Ediacara biota
Encyclopedia
Sessility (zoology)
In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
organisms which lived during the Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
Period (ca. 635-542 Ma). Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organism
Multicellular organism
Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. Most life that can be seen with the the naked eye is multicellular, as are all animals and land plants.-Evolutionary history:Multicellularity has evolved independently dozens of times...
s.Simple multicellular organisms such as red algae evolved at least . The Ediacara biota radiated in an event called the Avalon Explosion, , after the Earth had thawed from the Cryogenian
Cryogenian
The Cryogenian is a geologic period that lasted from . It forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran...
period's extensive glaciation
Snowball Earth
The Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 Ma . Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical...
, and largely disappeared contemporaneous with the rapid appearance of biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
known as the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
. Most currently existing body-plans of animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s first appeared only in the fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
record of the Cambrian rather than the Ediacaran. For macroorganisms, the Cambrian biota completely replaced the organisms that populated the Ediacaran fossil record.
The organisms of the Ediacaran Period first appeared around and flourished until the cusp of the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
when the characteristic communities of fossils vanished. The earliest reasonably diverse Ediacaran community was discovered in 1995 in Sonora, México, and is approximately 585 million years in age, roughly synchronous with the Gaskiers glaciation. While rare fossils that may represent survivors have been found as late as the Middle Cambrian (510 to 500 million years ago) the earlier fossil communities disappear from the record at the end of the Ediacaran leaving only curious fragments of once-thriving ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
s. Multiple hypotheses
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
exist to explain the disappearance of this biota, including preservation bias
Biased sample
In statistics, sampling bias is when a sample is collected in such a way that some members of the intended population are less likely to be included than others. It results in a biased sample, a non-random sample of a population in which all individuals, or instances, were not equally likely to...
, a changing environment, the advent of predators
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
and competition from other life-forms.
Determining where Ediacaran organisms fit in the tree of life has proven challenging; it is not even established that they were animals, with suggestions that they were lichens (animal-plant symbionts), algae, protists known as foraminifera, fungi or microbial colonies, to hypothetical intermediates between plants and animals. The morphology and habit of some taxa (e.g. Funisia dorothea) suggest relationships to Porifera or Cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
. Kimberella
Kimberella
Kimberella is a monospecific genus of bilaterian known only from rocks of the Ediacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the molluscs, although its affinity with this group is contentious.Specimens were first found in...
may show a similarity to molluscs, and other organisms have been thought to possess bilateral symmetry, though this is controversial. Most macroscopic fossil
Macrofossil
Macrofossils are preserved organic remains large enough to be visible without a microscope. Most fossils discussed in the article Fossil are macrofossils.-Macrofossil contrasted with Microfossil:...
s are morphologically
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
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 paleontologists have suggested that these represent completely extinct lineages that do not resemble any living organism. One paleontologist proposed a separate kingdom 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 independently evolving from unrelated single-celled organisms.
History
The first Ediacaran fossils discovered were the disc-shaped AspidellaAspidella
Aspidella is an Ediacaran disk-shaped fossil.- Morphology :Aspidella consists of disk-shaped fossils, with concentric rings and/or centripetal rays. The diameter of circular Aspidella varies from 1 to 180 mm. Most individuals are between 4 and 10 mm, however; smaller animals would...
terranovica in 1868. Their discoverer, A. Murray
Alexander Murray (geologist)
Alexander Murray, CMG was a Scottish geologist.Murray was born in Crieff, Perthshire, Scotland. He worked as a geologist in the United Kingdom and Canada, before coming to Newfoundland in 1864 to become the first director of the Geological Survey of Newfoundland...
a geological surveyor, found them useful aids for correlating the age of rocks around Newfoundland. However since they lay below the "Primordial Strata", the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
strata that were then thought to contain the very first signs of life, it took four years for anybody to dare propose they could be fossils. Elkanah Billings
Elkanah Billings
Elkanah Billings is often referred to as Canada's first paleontologist. Billings was born on a farm by the Rideau River outside Bytown, now known as Billings Estate. His parents were named Lamira and Braddish Billings. His family included an older sister named Sabra and an older brother Braddish II...
