The Ediacara (IPA: /ˌiːdɪˈækərə/, formerly Vendian) biota are ancient lifeforms of the Ediacaran Period, which represent the earliest known complex multicellular organisms. The Ediacaran Period (ˌiːdiˈækərən named after the Ediacara Hills of South Australia) is the last geological period of the Neoproterozoic Multicellular organisms are Organisms consisting of more than one cell, and having Differentiated cells that perform specialized functions 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. The Cryogenian (from Greek cryos "cold" and genesis "birth" is a Geologic period that lasted from. The Snowball Earth Hypothesis as it was originally proposed]] Evidence The Snowball Earth hypothesis was originally devised to explain the apparent presence of Biodiversity is the variation of Life forms within a given Ecosystem, Biome or for the entire Earth. The Cambrian explosion or Cambrian radiation was the seemingly rapid appearance of most major groups of complex Animals around, as evidenced by the FOSSIL is a standard protocol for allowing serial communication for Telecommunications programs under the DOS Operating system. 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. Biota is the total collection of Organisms of a geographic region or a time period from local geographic scales and instantaneous temporal scales all the way up to whole-planet

The organisms of the Ediacaran Period first appeared around 580 million years ago and flourished until the cusp of the Cambrian 542 million years ago, when the characteristic communities of fossils vanished. The Cambrian is a geologic period and system that began about Ma (million years ago at the end of the Proterozoic eon and ended about Ma with 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. The Middle Cambrian (also known as Albertan, Acadian, St David's, or Saint David's) is the second of three geological epochs of the An ecosystem is a natural unit consisting of all plants animals and micro-organisms( Biotic factors in an area functioning together with all of the non-living physical ( ^[1] Multiple hypotheses exist to explain this disappearance, including preservation bias, a changing environment, the advent of predators, and competition from other lifeforms. A hypothesis (from Greek) consists either of a suggested explanation for a phenomenon (an event that is observable or of a reasoned proposal suggesting a possible A biased sample is a statistical sample of a population in which some members of the population are less likely to be included than others

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. Kimberella is a Genus of Fossils known only from rocks of the Ediacaran period and only one Species, Kimberella quadrata Molluscs are animals belonging to the phylum Mollusca. There are around 250000 extant Species within the phylum with an estimated 70000 "Bilateral symmetry" redirects here For bilateral symmetry in mathematics see Reflection symmetry. Most microscopic fossils are morphologically distinct from later lifeforms: they resemble discs, mud-filled bags, or quilted mattresses. Micropaleontology (also sometimes spelled as micropalaeontology) is that branch of Paleontology which studies microfossils The term morphology in Biology refers to the outward appearance ( Shape, Structure, Colour, Pattern) of an Organism 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). In Biology, a species is one of the basic units of Biological classification and a Taxonomic rank. A fungus (ˈfʌŋgəs is a eukaryotic Organism that is a member of the kingdom Fungi (ˈfʌndʒaɪ Protists (ˈproʊtɨst are a diverse group of eukaryotic Microorganisms Historically protists were treated as the kingdom Protista but this Palaeontology redirects here For the Scientific journal, see Palaeontology (journal. ^[2] 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. ^[3]

Contents 1 History 2 Preservation 2.1 Microbial mats 2.2 Fossilisation 2.3 What is preserved? 3 Morphology 4 Classification and interpretation 4.1 Cnidarians 4.2 "The dawn of animal life" 4.3 New phylum 4.4 Lichens 4.5 Other interpretations 5 Origin 6 Disappearance 6.1 Preservation bias 6.2 Predation and grazing 6.3 Competition 6.4 Change in environmental conditions 7 Assemblages 7.1 Avalon-type assemblage 7.2 Ediacara-type assemblage 7.3 Nama-type assemblage 7.4 Significance of assemblages 8 Further reading 9 External links 10 See also 11 References

