The Ediacaran (; formerly Vendian) biota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period (). These were composed of enigmatic tubular and frond-shaped, mostly sessile, organisms. Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms. The Ediacaran biota may have undergone evolutionary radiation in a proposed event called the Avalon explosion, . This was after the Earth had thawed from the Cryogenian period's extensive glaciation. This biota largely disappeared with the rapid increase in

biodiversity Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic ('' genetic variability''), species ('' species diversity''), and ecosystem ('' ecosystem diversity'') ...

known as the Cambrian explosion. Most of the currently existing body plans of

animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...

s first appeared in the fossil record of the

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

rather than the Ediacaran. For macroorganisms, the Cambrian biota appears to have almost 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

, when the characteristic communities of fossils vanished. A diverse Ediacaran community was discovered in 1995 in Sonora, Mexico, and is approximately 555 million years in age, roughly coeval with Ediacaran fossils of the Ediacara Hills in South Australia and the White Sea on the coast of

Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-ei ...

. 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 ( ...

(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 ...

s. Multiple hypotheses exist to explain the disappearance of this biota, including preservation bias, a changing environment, the advent of predators and competition from other life-forms. A sampling, reported in 2018, of late Ediacaran strata across Baltica 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 has proven challenging; it is not even established that most of them were animals, with suggestions that they were

lichens A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.algae, protists known as

foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...

or microbial colonies, or hypothetical intermediates between plants and animals. The morphology and habit of some taxa (e.g. '' Funisia dorothea'') suggest relationships to Porifera or

Cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that ...

(e.g. ''

Auroralumina ''Auroralumina'' is a genus of cnidarians from the Ediacaran of Charnwood Forest; the only species is ''Auroralumina attenboroughii''. It is the earliest known animal predator. Fossil Biology The fossil, whose name recalls the Latin for "daw ...

''). '' Kimberella'' may show a similarity to molluscs, and other organisms have been thought to possess bilateral symmetry, 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 epoch (roughly 11,700 years before present). It includes the study of foss ...

s have suggested that these represent completely extinct lineages that do not resemble any living organism. Palaeontologist Adolf Seilacher proposed a separate subkingdom level category Vendozoa (now renamed

Vendobionta Vendobionts or Vendozoans (Vendobionta) are a proposed very high-level, extinct clade of benthic organisms that made up of the majority of the organisms that were part of the Ediacaran biota. It is a hypothetical group and at the same time, it w ...

) 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'', included

cholesterol Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell memb ...

, suggesting affinities to animals, fungi, or red algae.

History

The first Ediacaran fossils discovered were the disc-shaped '' Aspidella terranovica'' in 1868. Their discoverer, Scottish geologist Alexander Murray, found them useful aids for correlating the age of rocks around Newfoundland. However, since they lay below the "Primordial Strata" of the

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. His parents were named Lamira Dow a ...

that these simple forms represented fauna was dismissed by his peers. Instead, they were interpreted as gas escape structures or inorganic concretions. 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. Its western border is the Atlantic Ocean. It shares land borders with Zambia and Angola to the north, Botswana to the east and South Africa to the south and ea ...

but the firm belief that complex life originated in the

led to them being assigned to the Cambrian Period and no link to ''Aspidella'' was made. In 1946, Reg Sprigg noticed "jellyfishes" in the Ediacara Hills of Australia's Flinders Ranges but these rocks were believed to be Early Cambrian so, while the discovery sparked some interest, little serious attention was garnered. oldest multi-cellular animal

It was not until the British discovery of the iconic '' Charnia'' that the Precambrian was seriously considered as containing life. This frond-shaped fossil was found in England's

Charnwood Forest Charnwood Forest is a hilly tract in north-western Leicestershire, England, bounded by Leicester, Loughborough and Coalville. The area is undulating, rocky and picturesque, with barren areas. It also has some extensive tracts of woodland; ...

