The
Ediacara biota are ancient life-forms of the
EdiacaranThe Ediacaran Period 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, which represent the earliest known complex
multicellular organismMulticellular organisms are organisms consisting of more than one cell, and having differentiated cells that perform specialized functions in the organism...
s.
[Simple multicellular organisms such as red algae evolved at least .] They appeared soon after the Earth thawed from the
CryogenianThe Cryogenian is a geologic period that lasted from . The Cryogenian forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian period and followed by the Ediacaran...
period's
extensive glaciersSnowball Earth refers to hypotheses regarding paleoclimatic global-scale glaciation, claiming that the Earth's surface was nearly or entirely frozen at some points in its past. The occurrence of Snowball Earths remains controversial...
, and largely disappeared soon before the rapid appearance of
biodiversityBiodiversity is the variation of life forms within a given ecosystem, biome, or for the entire Earth. Biodiversity is often used as a measure of the health of biological systems...
known as the
Cambrian explosionThe Cambrian explosion or Cambrian radiation was the seemingly rapid appearance of most major groups of complex animals around , as evidenced by the fossil record. This was accompanied by a major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
, which saw the first appearance in the
fossilFossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous rock formations and sedimentary layers is known as the fossil record...
record of the basic patterns and body-plans that would go on to form the basis of modern
animalAnimals are a major group of mostly 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...
s. Little of the diversity of the Ediacara
biotaBiota is the total collection of organisms of a geographic region or a time period, from local geographic scales and instantaneous temporal scales all the way up to whole-planet and whole-timescale spatiotemporal scales. The biota of the Earth lives in the biosphere.-See also:* Biome* Fauna* Flora*...
would be incorporated in this new scheme, with a distinct Cambrian biota arising and usurping the organisms that dominated the Ediacaran fossil record.
The organisms of the Ediacaran Period first appeared around and flourished until the cusp of the
CambrianThe Cambrian is the first geological period of the Paleozoic era, lasting from ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux...
, when the characteristic communities of fossils vanished. While rare fossils that may represent survivors have been found as late as the Middle Cambrian (510 to 500 million years ago), the earlier fossil communities disappear from the record at the end of the Ediacaran, leaving only controversial fragments of once-thriving
ecosystemAn ecosystem is a system of interdependent organisms which share the same habitat, in an area functioning together with all of the physical factors of the environment. Ecosystems can be permanent or temporary. Ecosystems usually form a number of food webs...
s, if anything. Multiple
hypothesesA hypothesis is a proposed explanation for an observable 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 this disappearance, including
preservation biasA biased sample is a statistical sample of a population in which all participants are not equally balanced or objectively represented. It results from sampling bias , causing some members of the population to be less likely to be included than others...
, a changing environment, the advent of
predatorsIn 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 the prey...
, and competition from other life-forms.
Some Ediacaran organisms might have been closely related to groups that would rise to prominence later; for instance,
KimberellaKimberella is a genus of fossils known only from rocks of the Ediacaran period, and only one species, Kimberella quadrata, has been recognized. Specimens were first found in Australia's Ediacara Hills, but recent research has concentrated on the numerous finds near the White Sea in Russia, which...
shows some similarity to molluscs, and other organisms have been thought to show bilateral symmetry, though this is controversial. Most
macroscopic fossilMacrofossils 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
morphologicallyIn biology morphology is the form, structure and configuration of an organism.This includes aspects of the outward appearance as well as the form and structure of the internal parts like bones and organs...
distinct from later life-forms: they resemble discs, mud-filled bags, or quilted mattresses. Classification is difficult, and the assignment of some
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
even at the level of kingdom —
animalAnimals are a major group of mostly 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...
,
fungusA fungus is any member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. The Fungi are classified as a kingdom that is separate from plants, animals and bacteria...
,
protistProtists , are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista but this group is no longer recognized in modern taxonomy...
or something else — is uncertain: one paleontologist has even gained support for a separate kingdom
Vendozoa (now renamed
Vendobionta). Their strange form and apparent disconnectedness from later organisms have led some to consider them a "failed experiment" in multicellular life, with later multicellular life independently re-evolving from unrelated single-celled organisms.