History

The first Ediacaran fossils discovered were the disc-shaped Aspidella terranovica, in 1868. Aspidella is also a synonym for the Mushroom genus Amanita. Aspidella is an Ediacaran disk-shaped Their discoverer, A. Murray, a geological surveyor, found them useful aids for correlating the age of rocks around Newfoundland. Alexander Murray, CMG ( 2 June 1810 &ndash 18 December 1884) was a Scottish Geologist. Newfoundland — ˈn(jufənˌlænd (Terre-Neuve Talamh an Éisc — is a large island 15 km off the east coast of ^[6] 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. The Cambrian is a geologic period and system that began about Ma (million years ago at the end of the Proterozoic eon and ended about Ma with 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. Elkanah Billings ( May 5, 1820 &ndash June 14, 1876) is often referred to as Canada 's first ^[6] No similar structures elsewhere in the world were then known, and the one-sided debate soon fell into obscurity. ^[6] In 1933, Georg Gürich discovered specimens in Namibia,^[7] 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. Georg Julius Ernst Gürich ( Guttengau, Upper Silesia, 25 September 1859 – August 1938 was a German geologist Paleontologist and university teacher Namibia, officially the Republic of Namibia, is a country in Southern Africa on the Atlantic coast The Cambrian is a geologic period and system that began about Ma (million years ago at the end of the Proterozoic eon and ended about Ma with In 1946, Reg Sprigg noticed "jellyfishes" in the Ediacara Hills of Australia's Flinders Ranges^[8] but these rocks were believed to be Early Cambrian, so while the discovery sparked some interest, little serious attention was garnered. Reginald Sprigg, AO DSc hc ANU & Flinders University (born 1 March 1919, died 2 December 1994) Ediacara Hills are a range of low hills in the northern Flinders Ranges of South Australia, around 650 km north of Adelaide. The Flinders Ranges is South Australia 's largest Mountain range which starts approximately 200 km north west of Adelaide.

It was not until the British discovery of the iconic Charnia in 1957 that the pre-Cambrian was seriously considered as containing life. Charnia is the Genus name given to a Frond -like Precambrian lifeform with segmented ridges branching alternately to the right and left from a This frond-shaped fossil was found in England's Charnwood Forest,^[9] and due to the detailed geologic mapping of the British Geological Survey there was no doubt that these fossils sat in Precambrian rocks. A frond is a large Leaf with many divisions to it and the term is typically used for the leaves of palms Ferns or Cycads A frond is Charnwood Forest is an upland tract in north-western Leicestershire England, bounded by Leicester, Loughborough, and Coalville A geologic map or geological map is a special-purpose Map made to show Geological features The British Geological Survey (BGS is a partly publicly-funded body which aims to advance geoscientific knowledge of the United Kingdom landmass and its Continental Palæontologist Martin Glaessner finally made the connection between this and the earlier finds,^[10][11] and with a combination of improved dating of existing specimens and an injection of vigour into the search, many more instances were recognised. Martin Fritz Glaessner AM ( 25 December, 1906 &ndash 23 November, 1989) was a geologist and palaeontologist. ^[12]

However, all specimens discovered until 1967 were in coarse-grained sandstone that prevented preservation of fine details, making interpretation difficult. Sandstone is a Sedimentary rock composed mainly of Sand -size Mineral or rock grains. 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. Volcanic ash consists of small Tephra, which are bits of pulverized rock and glass created by volcanic eruptions less than in diameter ^[13][14]

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",^[15] for terminal-Precambrian rocks, names that were also applied to the lifeforms. Indigenous Australians are descendants of the first known human inhabitants of the Australian continent and its nearby islands. "Ediacaran" and "Ediacarian" were subsequently applied to the epoch or period of geologic time and its corresponding rocks. The geologic time scale is a chronologic schema (or idealized Model) relating Stratigraphy to time that is used by Geologists and other 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 International Union of Geological Sciences (IUGS is an international Non-governmental organization devoted to international cooperation in the field of Geology The Neoproterozoic Era is the unit of geologic time from 1000 to 542 +/- 0 ^[16]

Preservation

All but the smallest fraction of the fossil record is comprised of the robust skeletal matter of decayed corpses. In Biology, the skeleton is a strong and often a rigid framework that supports the body of an animal holding it upright and giving it shape and strength (Also skeletal 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;^[17] since after the evolution of these organisms in the Cambrian, soft-bodied impressions were usually disturbed before they could fossilize. In Oceanography and Limnology, bioturbation is the displacement and mixing of Sediment particles by benthic Fauna (animals or

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. ^[18] 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,^[19] and these communities are now limited to inhospitable refugia where predators cannot survive long enough to eat them. In biology a refugium (plural refugia is a location of an isolated or Relict Population of a once widespread animal or plant species