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 or geological map is a special-purpose map made to show various geological features. Rock units or geologic strata are shown by color or symbols. Bedding planes and structural features such as faults, folds, are shown with st ...

ping of the British Geological Survey, there was no doubt these fossils sat in Precambrian rocks. Palaeontologist

Martin Glaessner Martin Fritz Glaessner Order of Australia, AM (25 December 1906 – 23 November 1989) was a geologist and paleontology, palaeontologist. Born and educated in the Austro-Hungarian Empire, he spent the majority of his life in working for geoscientif ...

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 composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicat ...

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 fires. Specifically, ''ash'' refers to all non-aqueous, non-gaseous residues that remain after something burns. In analytical chemistry, to analyse the mineral and metal content of chemical samples, ash ...

-beds at the Mistaken Point assemblage in Newfoundland changed all this as the delicate detail preserved by the fine ash allowed the description of features that were previously undiscernible. 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 secondar ...

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 and its corresponding rocks. In March 2004, the International Union of Geological Sciences ended the inconsistency by formally naming the terminal period of the Neoproterozoic after the Australian locality. The term "Ediacaran biota" and similar ("Ediacara" / "Ediacaran" / "Ediacarian" / "Vendian" and "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 mats are areas of sediment stabilised by the presence of colonies of microbes that secrete sticky fluids or otherwise bind the sediment particles. They appear to migrate upwards when covered by a thin layer of sediment but this is an illusion caused by the colony's growth; individuals do not, themselves, move. If too thick a layer of sediment is deposited before they can grow or reproduce through it, parts of the colony will die leaving behind fossils with a characteristically wrinkled ("elephant skin") and tubercular texture. 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, such as the stromatolites found in Hamelin Pool Marine Nature Reserve in Shark Bay,

Western Australia Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to t ...

, 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 or Solnhofen Limestone, 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 or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares ...

. However, it is more common to find Ediacaran fossils under sandy beds deposited by storms or high-energy bottom-scraping ocean currents known as turbidites. Soft-bodied organisms today 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 (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...

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 grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definiti ...

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, which does not necessarily reflect their biological relationships. Form classification, generally restricted to palaeontology, reflects uncertainty; the goal of sc ...

: ; "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 (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...

-reducing bacteria akin to ''

Thiomargarita ''Thiomargarita'' is a genus (family Thiotrichaceae) which includes the vacuolate sulfur bacteria species ''Thiomargarita namibiensis'', ''Candidatus Thiomargarita nelsonii'', and ''Ca. Thiomargarita joergensii''. In 2022, scientists working ...

'',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 Doushantuo-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 dat ...

'', '' Cyclomedusa'' 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 (Wade ...

'' led to the initial identification of Ediacaran fossils as

cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that ...

, 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 organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the e ...

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'' 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 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. 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. :These organisms appear to form two groups: the fractal

rangeomorph The rangeomorphs are a form taxon of frondose Ediacaran fossils that are united by a similarity to '' Rangea''. Some researchers, such as Pflug and Narbonne, suggest that a natural taxon Rangeomorpha may include all similar-looking fossils. ...

s and the simpler erniettomorphs. Including such fossils as the iconic '' Charnia'' and '' Swartpuntia'', 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 Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region ...

. However, others argue against this. ; Non-Vendobionts : Some Ediacaran organisms have more complex details preserved, which has allowed them to be interpreted as possible early forms of

living Living or The Living may refer to: Common meanings *Life, a condition that distinguishes organisms from inorganic objects and dead organisms ** Living species, one that is not extinct *Personal life, the course of an individual human's life * H ...

phyla Phyla, the plural of ''phylum'', may refer to: * Phylum, a biological taxon between Kingdom and Class * by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another Phy ...

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 ''Ikaria wariootia'' is an early example of a wormlike, bilaterian organism. Its fossils are found in rocks of the Ediacara Member of South Australia that are estimated to be between 560 and 555 million years old. A representative of the Ediac ...