History
The first Ediacaran fossils discovered were the disc-shaped
AspidellaAspidella is an Ediacaran disk-shaped fossil.In 1872, Elkanah Billings described Aspidella terranovica fossils from Duckworth Street, St. John's, Newfoundland. They were in a Precambrian outcrop of black shale. Billings was the head paleontologist with the Geological Survey of Canada at the time...
terranovica, in 1868. Their discoverer,
A. MurrayAlexander 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
CambrianThe Cambrian is the first geological period of the Paleozoic era, lasting from ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux...
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 BillingsElkanah Billings is often referred to as Canada's first paleontologist. Billings was born on a farm by the Rideau River outside Bytown. He was originally educated in law and in 1845, he was called to the Canadian bar. In 1852, he founded the journal the Canadian Naturalist ...
' 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 GürichGeorg 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 Breslau/Wroclaw Georg...
discovered specimens in
NamibiaNamibia, officially the Republic of Namibia , is a country in Southern Africa whose western border is the Atlantic Ocean. It shares borders with Angola and Zambia to the north, Botswana and Zimbabwe to the east, and South Africa to the south and east...
, but the firm belief that life originated in the
CambrianThe Cambrian is the first geological period of the Paleozoic era, lasting from ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux...
led to them being assigned to the Cambrian Period, and no link to
Aspidella was made. In 1946,
Reg SpriggReginald Claude Sprigg, AO D.Sc., h.c. A.N.U. and Flinders University, . Sprigg was a well known Australian geologist. He is perhaps best remembered internationally as the discoverer of the Ediacara biota, an assemblage of some of the most ancient animal fossils known...
noticed "jellyfishes" in the
Ediacara HillsEdiacara 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 RangesFlinders Ranges is the largest mountain range in South Australia, which starts approximately 200 km north west of Adelaide. The discontinuous ranges stretch for over 430 km 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
CharniaCharnia 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, where the first specimen was found....
in 1957 that the pre-Cambrian was seriously considered as containing life. This
frondThe term frond is used to refer 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 ForestCharnwood Forest is an upland tract in north-western Leicestershire, England, bounded by Leicester, Loughborough, and Coalville. It is undulating, rocky, picturesque, with barren areas, and some extensive tracts of woodland; its elevation is generally 600 ft and upwards, the area exceeding this...
, and due to the detailed
geologic mapA 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 surface coverage. Structural features are shown with strike and dip symbols which consist of a long line, a number, and a short line...
ping of the
British Geological SurveyThe 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 GlaessnerMartin 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 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.
However, all specimens discovered until 1967 were in coarse-grained
sandstoneSandstone 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 color, but the most common colors are tan, brown, yellow,...
that prevented preservation of fine details, making interpretation difficult. S.B. Misra's discovery of fossiliferous
ashVolcanic 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 AustraliaSouth 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....
, which take their name from aborigine
Idiyakra, "water is present" — was added to the competing terms "Sinian" and "Vendian", for terminal-Precambrian rocks, names that were also applied to the life-forms. "Ediacaran" and "Ediacarian" were subsequently applied to the epoch or period of
geologic timeThe geologic time scale is a chronologic schema 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 SciencesThe 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
NeoproterozoicThe 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
skeletalIn biology, a skeleton is a rigid framework that provides protection and structure in many types of animal, particularly those of the phylum Chordata and of the superphylum Ecdysozoa. Exoskeletons are external, as is typical of many invertebrates; they enclose the soft tissues and organs of the...
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
disturbedIn oceanography, limnology and archaeology, bioturbation is the displacement and mixing of sediment particles by benthic fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves , gastropods, holothurians, or any other infaunal or epifaunal organisms...
before they could fossilize.
Microbial mats
Microbial matA 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...
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.
Some Ediacaran strata with the texture characteristic of microbial mats contain fossils, and Ediacaran fossils are almost never found in beds that do not contain these microbial mats.
Although microbial mats were once widespread, the evolution of grazing organisms in the Cambrian vastly reduced their numbers, and these communities are now limited to inhospitable
refugiaIn biology a refugium is a location of an isolated or relict population of a once widespread species.This isolation can be due to climatic changes or human activities such as deforestation and over-hunting...
where predators cannot survive long enough to eat them.
Fossilisation
The preservation of these fossils is one of their great fascinations to science. As soft-bodied organisms, they would normally not fossilise. Unlike later soft-bodied fossil biota (such as the
Burgess ShaleThe 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 LimestoneThe Solnhofen limestone is a Jurassic Konservat-Lagerstätte that preserves a rare assemblage of fossilized organisms, some of which, such as sea jellies, don't ordinarily fossilize at all. Others, like the early bird Archaeopteryx are preserved in such detail that they are among the most famous and...
