbonne maintains they were surface dwellers. [cite journal
title=The Ediacara Biota: Neoproterozoic Origin of Animals and Their Ecosystems
last=Narbonne
first=Guy M.
authorlink=
volume=33
pages=421–442
year=2005
doi=10.1146/annurev.earth.33.092203.122519
journal=Annual Review of Earth and Planetary Sciences] These beds are sandwiched between units comprising interbedded sandstones, siltstones and shales, with microbial mats, where present, usually containing fossils. The environment is interpreted as sand bars formed at the mouth of a delta's distributaries.

ignificance 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; [cite journal
author = Waggoner, B.
year = 1999
title = Biogeographic Analyses of the Ediacara Biota: A Conflict with Paleotectonic Reconstructions
journal = Paleobiology
volume = 25
issue = 4
pages = 440–458
issn = 0094-8373
doi = 10.1666/0094-8373(1999)025<0440:BAOTEB>2.3.CO;2
url = http://paleobiol.geoscienceworld.org/cgi/content/abstract/25/4/440
accessdate = 2007-06-22
format = abstract ] 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. An analysis of one of the White Sea fossil beds, where the layers cycle from continental sea-bed to inter-tidal to estuarine and back again a few times, found that a specific set of Ediacaran organisms was associated with each environment.

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.cite journal
author = Bambach, R.K.
coauthors = Bush, A.M., Erwin, D.H.
year = 2007
title = Autecology and the filling of Ecospace: Key metazoan radiations
journal = Palæontology
volume = 50
issue = 1
pages = 1–22
doi = 10.1111/j.1475-4983.2006.00611.x
url = http://www.blackwell-synergy.com/doi/full/10.1111/j.1475-4983.2006.00611.x
language =
accessdate = 2007-03-08] 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

*cite book |title=The Garden of Ediacara: Discovering the First Complex Life
author=Mark McMenamin
isbn=0231105584
year=1998
pages =368pp |publisher=Columbia University Press |location=New York |oclc=37588521 60159576 An outdated popular science account of these fossils, with a narrowed focus on only the Namibian Fossils.

*cite book |title=Palæobiology II: A synthesis
author=Derek Briggs & Peter Crowther (Editors)
isbn=0-632-05147-7
year=2001
pages = Chapter 1 |publisher=Blackwell Science |location=Malden, MA |oclc=43945263 51682981 Excellent further reading for the keen - includes many interesting chapters with macroevolutionary theme.

External links

* [http://geol.queensu.ca/museum/exhibits/ediac/drook/ "The oldest complex animal fossils"] - Queens' University, Canada
* [http://geol.queensu.ca/museum/exhibits/ediac/ediac.html "Ediacaran fossils of Canada"] - Queens' University, Canada
* [http://www.peripatus.gen.nz/paleontology/Ediacara.html "The Ediacaran Assemblage"] - Thorough, though slightly out-of-date, description

ee also

*List of Ediacaran genera
*Origin of life
*Cambrian explosion

References