'' 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 '' Kimberella'', '' 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 '' 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 fossil Small carbonaceous fossils (SCFs) are sub-millimetric organic remains of organisms preserved in sedimentary strata. This category of fossils has traditionally included robust or thick-walled entities such as plant spores, acritarchs and chitinozoa ...

s are also found in Ediacaran sediments, including the spiral-shaped ''

Cochleatina ''Cochleatina'' is an organic-walled microfossil ('Small Carbonaceous Fossil') known from the late Ediacaran period and early Cambrian Fortunian The Fortunian age marks the beginning of the Phanerozoic Eon, the Paleozoic Era, and the Cambrian P ...

'' which has been found to span the Ediacaran–Cambrian boundary. : Representatives of modern taxa existed in the Ediacaran, some of which are recognisable today.

Sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate throu ...

s, red and green

algæ Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...

s and

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

Platysolenites Platysolenites is a genus of agglutinated foraminifera known from Ediacaran and lower Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambria ...

'' are known from the latest Ediacaran of western Siberia, coexisting with ''Cloudina'' and ''

Namacalathus ''Namacalathus'' is a problematic metazoan fossil occurring in the latest Ediacaran. The first, and only described species, ''N. hermanastes'', was first described in 2000 from the Nama Group of central and southern Namibia. A U–Pb zircon age ...

''. ; 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

stolon In biology, stolons (from Latin '' stolō'', genitive ''stolōnis'' – "branch"), also known as runners, are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external s ...

ic outgrowths. A study of Brazilian Ediacaran fossils found filamentous microfossils, suggested to be eukaryotes or large 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 fossils : With the exception of some very simple 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 bilateral clade of

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'' are often found at the end of long pathways of trace fossils matching their shape; these fossils are thought to be associated with ciliary feeding but the precise method of formation of these disconnected and overlapping fossils largely remains a mystery. The potential mollusc ''Kimberella'' is associated with scratch marks, perhaps formed by a

radula The radula (, ; plural radulae or radulas) is an anatomical structure used by molluscs for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food ...

.According to
: For a more cynical perspective see
Diorama of the Ediacaran biota at the Field Museum in Chicago

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.

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 Mark A. S. McMenamin (born c. 1957) is an American paleontologist and professor of geology at Mount Holyoke College. He has contributed to the study of the Cambrian explosion and the Ediacaran biota. He is the author of several books, most re ...

claimed Ediacarans did not possess an embryonic stage, and thus could not be animals. He believed that they independently evolved a

nervous system In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes ...

and

brain A brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as Visual perception, vision. I ...

s, 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'', was an early animal.

Cnidarians

Since the most primitive eumetazoans—multi-cellular animals with tissues—are cnidarians, and the first recognized Ediacaran fossil '' Charnia'' looks very much like a sea pen, the first attempt to categorise these fossils designated them as

jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbre ...

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 ''Charniodiscus'' is an Ediacaran fossil that in life was probably a stationary filter feeder that lived anchored to a sandy sea bed. The organism had a holdfast, stalk and frond. The holdfast was bulbous shaped, and the stalk was flexible. The ...

'', 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. This leaves the Canadian polyp ''

Haootia ''Haootia quadriformis'' is an extinct animal belonging to the Ediacaran biota. Estimated to be about 560 million years old, ''H. quadriformis'' is identified as a cnidarian polyp, and represents the earliest known evidence for muscle tissue in ...

'', the British ''

'', the staurozoan ''

Corumbella ''Corumbella'' is an extinct genus of terminal-Ediacaran cnidarians. It is the only genus in the monotypic family Corumbellidae, and is represented by a single species ''Corumbella werneri''. It possessed a carapace made up of thick polygonal rin ...

'', and the early

conulariid Conulariida is a poorly understood fossil group that has possible affinity with the Cnidaria. Their exact position as a taxon of extinct medusozoan cnidarians is highly speculative. Members of the Conulariida are commonly referred to as conularii ...

