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The Ediacaran (; formerly Vendian) biota is a
taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific classification scheme. Originally used only about biological ...
period classification that consists of all life forms that were present on Earth during the
Ediacaran The Ediacaran Period ( ) is a geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of ...
Period (c. 635–541
Mya
Mya
). These were composed of enigmatic tubular and frond-shaped, mostly sessile, organisms.
Trace fossil A trace fossil, also ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language A clas ...
s of these organisms have been found worldwide, and represent the earliest known complex
multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biol ...
s.Simple multicellular organisms such as
red alga Red algae, or Rhodophyta ( , ; ), are one of the oldest groups of eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system t ...
e evolved at least . The status of the
Francevillian biota The Francevillian biota (also known as Gabon macrofossils or Gabonionta) is a group of 2.1-billion-year-old Palaeoproterozoic, macroscopic Organism, organisms known from Fossil, fossils found in Gabon in the Palaeoproterozoic Francevillian B Forma ...
of is unclear, but they may represent earlier multicellular forms of a more complex nature.
The Ediacaran biota may have undergone
evolutionary radiation An evolutionary radiation is an increase in taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific clas ...
in a proposed event called the
Avalon explosion The Avalon explosion, named from the Precambrian fauna of the Avalon Peninsula, is a proposed evolutionary radiation in the history of the Animalia, about 575 million years ago in the Ediacaran Period, some 33 million years earlier than the Cambria ...
, . This was after the Earth had thawed from the
Cryogenian The Cryogenian (, from grc, κρύος, krýos, meaning "cold" and , romanized: , meaning "birth") is a geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy ...
period's extensive glaciation. This biota largely disappeared with the rapid increase in
biodiversity Biodiversity is the biological variety and Genetic variability, variability of life, life on Earth. Biodiversity is a measure of variation at the Genetics, genetic, species, and ecosystem level. Terrestrial biodiversity is usually greater near ...

biodiversity
known as the Cambrian explosion. Most of the currently existing
body plan A body plan, ''Bauplan'' (German plural ''Baupläne''), or ground plan is a set of morphological features common to many members of a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ) ...
s of
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
s first appeared in the
fossil record A fossil (from Classical Latin: , literally 'obtained by digging') is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, Seashell, shells, exoskeletons, stone imprints of a ...

fossil record
of the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These peri ...
rather than the Ediacaran. For macroorganisms, the Cambrian biota appears to have completely replaced the organisms that dominated the Ediacaran fossil record, although relationships are still a matter of debate. The organisms of the Ediacaran Period first appeared around and flourished until the cusp of the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These peri ...
, when the characteristic communities of fossils vanished. A diverse Ediacaran community was discovered in 1995 in
Sonora Sonora (), officially Estado Libre y Soberano de Sonora ( en, Free and Sovereign State of Sonora), is one of the 31 states which, along with Mexico City Mexico City ( es, link=no, Ciudad de México, ; abbreviated as CDMX; nah, Āltepētl Mēx ...
, Mexico, and is approximately 555 million years in age, roughly coeval with Ediacaran fossils of the
Ediacara Hills Ediacara Hills are a range of low hills in the northern part of the Flinders Ranges of South Australia, around north of the state capital of Adelaide city centre, Adelaide. The area has many old copper and silver mining, mines from mining act ...
, South Australia and the White Sea, Russia. While rare fossils that may represent survivors have been found as late as the
Middle Cambrian Middle or The Middle may refer to: * Centre (geometry), the point equally distant from the outer limits. Places * Middle (sheading), a subdivision of the Isle of Man * Middle Bay (disambiguation) * Middle Brook (disambiguation) * Middle Creek (di ...
(510 to 500 million years ago), the earlier fossil communities disappear from the record at the end of the Ediacaran leaving only curious fragments of once-thriving
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...

ecosystem
s. Multiple hypotheses exist to explain the disappearance of this biota, including
preservation bias Taphonomy is the study of how organisms decay and become fossilized or preserved in the archaeological record. The term ''taphonomy'' (from the Greek language, Greek , meaning "burial", and , meaning "law") was introduced to paleontology in 1940 ...
, a changing environment, the advent of
predators Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation (which ...

predators
and competition from other life-forms. Recent (2018) sampling of late Ediacaran strata across Baltica (<560 mya) suggests the flourishing of the organisms coincided with conditions of low overall productivity with a very high percentage produced by bacteria, which may have led to high concentrations of dissolved organic material in the oceans. Determining where Ediacaran organisms fit in the
tree of life #REDIRECT Tree of life#REDIRECT Tree of life The tree of life is a fundamental widespread mytheme or archetype in many of the world's mythology, mythologies, religion, religious and philosophy, philosophical traditions. It is closely related ...
has proven challenging; it is not even established that they were animals, with suggestions that they were
lichens A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among Hypha, filaments of multiple Fungus, fungi species in a mutualism (biology), mutualistic relationship.
(fungus-alga symbionts),
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
,
protists A protist () is any eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...
known as
foraminifera Foraminifera (; Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...
,
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi
or
microbial A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
colonies, or hypothetical intermediates between plants and animals. The morphology and habit of some taxa (e.g. '' Funisia dorothea'') suggest relationships to
Porifera Sponges, the members of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. T ...