' proposal was dismissed by his peers on account of their simple form and they were instead declared gas escape structures, inorganic concretions, or even tricks played by a malicious God to promote unbelief. No similar structures elsewhere in the world were then known and the one-sided debate soon fell into obscurity. In 1933, Georg Gürich
Georg Gürich
Georg Julius Ernst Gürich was a German geologist, paleontologist and university teacher, who wrote on Paleozoic geological formations in Poland and ranged through Guinea, Tanzania and Southern Africa , in search of unrecorded new species.Georg Gürich studied geology in...
discovered specimens in Namibia
Namibia
Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...
but the firm belief that life originated in the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
led to them being assigned to the Cambrian Period and no link to Aspidella was made. In 1946 Reg Sprigg
Reg Sprigg
Reginald Claude Sprigg, AO, HonDSc ANU, HonDSc Flinders, MSc Adelaide, FTSE was an Australian geologist and conservationist. At age 17 he became the youngest Fellow of the Royal Society of South Australia. In 1946, in the Ediacara Hills, South Australia he discovered the Ediacara biota, an...
noticed "jellyfishes" in the Ediacara Hills
Ediacara Hills
Ediacara Hills are a range of low hills in the northern part of the Flinders Ranges of South Australia, around 650 km north of Adelaide. The area has many old copper and silver mines from mining activity in the late 19th century...
of Australia's Flinders Ranges
Flinders Ranges
Flinders Ranges is the largest mountain range in South Australia, which starts approximately north west of Adelaide. The discontinuous ranges stretch for over from Port Pirie to Lake Callabonna...
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
Charnia is the genus name given to a frond-like Ediacaran lifeform with segmented ridges branching alternately to the right and left from a zig-zag medial suture. The genus Charnia was named after Charnwood Forest in Leicestershire, England, where the first fossilised specimen was found.- Diversity...
in 1957 that the pre-Cambrian was seriously considered as containing life. This frond
Frond
The term frond refers to a large, divided leaf. In both common usage and botanical nomenclature, the leaves of ferns are referred to as fronds and some botanists restrict the term to this group...
-shaped fossil was found in England's Charnwood Forest
Charnwood Forest
Charnwood Forest is an upland 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; its elevation is generally 600 ft and upwards, the area...
, and due to the detailed geologic map
Geologic map
A geologic map or geological map is a special-purpose map made to show geological features. Rock units or geologic strata are shown by color or symbols to indicate where they are exposed at the surface...
ping of the British Geological Survey
British Geological Survey
The British Geological Survey is a partly publicly funded body which aims to advance geoscientific knowledge of the United Kingdom landmass and its continental shelf by means of systematic surveying, monitoring and research. The BGS headquarters are in Keyworth, Nottinghamshire, but other centres...
there was no doubt that these fossils sat in Precambrian rocks. Palæontologist Martin Glaessner
Martin Glaessner
Martin Fritz Glaessner AM was a geologist and palaeontologist. Born and educated in Austro-Hungarian Empire, he spent the majority of his life in working for oil companies in Russia, and studying the geology of the South Pacific in Australia...
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
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
that prevented preservation of fine details making interpretation difficult. S.B. Misra's discovery of fossiliferous ash
Volcanic ash
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...
-beds at the Mistaken Point assemblage in Newfoundland changed all this as the delicate detail preserved by the fine ash allowed the description of features that were previously invisible.
Poor communication, combined with the difficulty in correlating globally distinct formations, led to a plethora of different names for the biota.
In 1960 the French name "Ediacarien" — after the Ediacaran Hills in South Australia
South Australia
South Australia is a state of Australia in the southern central part of the country. It covers some of the most arid parts of the continent; with a total land area of , it is the fourth largest of Australia's six states and two territories.South Australia shares borders with all of the mainland...
, which take their name from aborigine Idiyakra, "water is present" — 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 geologic time
Geologic time scale
The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth...
and its corresponding rocks. In March 2004, the International Union of Geological Sciences
International Union of Geological Sciences
The International Union of Geological Sciences is an international non-governmental organization devoted to international cooperation in the field of geology.-About:...
ended the inconsistency by formally naming the terminal period of the Neoproterozoic
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...
after the Australian locality.
Preservation
All but the smallest fraction of the fossil record consists of the robust skeletalSkeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
matter of decayed corpses. Hence, since Ediacaran biota had soft bodies and no skeletons, their abundant preservation is surprising. The absence of burrowing creatures living in the sediments undoubtedly helped; since after the evolution of these organisms in the Cambrian, soft-bodied impressions were usually disturbed
Bioturbation
In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...
before they could fossilize.
Microbial mats
Microbial matMicrobial mat
A microbial mat is a multi-layered sheet of micro-organisms, mainly bacteria and archaea. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces but a few survive in deserts. They colonize environments ranging in temperature from –40°C to +120°C...
s are areas of sediment stabilised by the presence of colonies of microbes which secrete sticky fluids or otherwise bind the sediment particles. They appear to migrate upwards when covered by a thin layer of sediment but this is an illusion caused by the colony's growth; individuals do not, themselves, move. If too thick a layer of sediment is deposited before they can grow or reproduce through it parts of the colony will die leaving behind fossils with a characteristically wrinkled ("elephant skin") and tubercular texture.