'' Paraconularia ediacara'' as the only recognized cnidarian body fossils from the Ediacaran, both being sessile forms showing a quadrilateral symmetry of the body. So while this shows that Cnidarians were already well established in the Ediacaran period, the question remains if there were only sessile polyps or that a medusal stage had also developed.

Protozoans

Adolf Seilacher has suggested the Ediacaran sees animals usurping giant

s as the dominant life form. The modern

xenophyophore Xenophyophorea is a clade of foraminiferans. Members of this class are multinucleate unicellular organisms found on the ocean floor throughout the world's oceans, at depths of . They are a kind of foraminiferan that extract minerals from the ...

s are giant single-celled protozoans found throughout the world's oceans, largely on the

abyssal plain An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between and . Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth's surface ...

. A recent genetic study suggested that the xenophyophores are a specialised group of

Foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...

. 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 in diameter.

New phylum

Seilacher has suggested that the Ediacaran organisms represented a unique and extinct grouping of related forms descended from a common ancestor ( clade) and created the kingdom Vendozoa, named after the now-obsolete

Vendian The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and th ...

era. He later excluded fossils identified as

metazoa Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

ns and relaunched the phylum "Vendobionta". He described the Vendobionta as 'quilted' cnidarians lacking stinging cells. This absence precludes the current cnidarian method of feeding, so Seilacher suggested that the organisms may have survived by symbiosis with photosynthetic or

chemoautotrophic A Chemotroph is an organism that obtains energy by the oxidation of electron donors in their environments. These molecules can be organic ( chemoorganotrophs) or inorganic (chemolithotrophs). The chemotroph designation is in contrast to phototro ...

organisms.

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 has proposed that Ediacaran organisms were lichens. He argues that thin sections of Ediacaran fossils show lichen-like compartments and hypha-like wisps of ferruginized clay, and that Ediacaran fossils have been found in strata that he interprets as desert soils. The suggestion has been disputed by other scientists; some have described the evidence as ambiguous and unconvincing, for instance noting that ''Dickinsonia'' fossils have been found on rippled surfaces (suggesting a marine environment), while trace fossils like ''Radulichnus'' could not have been caused by needle ice as Retallack has proposed. Ben Waggoner notes that the suggestion would place the root of the

back from around 900 mya to between 1500 mya and 2000 mya, contradicting much other evidence. Matthew Nelsen, examining phylogenies of ascomycete fungi and

chlorophyte Chlorophyta or Prasinophyta is a taxon of green algae informally called chlorophytes. The name is used in two very different senses, so care is needed to determine the use by a particular author. In older classification systems, it refers to a ...

algae (components of lichens), calibrated for time, finds no support for the hypothesis that lichens predated the vascular plants.

Other interpretations

Several classifications have been used to accommodate the Ediacaran biota at some point, from algae, to

protozoans Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Hist ...

, to

l or 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 complex forms of life evolving as time progresses, with traces of earlier semi-complex life such as '' 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 Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

. Without a complex

circulatory system The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...

, 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 Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...

and

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

, existed in a reduced form that would react with any free oxygen produced by

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

ing 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 Donald Eugene Canfield (born 1957) is a geochemist and Professor of Ecology at the University of Southern Denmark known for his work on the evolution of Earth's atmosphere and oceans. The Canfield ocean, a sulfidic partially oxic ocean existing ...