Porifera
or
Cnidaria Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of ...

Cnidaria
. '''' may show a similarity to
mollusc Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number ...
s, and other organisms have been thought to possess
bilateral symmetry Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a pla ...
, although this is controversial. Most macroscopic fossils are morphologically distinct from later life-forms: they resemble discs, tubes, mud-filled bags or quilted mattresses. Due to the difficulty of deducing evolutionary relationships among these organisms, some
palaeontologist Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene The Holocene ( ) is the current Geologic time scale, geological epoch. ...
s have suggested that these represent completely extinct lineages that do not resemble any living organism. One
palaeontologist Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene The Holocene ( ) is the current Geologic time scale, geological epoch. ...
proposed a separate subkingdom level category Vendozoa (now renamed Vendobionta) in the Linnaean hierarchy for the Ediacaran biota. If these enigmatic organisms left no descendants, their strange forms might be seen as a "failed experiment" in multicellular life, with later multicellular life evolving independently from unrelated single-celled organisms. A 2018 study confirmed that one of the period's most-prominent and iconic fossils, ''
Dickinsonia ''Dickinsonia'' is an extinct genus of basal animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organi ...
'', included
cholesterol Cholesterol is any of a class of certain organic compound, organic molecules. A cholesterol is a sterol (or chemical modification, modified steroid), a type of lipid. Cholesterol is biosynthesis, biosynthesized by all animal Cell (biology)#Euk ...

cholesterol
, suggesting affinities to animals, fungi, or red algae. The concept of "Ediacaran Biota" is somewhat artificial as it cannot be defined geographically, stratigraphically, taphonomically, or biologically.


History

The first Ediacaran fossils discovered were the disc-shaped ''
Aspidella :Aspidella'' is also a synonym (taxonomy), synonym for the mushroom genus ''Saproamanita. ''Aspidella'' is an Ediacaran disk-shaped fossil of uncertain affinity. It is known from the single species ''A. terranovica''. Morphology ''Aspidella'' ...
terranovica'' in 1868. Their discoverer, Scottish geologist Alexander Murray, found them useful aids for correlating the age of rocks around
Newfoundland Newfoundland and Labrador (, ) is the easternmost provinces and territories of Canada, province of Canada, in the country's Atlantic Canada, Atlantic region. It is composed of the island of Newfoundland (island), Newfoundland and the continental ...
. However, since they lay below the "Primordial Strata" of the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These peri ...
that was then thought to contain the very first signs of animal life, a proposal four years after their discovery by
Elkanah Billings Elkanah Billings (May 5, 1820 – June 14, 1876) is often referred to as Canada's first paleontologist. Billings was born on a farm by the Rideau River outside Bytown (Ottawa), now known as Billings Estate Museum, Billings Estate. His parents w ...

Elkanah Billings
that these simple forms represented fauna was dismissed by his peers. Instead, they were interpreted as structures or inorganic
concretion A concretion is a hard, compact mass of matter formed by the precipitation of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition ...

concretion
s. No similar structures elsewhere in the world were then known and the one-sided debate soon fell into obscurity. In 1933, Georg Gürich discovered specimens in
Namibia Namibia (, ), officially the Republic of Namibia, is a country in Southern Africa Southern Africa is the south South is one of the cardinal directions or compass points. South is the opposite of north and is perpendicular to the east a ...

Namibia
but the firm belief that complex life originated in the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These peri ...
led to them being assigned to the Cambrian Period and no link to ''Aspidella'' was made. In 1946,
Reg Sprigg Reginald Claude Sprigg, (1 March 1919 – 2 December 1994) was an Australian geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes the Earth and other terrestrial planets, as well as the pr ...
noticed "jellyfishes" in the
Ediacara Hills Ediacara Hills are a range of low hills in the northern part of the Flinders Ranges of South Australia, around north of the state capital of Adelaide city centre, Adelaide. The area has many old copper and silver mining, mines from mining act ...
of Australia's
Flinders Ranges The Flinders Ranges are the largest mountain range A mountain range is a series of mountains ranged in a line and connected by high ground. A mountain system or mountain belt is a group of mountain ranges with similarity in form, structure, ...

Flinders Ranges
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 ''Charnia'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumsc ...

Charnia
'' that the pre-Cambrian was seriously considered as containing life. This
frond A frond is a large, divided leaf A leaf (plural leaves) is the principal lateral appendage of the vascular plant plant stem, stem, usually borne above ground and specialized for photosynthesis. The leaves, stem, flower and fruit tog ...

frond
-shaped fossil was found in England's
Charnwood Forest Charnwood Forest is an upland tract in north-western Leicestershire Leicestershire (; postal abbreviation Leics.) is a landlocked county A county is a geographical region of a country used for administrative or other purposesChamb ...
first by a 15-year-old girl in 1956 (Tina Negus, who was not believed) and then the next year by a group of three schoolboys including 15-year-old Roger Mason. Due to the detailed
geological map A geologic map is a special-purpose map made to show various geological features. Rock (geology), Rock units or Stratum, geologic strata are shown by color or symbols. Bed (geology), Bedding planes and structural features such as fault (geology), f ...
ping of the
British Geological Survey 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 shelf A continental shelf is a portion of a continent A continen ...