Hamelin Pool Marine Nature Reserve
Hamelin Pool Marine Nature Reserve is located in the Shark Bay World Heritage Site of Western Australia adjacent to the historic Hamelin Pool Telegraph Station about west of the Overlander Roadhouse on the North West Coastal Highway. Access is via Hamelin Pool Rd and then through the Hamelin Pool...
in Shark Bay
Shark Bay
Shark Bay is a World Heritage listed bay in Western Australia. The term may also refer to:* the locality of Shark Bay, now known as Denham* Shark Bay Marine Park* Shark Bay , a shark exhibit at Sea World, Gold Coast, Australia* Shire of Shark Bay...
, Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
where the salt levels can be as high as 1.5 or 2 times the normal level of the surrounding sea.
Fossilisation
The preservation of these fossils is one of their great fascinations to science. As soft-bodied organisms they would normally not fossilise and, unlike later soft-bodied fossil biota such as the Burgess ShaleBurgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
or Solnhofen Limestone
Solnhofen limestone
The Solnhofen Plattenkalk is a Jurassic Konservat-Lagerstätte that preserves a rare assemblage of fossilized organisms, including highly detailed imprints of soft bodied organisms such as sea jellies...
, the Ediacara biota is not found in a restricted environment subject to unusual local conditions: they were a global phenomenon. The processes that were operating must have been systemic and worldwide. There was something very different about the Ediacaran Period that permitted these delicate creatures to be left behind and it is thought that the fossils were preserved by virtue of rapid covering by ash or sand, trapping them against the mud or microbial mats on which they lived. Ash beds provide more detail and can readily be precisely dated to the nearest million years or better by means of radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
.
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
Turbidite
Turbidite geological formations have their origins in turbidity current deposits, which are deposits from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean.-The ideal turbidite sequence:...
s. Soft-bodied organisms today almost never fossilise during such events but the presence of widespread microbial mats probably aided preservation by stabilising their impressions in the sediment below.
What is preserved?
The rate of cementation of the overlying substrate relative to the rate of decomposition of the organism determines whether the top or bottom surface of an organism is preserved. Most disc-shaped fossils decomposed before the overlying sediment was cemented, the ash or sand then slumped in to fill the void, leaving a cast of the underside of the organism.Conversely, quilted fossils tend to decompose after the cementation of the overlying sediment; hence their upper surfaces are preserved. Their more resistant nature is reflected in the fact that in rare occasions quilted fossils are found within storm beds as the high-energy sedimentation did not destroy them as it would have the less-resistant discs. Further, in some cases, the bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
l precipitation
Precipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
of minerals formed a "death mask" creating a mould of the organism.
Morphology
| Forms of Ediacaran fossil | |
|---|---|
| The earliest discovered potential embryo, preserved within an acanthomorphic acritarch Acritarch Acritarchs are small organic fossils, present from approximately to the present. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.-Definition:In general, any small, non-acid soluble Acritarchs are small organic fossils, present from... . The term 'acritarch' describes a range of unclassified cell-like fossils. |
 |
| Tateana inflata (= Cyclomedusa Cyclomedusa Cyclomedusa is a circular fossil of the Ediacaran biota; it has a circular bump in the middle and as many as five circular growth ridges around it. Many specimens are small, but specimens in excess of 20cm are known. The concentric disks are not necessarily circular, especially when adjacent... ' radiata) is the attachment disk of an unknown organism. Metric scale. |
|
| A cast of the quilted Charnia, the first accepted complex Precambrian organism. Charnia was once interpreted as a relative of the sea pens. | |
| Spriggina Spriggina Fossils of Spriggina are known from the Ediacaran period, around . The segmented organism reached about 3 cm in length and may have been predatory... , a possible precursor to the trilobite Trilobite Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before... s, may be one of the predators that led to the demise of the Ediacaran fauna and subsequent diversification Cambrian explosion The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes... of animals. |
 |
| A late Ediacaran trace fossil preserved on a bedding plane. |  |
| A chain of trace fossils created by a grazing Yorgia Yorgia Yorgia waggoneri is a member of the Ediacara biota, and resembles a cross between the organisms Dickinsonia and Spriggina. It has a low, segmented body consisting of a short wide "head", no appendages, and a long body region, reaching a maximum length of... , terminating with a body fossil of the organism itself (right). |
 |
The Ediacaran biota exhibited a vast range of morphological
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
characteristics. Size ranged from millimetres to metres; complexity from "blob-like" to intricate; rigidity from sturdy and resistant to jelly-soft. Almost all forms of symmetry were present. These organisms differed from earlier fossils by displaying an organised, differentiated multicellular construction and centimetre-plus sizes.