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. See also Burgess shale type fauna The Burgess Shale is famous for the exceptional preservation of the fossils found within it in which the soft parts are preserved The Solnhofen limestone is a Jurassic Konservat-Lagerstätte that preserves a rare assemblage of Fossilized organisms some of which such as sea jellies 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. ^[20] Ash beds provide more detail, and can readily be precisely dated to the nearest million years or better by means of radiometric dating. Radiometric dating (often called radioactive dating) is a technique used to date materials usually based on a comparison between the observed abundance of a naturally occurring ^[21] 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. Turbidite Geological formations have their origins in Turbidity current deposits, which are deposits from a form of underwater Avalanche that ^[20] 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. ^[22]

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. The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have Precipitation is the formation of a Solid in a Solution during a Chemical reaction. ^[6]

Morphology

Forms of Ediacaran fossil
The earliest discovered potential embryo, preserved within an acanthomorphic acritarch. Acritarchs are small organic Fossils present from approximately to the present The term 'acritarch' describes a range of unclassified cell-like fossils.
Cyclomedusa, a disc-shaped fossil that has been interpreted as a microbial artefact. Cyclomedusa is a circular Fossil of the Ediacaran biota; it has a circular bump in the middle and as many as five circular growth ridges around it Metric scale.
A cast of the quilted Charnia, the first accepted complex Precambrian organism. Charnia was once interpreted as a relative of the sea-pens.
Spriggina, a possible precursor to the trilobites, may be one of the predators that led to the demise of the Ediacaran fauna[23] and subsequent diversification of animals. Fossils of Spriggina are known from the Ediacaran period around. Trilobites ("three-lobes" are extinct Arthropods that form the class Trilobita. The Cambrian explosion or Cambrian radiation was the seemingly rapid appearance of most major groups of complex Animals around, as evidenced by the [24]
A late Ediacaran trace fossil preserved on a bedding plane.

The Ediacaran biota exhibited a vast range of morphological characteristics. The term morphology in Biology refers to the outward appearance ( Shape, Structure, Colour, Pattern) of an Organism Size ranged from millimetres to metres; complexity from "blob-like" to intricate; rigidity from sturdy and resistant to jelly-soft. The Millimetre ( American spelling: millimeter, symbol mm) is a unit of Length in the Metric system, equal to The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International Almost all forms of symmetry were present. "Bilateral symmetry" redirects here For bilateral symmetry in mathematics see Reflection symmetry. ^[20] 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. Wastebasket taxon (also called a wastebin taxon or dustbin taxon) is a term used in taxonomic circles to refer to a Taxon that has the sole purpose The Doushantuo Formation is a lagerstätte in Guizhou Province China that is notable for being one of the oldest Fossil beds to contain highly Some finds^[25] generated intense media excitement^[26] though some have claimed they are instead inorganic structures formed by the precipitation of minerals on the inside of a hole. ^[27] Other "embryos" have been interpreted as the remains of the giant sulfur-reducing bacteria akin to Thiomargarita,^[28] a view which is highly contested yet gradually gaining supporters. Sulfur or sulphur (ˈsʌlfɚ see spelling below) is the Chemical element that has the Atomic number 16 Thiomargarita namibiensis ("Sulfur pearl of Namibia " is a Gram-negative Coccus Proteobacterium found in the ocean sediments ^[29][30]

Microfossils dating from 632.5 million years ago — just 3 million years after the end of the Cryogenian glaciations — may represent embryonic 'resting stages' in the life cycle of the earliest known animals. ^[31]

An alternative proposal is that these structures represent adult stages of the animals of this period. ^[32]

Discs 

Circular fossils, such as Ediacaria, Cyclomedusa, and Rugoconites led to the initial identification of Ediacaran fossils as cnidaria, which include jellyfish and corals. Ediacaria is a Fossil Genus dating to the Ediacaran Period of the Neoproterozoic Era Cyclomedusa is a circular Fossil of the Ediacaran biota; it has a circular bump in the middle and as many as five circular growth ridges around it A member of the Ediacaran biota which takes the form of a circular to oval impression preserved in hyporelief six or more centimeters in diameter Cnidaria (naɪˈdɛəriə is a phylum containing some 9000 Species of Animals found exclusively in aquatic mostly marine, environments ^[8] Further examination has provided alternative interpretations of all disc-shaped fossils: none is now confidently recognised as jellyfish. Alternate explanations include holdfasts, protists^[33] and sea anemones; the patterns displayed where two meet have led to many apparent individuals being recognised as microbial colonies. A holdfast is a Root -like structure that anchors aquatic Sessile organisms such as Seaweed, other sessile Algae, stalked Protists (ˈproʊtɨst are a diverse group of eukaryotic Microorganisms Historically protists were treated as the kingdom Protista but this Sea anemones are a group of water dwelling predatory animals of the order Actiniaria; they are named after the Anemone, a terrestrial Flower ^[34][35] 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". Pteridinium is a Fossil found in a number of Precambrian deposits worldwide The scientific community is a long way from reaching a consensus on their interpretation. ^[36]