British Geological Survey
, there was no doubt these fossils sat in Precambrian rocks. Palaeontologist
Martin Glaessner Martin Fritz Glaessner AM (25 December 1906 – 23 November 1989) was a geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes the Earth and other terrestrial planets, as well as the processes th ...
finally, in 1959, made the connection between this and the earlier finds and with a combination of improved dating of existing specimens and an injection of vigour into the search many more instances were recognised. All specimens discovered until 1967 were in coarse-grained
sandstone Sandstone is a clastic sedimentary rock of a clast (sand grain), derived from a basalt Basalt (, ) is a fine-grained extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron ('' mafic ' ...

sandstone
that prevented preservation of fine details, making interpretation difficult. S.B. Misra's discovery of fossiliferous
ash Ash or ashes are the solid remnants of fire BBQ. Fire is the rapid oxidation of a material in the exothermic chemical process of combustion, releasing heat, light, and various reaction Product (chemistry), products. Fire is hot because th ...
-beds at the
Mistaken Point Mistaken Point Ecological Reserve is a wilderness area and a UNESCO World Heritage Site located at the southeastern tip of Newfoundland's Avalon Peninsula in the Canadian Canadians (french: Canadiens) are people identified with the countr ...
assemblage in Newfoundland changed all this as the delicate detail preserved by the fine ash allowed the description of features that were previously undiscernible. It was also the first discovery of Ediacarans in deep water sediments. Poor communication, combined with the difficulty in correlating globally distinct
formation Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondary m ...
s, led to a plethora of different names for the biota. In 1960 the French name "Ediacarien" – after the Ediacara Hills – was added to the competing terms "Sinian" and "Vendian" for terminal-Precambrian rocks, and these names were also applied to the life-forms. "Ediacaran" and "Ediacarian" were subsequently applied to the epoch or period of
geological time The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth sciences, Earth scientists to describe ...

geological time
and its corresponding rocks. In March 2004, the
International Union of Geological Sciences The International Union of Geological Sciences (IUGS) is an international non-governmental organization A non-governmental organization, or simply an NGO, is an organization that is, generally, formed independent from government. They are t ...
ended the inconsistency by formally naming the terminal
period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Period, a descriptor for a historical or period drama ...
of the
Neoproterozoic The Neoproterozoic Era is the unit of geologic time Geology (from the Ancient Greek Ancient Greek includes the forms of the used in and the from around 1500 BC to 300 BC. It is often roughly divided into the following periods: ...
after the Australian locality. Reprint, 2004 original available her

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


Preservation


Microbial mats

Microbial mat The cyanobacterial algal mat, salty lake on the White Sea">algal_mat.html" ;"title="cyanobacterial algal mat">cyanobacterial algal mat, salty lake on the White Sea seaside A microbial mat is a multi-layered sheet of microorganisms, mainly bacteria ...
s are areas of sediment stabilised by the presence of colonies of microbes that secrete sticky fluids or otherwise bind the sediment particles. They appear to migrate upwards when covered by a thin layer of sediment but this is an illusion caused by the colony's growth; individuals do not, themselves, move. If too thick a layer of sediment is deposited before they can grow or reproduce through it, parts of the colony will die leaving behind fossils with a characteristically wrinkled ("elephant skin") and tubercular texture. Some Ediacaran strata with the texture characteristics of microbial mats contain fossils, and Ediacaran fossils are almost always found in beds that contain these microbial mats. Although microbial mats were once widespread, the evolution of grazing organisms in the Cambrian vastly reduced their numbers. These communities are now limited to inhospitable
refugia Refugium, plural refugia, the Latin for "refuge" or "hideaway", may refer to: * Refugium (fishkeeping), an appendage to a marine, brackish, or freshwater fish tank that shares the same water supply * Refugium (population biology), a location of an i ...
, such as the
stromatolites Stromatolites () or stromatoliths () are layered sedimentary Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's sur ...

stromatolites
found in
Hamelin Pool Marine Nature Reserve The Hamelin Pool Marine Nature Reserve is a state park, protected marine park, marine nature reserve located in the UNESCO World Heritage Site, World Heritagelisted Shark Bay in the Gascoyne (Western Australia), Gascoyne region of Western Aust ...
in
Shark Bay Shark Bay ( Malgana: ''Gutharraguda'', "two waters") is a World Heritage Site A World Heritage Site is a landmark or area with legal protection by an international convention administered by the United Nations Educational, Scientific an ...