These disparate morphologies can be broadly grouped into form taxa:
"Embryos" : Recent discoveries of Precambrian multicellular life have been dominated by reports of embryos, particularly from the Doushantuo Formation
Doushantuo Formation
The Doushantuo Formation is a Lagerstätte in Guizhou Province, China that is notable for being one of the oldest fossil beds to contain highly preserved fossils...
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
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
-reducing bacteria akin to Thiomargarita, a view which is highly contested yet gradually gaining supporters.
- 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 animals of this period.
Discs : Circular fossils, such as Ediacaria
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 from the Baikalian age of the Upper Riphean to 501 million...
, Cyclomedusa
Cyclomedusa
Cyclomedusa is a circular fossil of the Ediacaran biota; it has a circular bump in the middle and as many as five circular growth ridges around it. Many specimens are small, but specimens in excess of 20cm are known. The concentric disks are not necessarily circular, especially when adjacent...
and Rugoconites
Rugoconites
A member of the Ediacaran biota which takes the form of a circular to oval impression preserved in hyporelief, six or more centimeters in diameter. They are surrounded by a frill that has been interpreted as a set of tentacles...
led to the initial identification of Ediacaran fossils as cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
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 holdfast
Holdfast
A holdfast is a root-like structure that anchors aquatic sessile organisms, such as seaweed, other sessile algae, stalked crinoids, benthic cnidarians, and sponges, to the substrate. ...
s and protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
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 characters are often lacking because only the underside of the organism is preserved by fossilization.
Bags : Fossils such as Pteridinium
Pteridinium
Pteridinium is an erniettomorph found in a number of Precambrian deposits worldwide. It is a member of the Ediacaran biota.-Body plan:It has a three-lobed body which is generally smashed flat such that only two lobes are visible. Each lobe consists of a number of parallel ribs extending back to the...
preserved within sediment layers resemble "mud-filled bags". The scientific community is a long way from reaching a consensus on their interpretation.
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
Mattress
A mattress is a manufactured product to sleep or lie on, consisting of resilient materials and covered with an outer fabric or ticking. In the developed world it is typically part of a bed set and is placed upon a foundation....
. 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
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. Swartpuntia specimens range in length from 12 to 19 cm, and...
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 fractalFractalA fractal has been defined as "a rough or fragmented geometric shape that can be split into parts, each of which is a reduced-size copy of the whole," a property called self-similarity...
rangeomorphRangeomorphThe Rangeomorphs are a form taxon of frondose Ediacaran fossils that are united by a similarity to Rangea. Some workers, e.g. 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 CharniaCharniaCharnia is the genus name given to a frond-like Ediacaran lifeform with segmented ridges branching alternately to the right and left from a zig-zag medial suture. The genus Charnia was named after Charnwood Forest in Leicestershire, England, where the first fossilised specimen was found.- Diversity...
and SwartpuntiaSwartpuntiaSwartpuntia 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. Swartpuntia specimens range in length from 12 to 19 cm, and...
, the group is both the most iconic of the Ediacaran biota and the most difficult to place within the existing tree of lifeTree of lifeThe concept of a tree of life, a many-branched tree illustrating the idea that all life on earth is related, has been used in science , religion, philosophy, mythology, and other areas...
. 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 osmosisOsmosisOsmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...
.
Non-Ediacaran Ediacarans : Some Ediacaran organisms have more complex details preserved which has allowed them to be interpreted as possible early forms of living phyla
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
excluding them from some definitions of the Ediacaran biota.
- The earliest such fossil is the reputed bilaterian VernanimalculaVernanimalculaVernanimalcula guizhouena is a fossil believed by some to represent the earliest known member of the Bilateria . It is known from deposits dating to . The fossils are between 0.1 and 0.2 mm across...
claimed by some, however, to represent the infilling of an egg-sac or acritarchAcritarchAcritarchs are small organic fossils, present from approximately to the present. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.-Definition:In general, any small, non-acid soluble Acritarchs are small organic fossils, present from...
. Later examples are almost universally accepted as bilaterians and include the mollusc-like KimberellaKimberellaKimberella is a monospecific genus of bilaterian known only from rocks of the Ediacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the molluscs, although its affinity with this group is contentious.Specimens were first found in...
, SprigginaSprigginaFossils of Spriggina are known from the Ediacaran period, around . The segmented organism reached about 3 cm in length and may have been predatory...