Quilted organisms 

The organisms considered in A. Seilacher's revised definition of the Vendobionta^[2] share a "quilted" appearance, and resembled an inflatable mattress. A mattress is a mat or pad usually placed atop a Bed, upon which to sleep or lie 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. Swartpuntia is a monospecific Genus of Rangeomorph from the terminal Ediacaran period with at least three quilted leaf-shaped petaloids — ^[37]

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. Charnia is the Genus name given to a Frond -like Precambrian lifeform with segmented ridges branching alternately to the right and left from a Charniodiscus is an Ediacaran Fossil that was probably a stationary filter feeder that lived anchored to a sandy sea bed A tree of life is a mystical concept a Metaphor for common descent or a Motif in various world theologies and philosophies. 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 (plesiomorphy). In Evolutionary biology, a synapomorphy is a derived Character state shared by two or more terminal groups ( taxa included in a Cladistic analysis Cladistics is the hierarchical classification of Species based on evolutionary ancestry

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. Extant is a term commonly used to refer to Taxa (such as Species, genera or families) that are still in existence (living A phylum ( Plural: phyla) is a Taxonomic rank between Kingdom and above Class.

The earliest such fossil is the reputed bilaterian Vernanimalcula, claimed by some, however, to represent the infilling of an egg-sac or acritarch. Vernanimalcula is a fossil believed by some to represent the earliest known member of the Bilateria ( Animals with bilateral symmetry) Acritarchs are small organic Fossils present from approximately to the present ^[27][38] Later examples, almost universally accepted as bilaterians, include the mollusc-like Kimberella,^[39] Spriggina (pictured),^[23] and the shield-shaped Parvancorina,^[40] whose affinities are currently debated. Kimberella is a Genus of Fossils known only from rocks of the Ediacaran period and only one Species, Kimberella quadrata Fossils of Spriggina are known from the Ediacaran period around. Parvancorina is a Genus of shield-shaped Ediacaran fossils It has a raised ridge down the central axis of symmetry ^[41]

A suite of fossils known as the Small shelly fossils are represented in the Ediacaran, most famously by Cloudina,^[42] 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. The Cloudinids, early Metazoan family containing the Genus Cloudina, lived in the late Ediacaran period and

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. The sponges or poriferans (from Latin porus "pore" and ferre "to bear" are Animals Algae ( sing. alga are a large and diverse group of simple typically Autotrophic organisms ranging from Unicellular to Multicellular forms Protists (ˈproʊtɨst are a diverse group of eukaryotic Microorganisms Historically protists were treated as the kingdom Protista but this The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have

Trace fossils 

The only Ediacaran burrows are horizontal, or just below the surface. Trace fossils, also called ichnofossils (ˈɪknoʊfɒsɨl ιχνος or ikhnos meaning "trace" or "track" are geological records of biological 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. The Bilateria (ˌbaɪləˈtɪəriə are all animals having a bilateral symmetry, i ^[43] Putative "burrows" dating as far back as 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;^[44] however their uneven width and tapering ends make a biological origin difficult to defend. ^[45] 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;^[46] the method of formation of these disconnected and overlapping fossils largely remains a mystery. Yorgia waggoneri Ivantsov 1999is a member of the Ediacara biota, and resembles the organisms Dickinsonia and Spriggina. Dickinsonia is an iconic fossil of the Ediacaran biota. It (roughly resembles a bilaterally symmetrical ribbed oval The potential mollusc Kimberella is associated with scratch marks, perhaps formed by a radula,^[47] further traces from 555 million years ago appear to imply active crawling or burrowing activity. Molluscs are animals belonging to the phylum Mollusca. There are around 250000 extant Species within the phylum with an estimated 70000 The radula is a toothed Chitinous ribbon typically used for scraping cutting and chewing food before it enters the Esophagus. ^[47]