) the Ediacara biota is not found in a restricted environment subject to unusual local conditions: they were a global phenomenon. The processes that were operating must have been systemic and worldwide. There was something very different about the Ediacaran Period that permitted these delicate creatures to be left behind. It is thought that the fossils were preserved by virtue of rapid covering by ash or sand, trapping them against the mud or microbial mats on which they lived. Ash beds provide more detail, and can readily be precisely dated to the nearest million years or better by means of
radiometric datingRadiometric dating is a technique used to date materials, 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
turbiditeTurbidite 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, and the ash or sand slumped in to fill the void, leaving a cast of the underside of the organism.
Conversely, quilted fossils tend to decompose
after the cementation of the overlying sediment; hence their upper surfaces are preserved. Their more resistant nature is reflected in the fact that in rare occasions, quilted fossils are found
within storm beds, the high-energy sedimentation not having destroyed them as it would have the less-resistant discs. Further, in some cases, the
bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
l
precipitationPrecipitation is the formation of a solid in a solution during a chemical reaction. When the reaction occurs, the solid formed is called the precipitate, and the liquid remaining above the solid is called the supernate...
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 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 (= CyclomedusaCyclomedusa 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. |
|
SprigginaFossils 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 trilobiteTrilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. Trilobites first appear in the fossil record during the Early Cambrian period and flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the...
s, may be one of the predators that led to the demise of the Ediacaran fauna and subsequent diversificationThe Cambrian explosion or Cambrian radiation was the seemingly rapid appearance of most major groups of complex animals around , as evidenced by the fossil record. This was accompanied by a 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 YorgiaYorgia waggoneri Ivantsov, 1999is 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 25 cm...
, terminating with a body fossil of the organism itself (right). |
|
The Ediacaran biota exhibited a vast range of
morphologicalIn biology morphology is the form, structure and configuration of an organism.This includes aspects of the outward appearance as well as the form and structure of the internal parts like bones and organs...
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 FormationThe 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. The formation is of particular interest because it appears to cover the boundary between the problematic organisms of the Ediacaran...
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
sulfurSulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant, multivalent non-metal. Sulfur, in its native form, is a yellow crystalline solid. In nature, it can be found as the pure element and as sulfide and sulfate minerals...
-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
EdiacariaEdiacaria 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...
,
CyclomedusaCyclomedusa 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
RugoconitesA 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
cnidariaCnidaria is a phylum containing over 10,000 species of animals found exclusively in aquatic, mostly marine, environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey...
, 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
holdfastA 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,
protistProtists , are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista but this group is no longer recognized in modern taxonomy...
s and
sea anemoneSea anemones are a group of water dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower...
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
PteridiniumPteridinium 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.
Quilted organisms : The organisms considered in Seilacher's revised definition of the Vendobionta share a "quilted" appearance, and resembled an inflatable
mattressA mattress is a mat or pad, usually placed on top of a bed, upon which to sleep or lie.The word mattress is derived from Arabic words meaning "to throw" and "place where something is thrown" or "mat, cushion." During the Crusades, Europeans adopted the Arabic method of sleeping on cushions thrown...
. 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
SwartpuntiaSwartpuntia 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 fractal
A fractal is "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...
rangeomorphThe 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 CharniaCharnia 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, where the first specimen was found....
and SwartpuntiaSwartpuntia 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 groups is both the most iconic of the Ediacaran biota, and the most difficult to place within the existing tree of lifeThe concept of 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 osmosisOsmosis is the diffusion of water through a semi-permeable membrane. More specifically, it is the movement of water across a semi-permeable membrane from an area of high water potential to an area of low water potential...
.
Non-Ediacaran Ediacarans : Some Ediacaran organisms have more complex details preserved, which has allowed them to be interpreted as possible early forms of living
phylaIn biology, a phylum ["Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states.] is a taxonomic rank below Kingdom and above Class...
, excluding them from some definitions of the Ediacaran biota.