Shark Bay
,
Western Australia Western Australia (abbreviated as WA) is a state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ''The State'' (newspape ...

Western Australia
, where the salt levels can be twice those of the surrounding sea.


Fossilization

The preservation of these fossils is one of their great fascinations to science. As soft-bodied organisms, they would normally not fossilize and, unlike later soft-bodied fossil biota such as the
Burgess Shale The Burgess Shale is a fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. L ...
or
Solnhofen Limestone #REDIRECT Solnhofen Limestone The Solnhofen Plattenkalk, or Solnhofen Limestone, geologically known as the Altmühltal Formation, is a Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system ...
, the Ediacaran biota is not found in a restricted environment subject to unusual local conditions: they were a global phenomenon. The processes that were operating must have been systemic and worldwide. There was something very different about the Ediacaran Period that permitted these delicate creatures to be left behind and it is thought the fossils were preserved by virtue of rapid covering by ash or sand, trapping them against the mud or microbial mats on which they lived. Their preservation was possibly enhanced by the high concentration of silica in the oceans before silica-secreting organisms such as sponges and diatoms became prevalent. Ash beds provide more detail and can readily be dated to the nearest million years or better using
radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "di ...
. However, it is more common to find Ediacaran fossils under sandy beds deposited by storms or high-energy bottom-scraping ocean currents known as
turbidite A turbidite is the geologic Geology (from the Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly ...

turbidite
s. Soft-bodied organisms today rarely fossilize during such events, but the presence of widespread microbial mats probably aided preservation by stabilising their impressions in the sediment below.


Scale of preservation

The rate of cementation of the overlying substrate relative to the rate of decomposition of the organism determines whether the top or bottom surface of an organism is preserved. Most disc-shaped fossils decomposed before the overlying sediment was cemented, whereupon ash or sand slumped in to fill the void, leaving a cast of the organism's underside. Conversely, quilted fossils tended to decompose ''after'' the cementation of the overlying sediment; hence their upper surfaces are preserved. Their more resistant nature is reflected in the fact that, in rare occasions, quilted fossils are found ''within'' storm beds as the high-energy sedimentation did not destroy them as it would have the less-resistant discs. Further, in some cases, the
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
l
precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...
of minerals formed a "death mask", ultimately leaving a positive, cast-like impression of the organism.


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 Symmetry (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appro ...
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 Form classification is the classification of organisms based on their morphology (biology), morphology, which does not necessarily reflect their biological relationships. Form classification, generally restricted to palaeontology, reflects uncer ...
: ; "Embryos" : Recent discoveries of Precambrian multicellular life have been dominated by reports of embryos, particularly from the Doushantuo Formation in China. Some finds generated intense media excitement 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 Sulfur (in nontechnical British English: sulphur) is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: th ...

sulfur
-reducing bacteria akin to ''
Thiomargarita ''Thiomargarita'' is a genus (family Thiotrichaceae) which includes the vacuole, vacuolate Sulfate-reducing microorganisms, sulfur bacteria species ''Thiomargarita namibiensis'', ''Candidatus Thiomargarita nelsonii'', and ''Ca. Thiomargarita joer ...
'',e.g. , summarised by a view that, while it had enjoyed a notable gain of supportersXiao ''et al.''.'s response to Bailey ''et al.''.'s original paper : And Bailey ''et al.''.'s reply: as of 2007, has since suffered following further research comparing the potential Doushantuo embryos' morphologies with those of ''Thiomargarita'' specimens, both living and in various stages of decay. A recent discovery of comparable Ediacaran fossil embryos from the Portfjeld Formation in Greenland has significantly expanded the paleogeograpical occurrence of Doushanuto-type fossil "embryos" with similar biotic forms now reported from differing paleolatitudes. : Microfossils dating from  – just 3 million years after the end of the Cryogenian glaciations – may represent embryonic 'resting stages' in the life cycle of the earliest known animals. An alternative proposal is that these structures represent adult stages of the multicellular organisms of this period. Microfossils of '' Caveasphaera'' are thought to foreshadow the evolutionary origin of animal-like embryology. ; Discs : Circular fossils, such as ''
Ediacaria ''Ediacaria'' is a fossil genus dating to the Ediacaran Period of the Neoproterozoic Era. Unlike most Ediacaran biota, which disappeared almost entirely from the fossil record at the end of the Period, ''Ediacaria'' fossils have been found dating ...
'', '''' and ''
Rugoconites ''Rugoconites'' is a genus of Ediacaran biota found as fossils in the form of a circular or oval-like impression preserved in high relief, six or more centimeters in diameter. The fossils are surrounded by frills that have been interpreted (Mary ...