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. Charnia is the Genus name given to a Frond -like Precambrian lifeform with segmented ridges branching alternately to the right and left from a Eumetazoa is a Clade comprising all major Animal groups except sponges Characteristics of eumetazoans include true tissues organized into Cnidaria (naɪˈdɛəriə is a phylum containing some 9000 Species of Animals found exclusively in aquatic mostly marine, environments Jellyfish are free-swimming members of the phylum Cnidaria. They have several different basic morphologies that represent several different cnidarian classes including the Sea pens are colonial marine Cnidarians belonging to the order Pennatulacea. ^[48] However, detailed study of their growth pattern has discounted this hypothesis. ^[49][50]

"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. Martin Fritz Glaessner AM ( 25 December, 1906 &ndash 23 November, 1989) was a geologist and palaeontologist. ^[51] Adolf Seilacher responded by suggesting that the Ediacaran sees animals usurping giant protists as the dominant life form. Adolf "Dolf" Seilacher (b February 24 1925) is a German palaeontologist who has made major contributions to evolutionary and ecological palaeobiology in a career stretching Protists (ˈproʊtɨst are a diverse group of eukaryotic Microorganisms Historically protists were treated as the kingdom Protista but this ^[52]

In 1986 Mark McMenamin claimed that Ediacarans did not possess an embryonic stage, and thus could not be animals. Mark McMenamin is a professor of Geology at Mount Holyoke College. An embryo (from Greek:, plural, lit "that which grows" from en- "in" + bryein "to swell be full" is a multicellular 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. The nervous system is a Network of specialized cells that communicate information about an animal's surroundings and itself The brain is the center of the Nervous system in animals All Vertebrates and the majority of Invertebrates have a brain ^[33]

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,^[53][54] named after the now-obsolete Vendian era. A clade is a taxonomic group comprising a single Common ancestor and all the descendants of that ancestor In biological Taxonomy, a kingdom or regnum is a Taxonomic rank in either (historically the highest rank or (in the new three-domain system The Ediacaran Period (ˌiːdiˈækərən named after the Ediacara Hills of South Australia) is the last geological period of the Neoproterozoic He later excluded fossils identified as metazoans and relaunched the phylum "Vendobionta". A phylum ( Plural: phyla) is a Taxonomic rank between Kingdom and above Class.

He described the Vendobionta as quilted cnidarians lacking stinging cells. Cnidaria (naɪˈdɛəriə is a phylum containing some 9000 Species of Animals found exclusively in aquatic mostly marine, environments A cnidocyte, cnidoblast or nematocyte, is a type of Venomous cell unique to the Phylum Cnidaria ( Corals 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. This article is about the biological phenomenon for other uses see Symbiosis (disambiguation The term symbiosis (from the Greek Photosynthesis is a Metabolic pathway that converts Light Energy into Chemical energy. Chemotrophs are organisms that obtain Energy by the Oxidation of Electron donating Molecules in their environments ^[55]

Lichens

Gregory Retallack's hypothesis that Ediacaran organisms were lichens^[56] has failed to gain wide-spread acceptance. Lichens (ˈlaɪkən or /lɪtʃən/ are symbiotic associations of a Fungus (the mycobiont with a photosynthetic partner (the photobiont also known as ^[57][58] He argues that the fossils are not as squashed as jellyfish fossilised in similar situations, and their relief is closer to petrified wood. Petrified wood (from the Greek root "petro" meaning "rock" or "stone" literally "wood turned into stone" is a type of Fossil: it 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. Chitin ( C 8 H 13 O 5 N)n (ˈkaɪtən is a long-chain Polymer of a N-acetylglucosamine

Other interpretations

Almost every possible phylum has been used to accommodate the Ediacaran biota at some point,^[59] from algæ,^[60] to protists known as foraminifera,^[61] to fungi^[62] to bacterial or microbial colonies,^[34] to hypothetical intermediates between plants and animals. Algae ( sing. alga are a large and diverse group of simple typically Autotrophic organisms ranging from Unicellular to Multicellular forms Protists (ˈproʊtɨst are a diverse group of eukaryotic Microorganisms Historically protists were treated as the kingdom Protista but this The Foraminifera, ("Hole Bearers" or forams for short are a large group of Amoeboid Protists with reticulating Pseudopods fine A fungus (ˈfʌŋgəs is a eukaryotic Organism that is a member of the kingdom Fungi (ˈfʌndʒaɪ The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have A microorganism (also spelled micro organism or micro-organism and also called a microbe) is an Organism that is Microscopic (usually ^[63]