- The earliest such fossil is the reputed bilaterian Vernanimalcula
Vernanimalcula 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 acritarchAcritarchs 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, almost universally accepted as bilaterians, include the mollusc-like KimberellaKimberella is a genus of fossils known only from rocks of the Ediacaran period, and only one species, Kimberella quadrata, has been recognized. Specimens were first found in Australia's Ediacara Hills, but recent research has concentrated on the numerous finds near the White Sea in Russia, which...
, SprigginaFossils 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 ParvancorinaParvancorina 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 Cloudina
The 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, protist
Protists , are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista but this group is no longer recognized in modern taxonomy...
s and bacteriaThe bacteria are a large group of unicellular 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 fossilTrace 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 , footprints and feeding marks, and root cavities...
s : With the exception of some very simple
vertical burrowsSkolithos 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, 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
bilateralThe 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. The bilateralia are a subregnum of animals, including the majority...
clade of
animalAnimals are a major group of mostly 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...
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.
YorgiaYorgia waggoneri Ivantsov, 1999is 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 25 cm...
and
DickinsoniaDickinsonia 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".-Morphology:The...
are often found at the end of long pathways of trace fossils matching their shape; these fossils are thought to be associated with cilliarty 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
radulaThe radula is an anatomical structure found in mollusks and used for feeding. It is a minutely toothed, chitinous ribbon. It is typically used for scraping or cutting food before the food enters the esophagus...
.
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 are free-swimming members of the phylum Cnidaria. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa , Staurozoa , Cubozoa , and Hydrozoa Jellyfish (also known as jellies or sea jellies) are free-swimming members of...
and
sea-pensSea 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...
. However, detailed study of their growth pattern has discounted this hypothesis.
"The dawn of animal life"
Martin GlaessnerMartin 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 groupA crown group is the smallest monophyletic group, or "clade", to contain the last common ancestor of all members, and all of that ancestor's descendants...
members of modern phyla, but were unfamiliar because they had yet to evolve the characteristic features we use in modern classification.
Adolf SeilacherAdolf "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...
responded by suggesting that the Ediacaran sees animals usurping giant
protistProtists , are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista but this group is no longer recognized in modern taxonomy...
s as the dominant life form.
In 1986
Mark McMenaminMark McMenamin is a professor of geology at Mount Holyoke College. His main research interests are early fossil life, particularly the Ediacaran biota...
claimed that Ediacarans did not possess an
embryoAn 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 systemThe nervous system is a network of specialized cells that communicate information about an organism's surroundings and itself. It processes this information and causes reactions in other parts of the body. It is composed of neurons and other specialized cells called glial cells that aid in the...
and
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
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 (
cladeA clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.[The term "monophyletic group" is used in this article...]
) and created the
kingdomIn biological taxonomy, kingdom and/or regnum is a taxonomic rank in either the highest rank, or the rank below domain. Each kingdom is divided into smaller groups called phyla...
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 cellA 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
symbiosisThe term symbosis commonly describes close and often long-term interactions between different biological species...
with
photosyntheticPhotosynthesis is a 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...
or chemoautotrophic organisms.
Lichens
Gregory Retallack's hypothesis that Ediacaran organisms were
lichenLichens are composite organisms consisting of a symbiotic association 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 woodPetrified wood is a type of fossil: it consists of fossil wood where all the organic materials have been replaced with minerals , while retaining the original structure of the wood...
. He points out the
chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world. It is the main component of the cell walls of fungi, the exoskeletons of arthropods, such as crustaceans Chitin...
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
algæAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds. They are photosynthetic, like plants, and "simple" because they lack the many distinct organs found in...
, to protists known as
foraminiferaThe Foraminifera, or forams for short, are a large group of amoeboid protists with reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net. They typically produce a test, or shell, which can have either one or multiple chambers, some becoming quite elaborate...
, to fungi to
bacteriaThe bacteria are a large group of unicellular 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 evolvingThe evolution of 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 cell types...
as time progresses, with traces of earlier semi-complex life such as
NimbiaNimbia 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, 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
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
. Without a complex
circulatory system- [Headline text]--75.137.171.242 01:21, 21 October 2009 :bThe circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, nitrogen waste products, etc. to and from cells in the body to help fight diseases and help stabilize body temperature and pHb to maintain...