(pictured) and the shield-shaped ParvancorinaParvancorinaParvancorina is a genus of shield-shaped Ediacaran fossils. It has a raised ridge down the central axis of symmetry. This ridge can be high in unflattened fossils. At the 'head' end of the ridge there are two quarter circle shaped raised arcs attached. In front of this are two nested...
whose affinities are currently debated. - A suite of fossils known as the Small shelly fossils are represented in the Ediacaran, most famously by CloudinaCloudinidThe Cloudinids, an early metazoan family containing the genus Cloudina, lived in the late Ediacaran period and became extinct at the base of the Cambrian. They formed millimetre-scale conical fossils consisting of calcareous cones nested within one another; the appearance of the organism itself...
a shelly tube-like fossil that often shows evidence of predatory boring, suggesting that whilst predation may not have been common in the Ediacaran Period it was at least present. - Representatives of modern taxa existed in the Ediacaran, some of which are recognisable today. Sponges, red and green algæ, protistProtistProtists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
s and bacteriaBacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
are all easily recognisable with some pre-dating the Ediacaran by thousands of millions of years . Possible arthropods have also been described.
Trace fossil
Trace fossil
Trace fossils, also called ichnofossils , are geological records of biological activity. Trace fossils may be impressions made on the substrate by an organism: for example, burrows, borings , urolites , footprints and feeding marks, and root cavities...
s : With the exception of some very simple vertical burrows
Skolithos
Skolithos is a common trace fossil ichnogenus whose original form consisted of approximately vertical cylinders. One well-known occurrence of Cambrian trace fossils is the famous 'Pipe Rock' of northwest Scotland...
the only Ediacaran burrows are horizontal lying on or just below the surface. 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
Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
of animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
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 which 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. YorgiaYorgiaYorgia waggoneri is a member of the Ediacara biota, and resembles a cross between the organisms Dickinsonia and Spriggina. It has a low, segmented body consisting of a short wide "head", no appendages, and a long body region, reaching a maximum length of...
and DickinsoniaDickinsoniaDickinsonia is an iconic fossil of the Ediacaran biota. It resembles a bilaterally symmetrical ribbed oval. Its affinities are presently unknown; most interpretations consider it to be an animal, although others suggest it may be fungal, or a member of an "extinct kingdom".-Species variety:A...
are often found at the end of long pathwayFossil trackwayA fossil trackway is a type of trace fossil, a trackway made by an organism. Many fossil trackways were made by dinosaurs, early tetrapods, and other quadrupeds and bipeds on land...
s of trace fossils matching their shape; these fossils are thought to be associated with ciliaryCiliumA cilium is an organelle found in eukaryotic cells. Cilia are slender protuberances that project from the much larger cell body....
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 radulaRadulaThe radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...
.
- Biologist Mikhail Matz from The University of Texas at Austin and his colleagues recently discovered grape-sized protists, single-celled organisms, which are able to make tracks resembling those of multi-cellular animals. Although these organisms are slow-moving their movement is persistent. This suggests that some of the trace fossils of the Ediacara may have been from protists making it difficult to find distinctions between single- and multi-cellular life in the early fossil record.
Classification and interpretation
Classification of the Ediacarans is difficult, and hence a variety of theories exist as to their placement on the tree of life.Cnidarians
Since the most primitive eumetazoans — multi-cellular animals with tissues — are cnidarians, the first attempt to categorise these fossils designated them as jellyfishJellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
and Sea pen
Sea pen
Sea pens are colonial marine cnidarians belonging to the order Pennatulacea. There are 14 families within the order; they are thought to have a cosmopolitan distribution in tropical and temperate waters worldwide...
s. 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.
"The dawn of animal life"
Martin Glaessner
Martin Fritz Glaessner AM was a geologist and palaeontologist. Born and educated in Austro-Hungarian Empire, he spent the majority of his life in working for oil companies in Russia, and studying the geology of the South Pacific in Australia...
proposed in The Dawn of Animal Life (1984) that the Ediacara biota were recognisable crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
members of modern phyla, but were unfamiliar because they had yet to evolve the characteristic features we use in modern classification.