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 3,460 million years ago,^[64][65] the first uncontroversial evidence for life is found 2,700 million years ago,^[66] and cells with nuclei certainly existed by 1,200 million years ago:^[67] 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 610 million-year-old Twitya formation,^[68] possibly displaying the most complex morphology of the time. The evolution of complexity is an important outcome of the process of Evolution. Nimbia occlusa is a form of Ediacaran fossil shaped like a circular or oval disk with a thick rim around the margin

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. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Without a complex circulatory system, low concentrations of oxygen cannot reach the centre of an organism quickly enough to supply its metabolic demand. This is an article about the rock music band "Circulatory System"

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. Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 Uranium (jʊˈreɪniəm is a silvery-gray Metallic Chemical element in the Redox (shorthand for reduction-oxidation reaction describes all Chemical reactions in which atoms have their Oxidation number ( Oxidation state Photosynthesis is a Metabolic pathway that converts Light Energy into Chemical energy. 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. An atmosphere (from Greek ατμός - atmos, " Vapor " + σφαίρα - sphaira, " Sphere " Banded iron formations (also known as banded ironstone formations or BIF s are a distinctive type of rock often found in primordial ( Precambrian) Sedimentary Donald Canfield detected records of the first significant quantities of atmospheric oxygen just before the first Ediacaran fossils appeared^[69] — and the presence of atmospheric oxygen was soon heralded as a possible trigger for the Ediacaran radiation. Donald Canfield is a geologist born in 1958 most famous for his work on ancient ocean chemistry An adaptive radiation is a rapid Evolutionary radiation characterized by an increase in the morphological and ecological diversity of a single rapidly diversifying lineage ^[70] 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. ^[71] 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. ^[72]

Periods of intense cold have also been suggested as a barrier to the evolution of multicellular life. A glacial period is an interval of time within an Ice age that is marked by colder temperatures and Glacier advances 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. The Doushantuo Formation is a lagerstätte in Guizhou Province China that is notable for being one of the oldest Fossil beds to contain highly The Snowball Earth Hypothesis as it was originally proposed]] Evidence The Snowball Earth hypothesis was originally devised to explain the apparent presence of ^[73] 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. ^[18] However, if they were common, more than the occasional specimen^[1] might be expected in exceptionally preserved fossil assemblages (Konservat-Lagerstätten) such as the Burgess Shale and Chengjiang^[74] — unless such assemblages represent an environment never occupied by the Ediacaran biota, or unsuitable conditions for their preservation. A Lagerstätte ( German; literally place of storage; plural Lagerstätten) is a sedimentary deposit that exhibits extraordinary fossil See also Burgess shale type fauna The Burgess Shale is famous for the exceptional preservation of the fossils found within it in which the soft parts are preserved In the Maotianshan shales is a lower Cambrian Konservat Lagerstätte named for Maotianshan Hill ( in Chengjiang County Yunnan Province

Predation and grazing

It is suggested that by the Early Cambrian, organisms higher in the food chain caused the microbial mats to largely disappear. In Ecology, trophic dynamics is the system of trophic levels ( Greek trophē, food which describe the position that an organism occupies 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. ^[33] 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". Kimberella is a Genus of Fossils known only from rocks of the Ediacaran period and only one Species, Kimberella quadrata ^[39]

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. The Cambrian is a geologic period and system that began about Ma (million years ago at the end of the Proterozoic eon and ended about Ma with

Competition

It is possible that increased competition due to the evolution of key innovations amongst other groups, perhaps as a response to predation,^[17] 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. Bivalves are Molluscs belonging to the class Bivalvia. They have two-part shells and typically both valves are symmetrical along the hinge line Brachiopods (from Latin brachium, arm + New Latin -poda, foot are a small phylum of Benthic Invertebrates Also ^[75]

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,^[76] rising sea levels (creating shallow, "life-friendly" seas),^[77] a nutrient crisis,^[78] fluctuations in atmospheric composition, including oxygen and carbon dioxide levels,^[79] and changes in ocean chemistry^[80] (promoting biomineralisation)^[81] could all have played a part. For the Genus of Metalmark butterflies, see Rodinia (butterfly. Biomineralisation is the process by which living organisms produce minerals often to harden or stiffen existing tissues