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 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. Periods of intense cold have also been suggested as a barrier to the evolution of multicellular life. The earliest known embryos, from China's Doushantuo Formation, appear just a million years after the Earth emerged from a global glaciation, suggesting that ice cover and cold oceans may have prevented the emergence of multicellular life. 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 preserved fossil assemblages (Konservat- Lagerstätten) such as the Burgess Shale and Chengjiang. Although no reports of Ediacara-type organisms in the Cambrian period are widely accepted at present, a few disputed reports have been made, 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 food chain caused the microbial mats to largely disappear. If these grazers first appeared as the Ediacaran biota started to decline, then it may suggest that they destabilised the microbial substrate, leading to displacement or detachment of the biota; or that the destruction of the mat destabilised the ecosystem, causing extinctions. Alternatively, skeletonised animals could have fed directly on the relatively undefended Ediacaran biota. 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". Little evidence has been identified for any trace fossils in the Ediacaran Period, which could speak against the active grazing theory. Further, the onset of the

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 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 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 depositional conditions, and are commonly grouped into three main types, known as 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 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

. 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

s such as '' Charnia'', all of which share a fractal growth pattern. They were probably preserved ''in situ'' (without post-mortem transportation), although this point is not universally accepted. The assemblage, while less diverse than the Ediacara- or Nama-types, resembles Carboniferous suspension-feeding communities, which may suggest filter feeding – 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.

Ediacara-type assemblage

The Ediacara-type assemblage is named after Australia's Ediacara Hills, and consists of fossils preserved in facies of coastal lagoons and rivers. They are typically found in red gypsiferous and calcareous paleosols formed on loess and flood deposits in an arid cool temperate paleoclimate. 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

. 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 delta's distributaries. Mattress-like vendobionts ('' Ernietta'', '' Pteridinium'', '' Rangea'') in these sandstones form a very different assemblage from vermiform fossils ('' Cloudina'', ''

'') of Ediacaran "wormworld" in marine

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

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 communities. Since they are globally distributed – described on all continents except

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...

– geographical boundaries do not appear to be a factor; the same fossils are found at all

palaeolatitude Paleomagnetism (or palaeomagnetismsee ), is the study of magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.'' Certain magnetic minerals in rocks ...

s (the latitude where the fossil was created, accounting for continental drift) and in separate

sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsiden ...

s. 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 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

allowed the number of lifestyles occupied to rise to 30.

Notes

References

External links

* — Ediacaran biota review article, with nice illustrations * — Thorough, though slightly out-of-date, description * {{cite web , title=Database of Ediacaran biota , date=July 2010 , website=complex-life.org , url=http://www.complex-life.org/database , url-status=dead , archive-url=https://web.archive.org/web/20110725191154/http://www.complex-life.org/database — compilation up to 7/2010 * {{cite web , title=The oldest complex animal fossils , year=2007 , publisher= Queen's University , place=Kingston, Ontario , url=http://geol.queensu.ca/museum/exhibits/ediac/drook , url-status=dead , archive-url=https://web.archive.org/web/20070528175308/http://geol.queensu.ca/museum/exhibits/ediac/drook/ , archive-date=2007-05-28 * {{cite web , title=Ediacaran fossils of Canada , year=2007 , publisher= Queen's University , place=Kingston, Ontario , url=http://geol.queensu.ca/museum/exhibits/ediac/ediac.html , url-status=dead , archive-url=https://web.archive.org/web/20070405091608/http://geol.queensu.ca/museum/exhibits/ediac/ediac.html , archive-date=2007-04-05 * {{cite news , title=Earth's oldest animal ecosystem held in fossils at Nilpena Station in SA outback , date=5 August 2013 , publisher=

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, url=http://www.abc.net.au/news/2013-08-03/sa-pastoral-property-nilpena-holds-animal-fossils/4862432 , access-date=6 August 2013 * {{cite AV media , title=Meet the fossils , series=''

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, url=http://www.abc.net.au/tv/programs/landline/old-site/content/2013/s3817544.htm , access-date=28 December 2018 — ABC TV show on Ediacaran fossils at Nilpena {{Spoken Wikipedia, Ediacara biota.ogg, date=2009-08-29 * {{In Our Time, Ediacara Biota, b00lh2s3, Ediacara_Biota, 2009, radio program {{DEFAULTSORT:Ediacaran biota Biota Proterozoic life