Rugoconites
'' led to the initial identification of Ediacaran fossils as
cnidaria Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of ...

cnidaria
, which include jellyfish and corals. Further examination has provided alternative interpretations of all disc-shaped fossils: not one is now confidently recognised as a jellyfish. Alternate explanations include holdfasts and
protist A protist () is any eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are c ...
s; the patterns displayed where two meet have led to many 'individuals' being identified as microbial colonies, and yet others may represent scratch marks formed as stalked organisms spun around their holdfasts. Useful diagnostic characteristics are often lacking because only the underside of the organism is preserved by fossilisation. ; Bags : Fossils such as ''
Pteridinium ''Pteridinium'' is an erniettomorph found in a number of Precambrian deposits worldwide. It is a member of the Ediacaran biota. Body plan The three-lobed body is generally flat such that only two lobes are visible. Each lobe consists of a number ...
'' 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: ; Toroids : The fossil ''Vendoglossa tuberculata'' from the Nama Group, Namibia, has been interpreted as a dorso-ventrally compressed stem-group metazoan, with a large gut cavity and a transversely ridged
ectoderm The ectoderm is one of the three primary germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans ( ...

ectoderm
. The organism is in the shape of a flattened torus, with the long axis of its toroidal body running through the approximate center of the presumed gut cavity. ; Quilted organisms : The organisms considered in Seilacher's revised definition of the Vendobionta share a "quilted" appearance and resembled an inflatable
mattress A mattress is a large, usually rectangular pad for supporting a lying A lie is an assertion that is believed to be false, typically used with the purpose of deceiving someone. The practice of communicating lies is called lying. A person who ...

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 ''Swartpuntia'' is a monospecific genus of erniettomorph from the terminal Ediacaran period, with at least three quilted, leaf-shaped petaloids — probably five or six. The petaloids comprise vertical sheets of tubes filled with sand. ''Swartpu ...
germsi'' could only be recognised in a posthumously damaged specimen – usually multiple fronds were hidden as burial squashed the organisms flat. :These organisms appear to form two groups: the fractal rangeomorphs and the simpler erniettomorphs. Including such fossils as the iconic ''
Charnia ''Charnia'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumsc ...

Charnia
'' and ''
Swartpuntia ''Swartpuntia'' is a monospecific genus of erniettomorph from the terminal Ediacaran period, with at least three quilted, leaf-shaped petaloids — probably five or six. The petaloids comprise vertical sheets of tubes filled with sand. ''Swartpu ...
'', the group is both the most iconic of the Ediacaran biota and the most difficult to place within the existing tree of life. Lacking any mouth, gut, reproductive organs, or indeed any evidence of internal anatomy, their lifestyle was somewhat peculiar by modern standards; the most widely accepted hypothesis holds that they sucked nutrients out of the surrounding seawater by osmotrophy or osmosis. However, others argue against this. ; Non-Vendobionts : Some Ediacaran organisms have more complex details preserved, which has allowed them to be interpreted as possible stem group, early forms of Extant taxon, living phylum, 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. In 2020, ''Ikaria wariootia'' was claimed to represent one of the oldest organisms with anterior and posterior differentiation. Later examples are almost universally accepted as bilaterians and include the mollusc-like '''', ''Spriggina'' (pictured) and the shield-shaped ''Parvancorina'' whose affinities are currently debated. :A suite of fossils known as the small shelly fossils are represented in the Ediacaran, most famously by ''Cloudinid, Cloudina'' a shelly tube-like fossil that often shows evidence of predatory boring, suggesting that, while predation may not have been common in the Ediacaran Period, it was at least present. :Organic microfossils known as small carbonaceous fossils are also found in Ediacaran sediments, including the spiral-shaped ''Cochleatina'' which has been found to span the Ediacaran–Cambrian boundary. : Representatives of modern taxa existed in the Ediacaran, some of which are recognisable today. Sponges, red and green alga, algæ,
protist A protist () is any eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are c ...
s and
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
are all easily recognisable with some pre-dating the Ediacaran by nearly three billion years. Possible arthropods have also been described. :Fossils of the hard-shelled
foraminifera Foraminifera (; Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...
''Platysolenites'' are known from the latest Ediacaran of western Siberia, coexisting with ''Cloudina'' and ''Namacalathus''. ; Filaments :Filament-shaped structures in Precambrian fossils have been observed on many occasions. Frondose fossils in Newfoundland have been observed to have developed filamentous bedding planes, inferred to be stolonic outgrowths. A study of Brazilian Ediacaran fossils found filamentous microfossils, suggested to be eukaryotes or large Microbial metabolism, sulfur-oxidizing-bacteria (SOBs). Fungus-like filaments found in the Doushantuo Formation have been interpreted as eukaryotes and possibly fungi, providing possible evidence for the evolution and terrestrialization of fungi ~635 Ma. ;
Trace fossil A trace fossil, also ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language A clas ...
s : With the exception of some very simple Skolithos, vertical burrows the only Ediacaran burrows are horizontal, lying on or just below the surface of the seafloor. Such burrows have been taken to imply the presence of motile organisms with heads, which would probably have had a bilateral symmetry. This could place them in the bilateria, bilateral clade of
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
s but they could also have been made by simpler organisms feeding as they slowly rolled along the sea floor. Putative "burrows" dating as far back as may have been made by animals that fed on the undersides of microbial mats, which would have shielded them from a chemically unpleasant ocean; however their uneven width and tapering ends make a biological origin so difficult to defend that even the original proponent no longer believes they are authentic. : 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 ''Dickinsonia'' is an extinct genus of basal animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organi ...
'' are often found at the end of fossil trackway, long pathways of trace fossils matching their shape; these fossils are thought to be associated with cilium, ciliary feeding but the precise method of formation of these disconnected and overlapping fossils largely remains a mystery.A. Yu. Ivantsov. (2008)
"Feeding traces of the Ediacaran animals"
HPF-17 Trace fossils : ichnological concepts and methods. International Geological Congress – Oslo 2008.
The potential
mollusc Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number ...
''Kimberella'' is associated with scratch marks, perhaps formed by a radula.According to
For a more cynical perspective see