Adolf Seilacher
Adolf Seilacher
Adolf "Dolf" Seilacher is a German palaeontologist who has made major contributions to evolutionary and ecological palaeobiology in a career stretching over 60 years. He won the Crafoord Prize in 1992, the Paleontological Society Medal in 1994 and the Palaeontological Association's Lapworth Medal...
responded by suggesting that the Ediacaran sees animals usurping giant protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
s as the dominant life form. The modern xenophyophore
Xenophyophore
Xenophyophores are marine protozoa, giant single-celled organisms found throughout the world's oceans, at depths of up to 10,641 meters . Xenophyophores are found in the greatest numbers on the abyssal plains of the deep ocean. They were first described as sponges in 1889, then as testate...
s are giant single-celled protozoans found throughout the world's oceans, largely on the abyssal plain
Abyssal plain
An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between 3000 and 6000 metres. 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. They are among the flattest, smoothest...
. A recent genetic study suggested that the xenophyophores are a specialized group of Foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...
. There are approximately 42 recognized species in 13 genera and 2 orders; one of which, Syringammina fragilissima
Syringammina fragilissima
Syringammina fragilissima is a xenophyophore found off the coast of Scotland, near Rockall. It is the largest single-celled organism known, at up to across...
, is among the largest known protozoans at up to 20 centimeters in diameter.
In 1998 Mark McMenamin
Mark McMenamin
Mark McMenamin is a tenured professor of geology at Mount Holyoke College. His research is primarily focused on paleontology, particularly the Ediacaran biota....
claimed that Ediacarans did not possess an embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
nic stage, and thus could not be animals. He believed that they independently evolved a nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
and brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
s, meaning that "the path toward intelligent life was embarked upon more than once on this planet", though this idea has not been widely accepted.
New phylum
Seilacher most famously suggested that the Ediacaran organisms represented a unique and extinct grouping of related forms descended from a common ancestor (cladeClade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
) and created the kingdom
Kingdom (biology)
In biology, kingdom is a taxonomic rank, which is either the highest rank or in the more recent three-domain system, the rank below domain. Kingdoms are divided into smaller groups called phyla or divisions in botany...
Vendozoa, named after the now-obsolete Vendian era. He later excluded fossils identified as metazoans and relaunched the phylum "Vendobionta".
He described the Vendobionta as quilted cnidarians lacking stinging cell
Cnidocyte
A cnidocyte, cnidoblast, or nematocyte is a type of venomous cell unique to the phylum Cnidaria . The cnidocyte cell provides a means for them to catch prey and defend themselves from predators. Despite being morphologically simple, lacking a skeleton and usually being sessile, cnidarians prey on...
s. This absence precludes the current cnidarian method of feeding, so Seilacher suggested that the organisms may have survived by symbiosis
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
with photosynthetic
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
or chemoautotrophic organisms. Mark McMenamin
Mark McMenamin
Mark McMenamin is a tenured professor of geology at Mount Holyoke College. His research is primarily focused on paleontology, particularly the Ediacaran biota....
saw such feeding strategies as characteristic for the entire biota, and referred to the marine biota of this period as a "Garden of Ediacara".
Lichens
Gregory Retallack's hypothesis that Ediacaran organisms were lichenLichen
Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...
s has failed to gain widespread acceptance. He argues that the fossils are not as squashed as jellyfish fossilised in similar situations, and their relief is closer to petrified wood
Petrified wood
Petrified wood is the name given to a special type of fossilized remains of terrestrial vegetation. It is the result of a tree having turned completely into stone by the process of permineralization...
. He points out the chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
ous walls of lichen colonies would provide a similar resistance to compaction, and claims the large size of the organisms — sometimes over a metre across, far larger than any of the preserved burrows — also hints against a classification with the animals.
Other interpretations
Almost every possible phylum has been used to accommodate the Ediacaran biota at some point, from algaeAlgae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
, to protists known as foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...
, to fungi to bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
l or microbial colonies, to hypothetical intermediates between plants and animals.
Origin
It took almost 4 billion years from the formation of the Earth for the Ediacaran fossils to first appear, 655 million years ago. Whilst putative fossils are reported from , the first uncontroversial evidence for life is found , and cells with nuclei certainly existed by : Why did it take so long for forms with an Ediacaran grade of organisation to appear?It could be that no special explanation is required: the slow process of evolution simply required 4 billion years to accumulate the necessary adaptations. Indeed, there does seem to be a slow increase in the maximum level of complexity seen over this time, with more and more complex forms of life evolving
Evolution of complexity
The evolution of biological complexity is an important outcome of the process of evolution. Evolution has produced some remarkably complex organisms - although the actual level of complexity is very hard to define or measure accurately in biology, with properties such as gene content, the number of...
as time progresses, with traces of earlier semi-complex life such as Nimbia
Nimbia
Nimbia occlusa is a form of Ediacaran fossil shaped like a circular or oval disk, with a thick rim around the margin. Within the rim the fossil is usually flat, but may have a central nipple or dimple...