Assemblages

Ediacaran-type fossils are recognised globally in 25 localities^[16] and a variety of depositional conditions, and are commonly grouped into three main types, named after typical localities. Deposition is the geological process by which material is added to a Landform or land mass

Avalon-type assemblage

The Avalon-type assemblage is defined at Mistaken Point in Canada, the oldest locality with a large quantity of Ediacaran fossils. ^[82] 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. Zircon is a Mineral belonging to the group of nesosilicates. Its chemical name is Zirconium silicate and its corresponding chemical formula is Radiometric dating (often called radioactive dating) is a technique used to date materials usually based on a comparison between the observed abundance of a naturally occurring These fine-grained ash beds also preserve exquisite detail.

The biota comprises deep sea dwelling rangeomorphs^[83] such as Charnia, all of which share a fractal growth pattern. The Rangeomorphs are a Form taxon of frondose Ediacaran fossils that are united by a similarity to Rangea. A fractal is generally "a rough or fragmented geometric shape that can be split into parts each of which is (at least approximately a reduced-size copy of the whole" 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^[84] — by most interpretations, the assemblage is found in water too deep for photosynthesis. The Carboniferous is a geologic period and system that extends from the end of the Devonian period about 359 Filter feeders (also known as suspension feeders) are Animals that feed by straining suspended matter and food particles from water typically by passing the water 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. Speciation is the Evolutionary process by which new biological Species arise Opinion is currently divided between these conflicting hypotheses. ^[85]

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). Ediacara Hills are a range of low hills in the northern Flinders Ranges of South Australia, around 650 km north of Adelaide. A delta is a Landform where the mouth of a River flows into an Ocean, Sea, Estuary, Lake or another river The term " Facies " can also refer to distinctive Facial expressions associated with conditions such as Williams syndrome. 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. ^[85]

Biota ranges[85]
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Axis scale: millions of years ago, dated with U/Pb of zircons

Nama-type assemblage

The Nama assemblage is best represented in Namibia. Namibia, officially the Republic of Namibia, is a country in Southern Africa on the Atlantic coast Three-dimensional preservation is most common, with organisms preserved in sandy beds containing internal bedding. Dima Grazhdankin believes that these organisms represent burrowing organisms,^[36] while Guy Narbonne maintains they were surface dwellers. ^[86] These beds are sandwiched between units comprising interbedded sandstones, siltstones and shales, with microbial mats, where present, usually containing fossils. Shale (also called mudstone) is a fine-grained Sedimentary rock whose original constituents were Clay minerals or Muds It is characterized by The environment is interpreted as sand bars formed at the mouth of a delta's distributaries. A delta is a Landform where the mouth of a River flows into an Ocean, Sea, Estuary, Lake or another river A distributary, or a distributary channel, is a Stream that branches off and flows away from a main stream channel ^[85]

Significance of assemblages

In the White Sea region of Russia, all three assemblage types have been found in close proximity. The White Sea (Бе́лое мо́ре Vienanmeri is an Inlet of the Barents Sea on the northwest coast of Russia. This, and the faunas' considerable temporal overlap, makes it unlikely that they represent evolutionary stages or temporally distinct communities. In Ecology, a community is an assemblage of Populations of different Species, interacting with one another Since they are globally distributed — described on all continents except Antarctica — geographical boundaries do not appear to be a factor;^[87] the same fossils are found at all palæolatitudes (the latitude where the fossil was created, accounting for continental drift) and in separate sedimentary basins. Latitude, usually denoted symbolically by the Greek letter phi ( Φ) gives the location of a place on Earth (or other planetary body north or south of the Continental drift is the movement of the Earth 's Continents relative to each other The term sedimentary basin is used to refer to any geographical feature exhibiting Subsidence and consequent infilling by sedimentation ^[85]

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. Guilds are groups of Species that exploit the same resources in the same way therefore sharing a similar Ecological niche. 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. ^[88] 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. The Cambrian is a geologic period and system that began about Ma (million years ago at the end of the Proterozoic eon and ended about Ma with

Further reading

• Mark McMenamin (1998). The Garden of Ediacara: Discovering the First Complex Life, 368pp. ISBN 0231105584.   An outdated popular science account of these fossils, with a narrowed focus on only the Namibian Fossils.

• Derek Briggs & Peter Crowther (Editors) (2001). Palæobiology II: A synthesis, Chapter 1. ISBN 0-632-05147-7.   Excellent 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

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