Classification and interpretation

Classification of the Ediacarans is difficult, and hence a variety of theories exist as to their placement on the tree of life.
Martin Glaessner Martin Fritz Glaessner AM (25 December 1906 – 23 November 1989) was a geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes the Earth and other terrestrial planets, as well as the processes th ...
proposed in ''The Dawn of Animal Life'' (1984) that the Ediacaran biota were recognizable crown group members of modern phyla, but were unfamiliar because they had yet to evolve the characteristic features we use in modern classification. In 1998 Mark McMenamin claimed Ediacarans did not possess an embryonic stage, and thus could not be animals. He believed that they independently evolved a nervous system and brains, meaning that "the path toward intelligent life was embarked upon more than once on this planet". In 2018 analysis of ancient sterols was taken as evidence that one of the period's most-prominent and iconic fossils, ''
Dickinsonia ''Dickinsonia'' is an extinct genus of basal animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organi ...
'', was an early animal.


Cnidarians

Since the most primitive eumetazoans—multi-cellular animals with tissues—are cnidarians, the first attempt to categorise these fossils designated them as jellyfish and sea pens. However, more recent discoveries have established that many of the circular forms formerly considered "cnidarian medusa" are actually holdfasts – sand-filled vesicles occurring at the base of the stem of upright frond-like Ediacarans. A notable example is the form known as ''Charniodiscus'', a circular impression later found to be attached to the long 'stem' of a frond-like organism that now bears the name. The link between certain frond-like Ediacarans and sea pens has been thrown into doubt by multiple lines of evidence; chiefly the derived nature of the most frond-like pennatulacean octocorals, their absence from the fossil record before the Tertiary, and the apparent cohesion between segments in Ediacaran frond-like organisms. Some researchers have suggested that an analysis of "growth poles" discredits the pennatulacean nature of Ediacaran fronds.


Protozoans

Adolf Seilacher has suggested the Ediacaran sees animals usurping giant
protist A protist () is any eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are c ...
s as the dominant life form. The modern xenophyophores are giant single-celled protozoans found throughout the world's oceans, largely on the abyssal plain. A recent genetic study suggested that the xenophyophores are a specialised group of Foraminifera. There are approximately 42 recognised species in 13 genera and 2 orders; one of which, ''Syringammina fragilissima'', is among the largest known protozoans at up to 20 centimetres in diameter.


New phylum

Seilacher has suggested that the Ediacaran organisms represented a unique and extinct grouping of related forms descended from a common ancestor (clade) and created the kingdom (biology), kingdom Vendozoa, named after the now-obsolete Vendian era. He later excluded fossils identified as metazoans and relaunched the phylum (biology), phylum "Vendobionta". He described the Vendobionta as quilted cnidarians lacking cnidocyte, stinging cells. This absence precludes the current cnidarian method of feeding, so Seilacher suggested that the organisms may have survived by symbiosis with photosynthesis, photosynthetic or chemoautotrophic organisms. Mark McMenamin saw such feeding strategies as characteristic for the entire biota, and referred to the marine biota of this period as a "Garden of Ediacara".


Lichen hypothesis

Greg Retallack's hypothesis that Ediacaran organisms were lichens has been controversial. He argues that the fossils are not as squashed as known fossil jellyfish, and their relief is closer to compressed woody branches whose compaction can be estimated as compressed cylinders. He points out the chitinous walls of lichen colonies would provide a similar resistance to compaction, and claims the large size of the organisms (up to 1.5 metres long, far larger than any of the preserved burrows) also hints against classification with animals. Thin sections of Ediacaran fossils show lichen-like compartments and hypha-like wisps of ferruginized clay. Finally, Ediacaran fossils from classic localities of the Flinders Ranges have been found in strata that Rettalack interprets to be both growth position within strata that he controversially interprets to be red calcareous and Gypsum, gypsiferous paleosols and possibly well-drained temperate desert soils. According to Retallack's interpretations, such habitats limit interpretive options for fractal Ediacaran fossils such as ''
Dickinsonia ''Dickinsonia'' is an extinct genus of basal animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organi ...
'' to lichenised or unlichenised fungi, but other Ediacaran fossils could have been slime moulds or microbial colonies.