, found in the Twitya formation, (and possibly older rocks dating to ) possibly displaying the most complex morphology of the time.
A primary size-limiting factor is the amount of atmospheric oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. Without a complex circulatory system
Circulatory system
The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...
, 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
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
existed in a reduced
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
form; these would react with any free oxygen produced by photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
ing organisms. Oxygen would not be able to build up in the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
until all the iron had rusted (producing banded iron formation
Banded iron formation
Banded iron formations are distinctive units of sedimentary rock that are almost always of Precambrian age. A typical BIF consists of repeated, thin layers of iron oxides, either magnetite or hematite , alternating with bands of iron-poor shale and chert...
s), and other reactive elements had also been oxidised.
Donald Canfield
Donald Canfield
Donald Canfield is a geologist born in 1958, most famous for his work on ancient ocean chemistry. The Canfield ocean, a sulfidic partially oxic ocean existing between the Archean and Ediacaran periods, takes its name from his seminal paper....
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
Adaptive radiation
In evolutionary biology, adaptive radiation is the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage. Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different...
. 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
Glacial period
A glacial period is an interval of time within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate within an ice age...
have also been suggested as a barrier to the evolution of multicellular life.
The earliest known embryos, from China's Doushantuo Formation
Doushantuo Formation
The Doushantuo Formation is a Lagerstätte in Guizhou Province, China that is notable for being one of the oldest fossil beds to contain highly preserved fossils...
, appear just a million years after the Earth emerged from a global glaciation
Snowball Earth
The Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 Ma . Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical...
, 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
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
.
Disappearance
The low resolution of the fossil record means that the disappearance of the Ediacarans remains something of a mystery. There appears to have been a relatively abrupt disappearance at the end of the Ediacaran period; reports of Cambrian "Ediacarans" like ThaumaptilonThaumaptilon
Thaumaptilon is a fossil from the middle Cambrian Burgess Shale which some authors have compared to members of the Ediacaran biota, generally believed to have disappeared at the start of the Cambrian,...
are not universally accepted. The cause — and reality — of this disappearance is open to debate.
Preservation bias
The sudden vanishing of Ediacaran fossils at the Cambrian boundary could simply be because conditions no longer favoured the fossilisation of Ediacaran organisms, which may have continued to thrive unpreserved.However, if they were common, more than the occasional specimen might be expected in exceptionally preserved fossil assemblages (Konservat-Lagerstätten) such as the Burgess Shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
and Chengjiang
Maotianshan shales
The Maotianshan Shales are a series of lower Cambrian deposits in the Chiungchussu formation, famous for their Konservat Lagerstätten, or high number of fossils preserved in place...
— unless such assemblages represent an environment never occupied by the Ediacaran biota, or unsuitable conditions for their preservation.
Predation and grazing
It is suggested that by the Early Cambrian, organisms higher in the food chainTrophic level
The trophic level of an organism is the position it occupies in a food chain. The word trophic derives from the Greek τροφή referring to food or feeding. A food chain represents a succession of organisms that eat another organism and are, in turn, eaten themselves. The number of steps an organism...
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
Substrate (marine biology)
Stream substrate is the material that rests at the bottom of a stream. There are several classification guides. One is:*Mud – silt and clay.*Sand – Particles between 0.06 and 2 mm in diameter.*Granule – Between 2 and 4 mm in diameter....
, 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
Kimberella
Kimberella is a monospecific genus of bilaterian known only from rocks of the Ediacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the molluscs, although its affinity with this group is contentious.Specimens were first found in...
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
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
Period is defined by the appearance of a worldwide trace fossil assemblage, quite distinct from the activity-barren Ediacaran Period.
Competition
It is possible that increased competition due to the evolution of key innovations amongst other groups, perhaps as a response to predation, drove the Ediacaran biota from their niches.However, this argument has not successfully explained similar phenomena. For instance, the bivalve molluscs'
Bivalvia
Bivalvia is a taxonomic class of marine and freshwater molluscs. This class includes clams, oysters, mussels, scallops, and many other families of molluscs that have two hinged shells...
"competitive exclusion" of brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s 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 supercontinentRodinia
In geology, Rodinia is the name of a supercontinent, a continent which contained most or all of Earth's landmass. According to plate tectonic reconstructions, Rodinia existed between 1.1 billion and 750 million years ago, in the Neoproterozoic era...
s, 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
Biomineralisation
Biomineralization is the process by which living organisms produce minerals, often to harden or stiffen existing tissues. Such tissues are called mineralized tissues. It is an extremely widespread phenomenon; all six taxonomic kingdoms contain members that are able to form minerals, and over 60...