Other interpretations

Several classifications have been used to accommodate the Ediacaran biota at some point, from
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
, to protozoans, to
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi
to
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
l or microbe, microbial colonies, to hypothetical intermediates between plants and animals. A new extant genus discovered in 2014, ''Dendrogramma'', which at the time of discovery appeared to be a basal metazoan but of unknown taxonomic placement, had been noted to have similarities with the Ediacaran fauna. It has since been found to be a siphonophore, possibly even sections of a more complex species, though this in turn has raised suspicions for a similar status for at least some Ediacaran organisms.


Origin

It took almost 4 billion years from the formation of the Earth for the Ediacaran fossils to first appear, 655 million years ago. While putative fossils are reported from , the first uncontroversial evidence for life is found , and cells with nuclei certainly existed by : The reason why it took so long for forms with an Ediacaran grade of organisation to appear is uncertain. It could be that no special explanation is required: the slow process of evolution simply required 4 billion years to accumulate the necessary adaptations. Indeed, there does seem to be a slow increase in the maximum level of complexity seen over this time, with more and more evolution of complexity, complex forms of life evolving as time progresses, with traces of earlier semi-complex life such as ''Nimbia occlusa, Nimbia'', found in the Twitya formation, and older rocks dating to in Kazakhstan, 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 redox, reduced form that would react with any free oxygen produced by photosynthesising organisms. Oxygen would not be able to build up in the atmosphere until all the iron had rusted (producing banded iron formations), and all the other reactive elements had been oxidised. Donald Canfield detected records of the first significant quantities of atmospheric oxygen just before the first Ediacaran fossils appeared – and the presence of atmospheric oxygen was soon heralded as a possible trigger for the Ediacaran adaptive radiation, radiation. 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. However, the assumptions underlying the reconstruction of atmospheric composition have attracted some criticism, with widespread anoxia having little effect on life where it occurs in the Early Cambrian and the Cretaceous. Glacial period, 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 Snowball Earth, global glaciation, suggesting that ice cover and cold oceans may have prevented the emergence of multicellular life. Potentially, complex life may have evolved before these glaciations, and been wiped out. However, the diversity of life in modern Antarctica has sparked disagreement over whether cold temperatures increase or decrease the rate of evolution. In early 2008 a team analysed the range of basic body structures ("disparity") of Ediacaran organisms from three different fossil beds: Avalon in Canada, to ; White Sea in Russia, to ; and Nama in Namibia, to , immediately before the start of the Cambrian. They found that, while the White Sea assemblage had the most species, there was no significant difference in disparity between the three groups, and concluded that before the beginning of the Avalon timespan these organisms must have gone through their own evolutionary "explosion", which may have been similar to the famous Cambrian explosion .


Preservation bias

The paucity of Ediacaran fossils after the Cambrian could simply be due to conditions that no longer favoured the fossilisation of Ediacaran organisms, which may have continued to thrive unpreserved. However, if they were common, more than the occasional specimen might be expected in exceptionally taphonomy, preserved fossil assemblages (Konservat-Lagerstatten, Lagerstätten) such as the
Burgess Shale The Burgess Shale is a fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. L ...
and Maotianshan shales, Chengjiang. There are at present no widely accepted reports of Ediacara-type organisms in the Cambrian period, though there are a few disputed reports, as well as unpublished observations of 'vendobiont' fossils from 535 Ma Orsten-type deposits in China.


Predation and grazing

It is suggested that by the Early Cambrian, organisms higher in the trophic level, 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 (marine biology), 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. However, if the interpretation of the Ediacaran age '''' 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 The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These peri ...
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 among other groups, perhaps as a response to predation, drove the Ediacaran biota from their niches. However, this argument has not successfully explained similar phenomena. For instance, the Bivalvia, bivalve molluscs' "competitive exclusion" of brachiopods was eventually deemed to be a coincidental result of two unrelated trends.


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 Rodinia, supercontinents, rising sea levels (creating shallow, "life-friendly" seas), a nutrient crisis, fluctuations in atmospheric composition, including oxygen and carbon dioxide levels, and changes in ocean chemistry (promoting biomineralisation) could all have played a part.


Assemblages

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


Avalon-type assemblage

The Avalon-type assemblage is defined at Mistaken Point (Newfoundland and Labrador), Mistaken Point in Canada, the oldest locality with a large quantity of Ediacaran fossils. 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 Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "di ...
. 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. One interpretation of the biota is as deep-sea-dwelling rangeomorphs such as ''
Charnia ''Charnia'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumsc ...

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 feeder, filter feeding – by most interpretations, the assemblage is found in water too deep for photosynthesis. The low diversity may reflect the depth of water – which would restrict speciation opportunities – or it may just be too young for a rich biota to have evolved. Opinion is currently divided between these conflicting hypotheses. An alternative explanation for the distinct composition of the Avalon-type assemblage is that it was a terrestrial assemblage of volcaniclastic coastal soils near a continental volcanic arc. This view is based on geochemical studies of the substrates of Mistaken Point fossils and associated matrix supported tuffs and volcanic bombs that could only form on land. Some of these fossils such as ''Fractofusus'' and ''Charniodiscus'' were found in strata that Retallack interprets to be red well drained paleosols of coastal plains, but others such as ''
Aspidella :Aspidella'' is also a synonym (taxonomy), synonym for the mushroom genus ''Saproamanita. ''Aspidella'' is an Ediacaran disk-shaped fossil of uncertain affinity. It is known from the single species ''A. terranovica''. Morphology ''Aspidella'' ...
'' were found in strata that Retallack interprets to be intertidal paleosols.


Ediacara-type assemblage

The Ediacara-type assemblage is named after Australia's
Ediacara Hills Ediacara Hills are a range of low hills in the northern part of the Flinders Ranges of South Australia, around north of the state capital of Adelaide city centre, Adelaide. The area has many old copper and silver mining, mines from mining act ...
, and consists of fossils preserved in facies of coastal lagoons and rivers. They are typically found in red gypsiferous and calcareous paleosols formed on loess and flood deposits in an arid cool temperate paleoclimate. Most fossils are preserved as imprints in microbial earths, but a few are preserved ''within'' sandy units. (Source of data for Timeline synthesis, p. 218. Further citations available in caption to Fig. 8.)


Nama-type assemblage

The Nama assemblage is best represented in
Namibia Namibia (, ), officially the Republic of Namibia, is a country in Southern Africa Southern Africa is the south South is one of the cardinal directions or compass points. South is the opposite of north and is perpendicular to the east a ...

Namibia
. Three-dimensional preservation is most common, with organisms preserved in sandy beds containing internal bedding. Dima Grazhdankin believes that these fossils represent burrowing organisms, while Guy Narbonne maintains they were surface dwellers. These beds are sandwiched between units comprising interbedded sandstones, siltstones and shales – with microbial mats, where present, usually containing the fossils. The environment is interpreted as sand bars formed at the mouth of a river delta, delta's distributary, distributaries. Mattress-like vendobionts (''Ernietta'', ''
Pteridinium ''Pteridinium'' is an erniettomorph found in a number of Precambrian deposits worldwide. It is a member of the Ediacaran biota. Body plan The three-lobed body is generally flat such that only two lobes are visible. Each lobe consists of a number ...
'', ''Rangea'') in these sandstones form a very different assemblage from vermiform fossils (''Cloudina'', ''Namacalathus'') of Ediacaran "wormworld" in marine Dolomite (rock), dolomite of Namibia.


Significance of assemblages

In the White Sea region of Russia, all three assemblage types have been found in close proximity. This, and the faunas' considerable temporal overlap, makes it unlikely that they represent evolutionary stages or temporally distinct community (ecology), communities. Since they are globally distributed – described on all continents except Antarctica – geographical boundaries do not appear to be a factor; the same fossils are found at all palaeolatitudes (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 seabed to inter-tidal to estuarine and back again a few times, found that a specific set of Ediacaran organisms was associated with each environment. As the Ediacaran biota represent an early stage in multicellular life's history, it is unsurprising that not all possible guild (ecology), 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. 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 The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These peri ...
allowed the number of lifestyles occupied to rise to 30.


See also

* Cambrian explosion * Large ornamented Ediacaran microfossil * List of Ediacaran genera * Origin of life *
Francevillian biota The Francevillian biota (also known as Gabon macrofossils or Gabonionta) is a group of 2.1-billion-year-old Palaeoproterozoic, macroscopic Organism, organisms known from Fossil, fossils found in Gabon in the Palaeoproterozoic Francevillian B Forma ...


Notes


References


Further reading

* Excellent further reading for the keen – includes many interesting chapters with macroevolutionary theme. * A popular science account of these fossils, with a particular focus on the Namibian fossils. * Rachel Wood (geologist), Wood, Rachel A., "The Rise of Animals: New fossils and analyses of ancient ocean chemistry reveal the surprisingly deep roots of the Cambrian explosion", ''Scientific American'', vol. 320, no. 6 (June 2019), pp. 24–31.


External links


Ediacaran biota review article
28 October 2020, ''Nature'' news feature, with nice illustrations

nbsp;– Thorough, though slightly out-of-date, description, 2010
"Database of Ediacaran Biota"
compilation up to 7/2010
"The oldest complex animal fossils"
nbsp;– Queen's University, Canada, 2007
"Ediacaran fossils of Canada"
nbsp;– Queen's University, Canada, 2007
Earth's oldest animal ecosystem held in fossils at Nilpena Station in SA outback
''ABC News (Australia), ABC News'', 5 August 2013. Accessed 6 August 2013.
Meet the fossils
ABC ''Landline (TV series), Landline'' TV program on Ediacaran fossils at Nilpena (audio + transcript). First broadcast 3 August 2013. Accessed 28 December 2018. * , 2009, radio program {{DEFAULTSORT:Ediacaran biota Ediacaran life, Ediacaran, Biota Proterozoic life