) could all have played a part.
Assemblages
Ediacaran-type fossils are recognised globally in 25 localities and a variety of depositionalDeposition (geology)
Deposition is the geological process by which material is added to a landform or land mass. Fluids such as wind and water, as well as sediment flowing via gravity, transport previously eroded sediment, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of...
conditions, and are commonly grouped into three main types, named after typical localities. Each assemblage tends to occupy its own region of morphospace, and after an initial burst of diversification changes little for the rest of its existence.
Avalon-type assemblage
The Avalon-type assemblage is defined at Mistaken Point in Canada, the oldest locality with a large quantity of Ediacaran fossils.The assemblage is easily dated because it contains many fine ash-beds, which are a good source of zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
s used in the uranium-lead method of radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
. 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.
The biota comprises deep sea dwelling rangeomorph
Rangeomorph
The Rangeomorphs are a form taxon of frondose Ediacaran fossils that are united by a similarity to Rangea. Some workers, e.g. Pflug and Narbonne, suggest that a natural taxon Rangeomorpha may include all similar-looking fossils....
s
such as Charnia, all of which share a fractal
Fractal
A fractal has been defined as "a rough or fragmented geometric shape that can be split into parts, each of which is a reduced-size copy of the whole," a property called self-similarity...
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
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
suspension-feeding communities, which may suggest filter feeding
Filter feeder
Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
— 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
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
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 HillsEdiacara Hills
Ediacara Hills are a range of low hills in the northern part of the Flinders Ranges of South Australia, around 650 km north of Adelaide. The area has many old copper and silver mines from mining activity in the late 19th century...
, and consist of fossils preserved in areas near the mouths of rivers (prodeltaic
River delta
A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...
facies
Facies
In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....
). They are typically found in interbedded sandy and silty layers formed below the normal base of wave-related water motion, but in waters shallow enough to be affected by wave motion during storms. Most fossils are preserved as imprints in microbial mats, but a few are preserved within sandy units.
| Biota ranges |
|---|
| Axis scale: millions of years ago, dated with U/Pb of zircons |
Nama-type assemblage
The Nama assemblage is best represented in NamibiaNamibia
Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...
. Three-dimensional preservation is most common, with organisms preserved in sandy beds containing internal bedding. Dima Grazhdankin believes that these organisms represent burrowing organisms, while Guy Narbonne maintains they were surface dwellers. These beds are sandwiched between units comprising interbedded sandstones, siltstones and shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
s – with microbial mats, where present, usually containing the fossils. The environment is interpreted as sand bars formed at the mouth of a delta
River delta
A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...
's distributaries
Distributary
A distributary, or a distributary channel, is a stream that branches off and flows away from a main stream channel. They are a common feature of river deltas. The phenomenon is known as river bifurcation. The opposite of a distributary is a tributary...
.
Significance of assemblages
In the White SeaWhite Sea
The White Sea is a southern inlet of the Barents Sea located on the northwest coast of Russia. It is surrounded by Karelia to the west, the Kola Peninsula to the north, and the Kanin Peninsula to the northeast. The whole of the White Sea is under Russian sovereignty and considered to be part of...
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
Community (ecology)
In ecology, a community is an assemblage of two or more populations of different species occupying the same geographical area. The term community has a variety of uses...
. 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 palæolatitudes (the latitude where the fossil was created, accounting for continental drift
Continental drift
Continental drift is the movement of the Earth's continents relative to each other. The hypothesis that continents 'drift' was first put forward by Abraham Ortelius in 1596 and was fully developed by Alfred Wegener in 1912...
) and in separate sedimentary basin
Sedimentary basin
The term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification...
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
Guild (ecology)
A guild is any group of species that exploit the same resources, often in related ways. As can be seen from the list of examples below, it does not follow that the species within a guild occupy the same, or even similar, ecological niches...
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
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
allowed the number of lifestyles occupied to rise to 30.
Further reading
A popular science account of these fossils, with a particular focus on the Namibian fossils. Excellent further reading for the keen - includes many interesting chapters with macroevolutionary theme.External links
- "The oldest complex animal fossils" – Queen's UniversityQueen's UniversityQueen's University, , is a public research university located in Kingston, Ontario, Canada. Founded on 16 October 1841, the university pre-dates the founding of Canada by 26 years. Queen's holds more more than of land throughout Ontario as well as Herstmonceux Castle in East Sussex, England...
, Canada - "Ediacaran fossils of Canada" – Queen's University, Canada
- "The Ediacaran Assemblage" – Thorough, though slightly out-of-date, description
- "Database of Ediacaran Biota" Advent of Complex Life