, 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
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
and
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. Besides its 92 protons, a uranium nucleus can have between 141 and 146 neutrons. The most common uranium isotopes are U-238 and U-235 . A uranium atom has...
existed in a
reducedRedox describes all chemical reactions in which atoms have their oxidation number changed....
form; these would react with any free oxygen produced by
photosynthesisPhotosynthesis is a 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...
ing organisms. Oxygen would not be able to build up in the
atmosphereAn atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low...
until all the iron had rusted (producing
banded iron formationBanded iron formations are a distinctive type of rock often found in primordial sedimentary rocks. The structures consist 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 CanfieldDonald 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
radiationAn adaptive radiation is a rapid evolutionary radiation characterized by an increase in the morphological and ecological diversity of a single, rapidly diversifying lineage. Phenotypes adapt in response to the environment, with new and useful traits arising...
. 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 inin the Early Cambrian and the Cretaceous.
Periods of intense coldA 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 FormationThe 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. The formation is of particular interest because it appears to cover the boundary between the problematic organisms of the Ediacaran...
, appear just a million years after the Earth emerged from a
global glaciationSnowball Earth refers to hypotheses regarding paleoclimatic global-scale glaciation, claiming that the Earth's surface was nearly or entirely frozen at some points in its past. The occurrence of Snowball Earths remains controversial...
, 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 explosionThe Cambrian explosion or Cambrian radiation was the seemingly rapid appearance of most major groups of complex animals around , as evidenced by the fossil record. This was accompanied by a 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" 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 ShaleThe 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
ChengjiangThe Maotianshan Shales area a series of lower Cambrian deposits, famous for their Konservat Lagerstätten, or high number of fossils. They take their name from Maotianshan Hill in Chengjiang County, Yunnan Province, China....
— 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 chainIn ecology, trophic dynamics is the system of trophic levels , which describes the position that an organism occupies in a food chain — what an organism eats, and what eats the organism.- Energy economy :...
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
substrateStream 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
KimberellaKimberella is a genus of fossils known only from rocks of the Ediacaran period, and only one species, Kimberella quadrata, has been recognized. Specimens were first found in Australia's Ediacara Hills, but recent research has concentrated on the numerous finds near the White Sea in Russia, which...
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
CambrianThe Cambrian is the first geological period of the Paleozoic era, lasting from ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux...
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'Bivalves are marine and freshwater molluscs belonging to the class Bivalvia. Other names for the class include Bivalva, Pelecypoda, and Lamellibranchia...
"competitive exclusion" of
brachiopodBrachiopods are a small phylum of benthic invertebrates. Also known as lamp shells , "brachs" or Brachiopoda, they are sessile, two-valved, marine animals with an external morphology superficially resembling bivalves to which they are not closely related...
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
supercontinentIn geology, Rodinia is the name of a supercontinent, a continent which contained most or all of Earth's landmass...
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
biomineralisationBiomineralisation is the process by which living organisms produce minerals, often to harden or stiffen existing tissues. It is an extremely widespread phenomenon; all five taxonomic kingdoms contain members that are able to form minerals, and over 60 different minerals have been identified in...
) could all have played a part.
Assemblages
Ediacaran-type fossils are recognised globally in 25 localities and a variety of
depositionalDeposition 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 gravity flows, 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
zirconZircon 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 datingRadiometric dating is a technique used to date materials, 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
rangeomorphThe 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
fractalA fractal is "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
CarboniferousThe Carboniferous is a geologic period and system that extends from the end of the Devonian period, about 359.2 ± 2.5 Ma , to the beginning of the Permian period, about 299.0 ± 0.8 Ma ....
suspension-feeding communities, which may suggest
filter feedingFilter 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, some fish and sharks, and baleen whales. Some birds, such...
— 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
speciationSpeciation 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 evolution to rich biota. Opinion is currently divided between these conflicting hypotheses.
Ediacara-type assemblage
The Ediacara-type assemblage is named after Australia's
Ediacara HillsEdiacara 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 (
prodeltaicA delta is a landform that is created 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...
faciesIn geology, facies are a body of rock with specified characteristics. [Reading ] 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 |
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| Axis scale: millions of years ago, dated with U/Pb of zircons |
Nama-type assemblage
The Nama assemblage is best represented in
NamibiaNamibia, officially the Republic of Namibia , is a country in Southern Africa whose western border is the Atlantic Ocean. It shares borders with Angola and Zambia to the north, Botswana and Zimbabwe to the east, and South Africa to the south and east...
. 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
shaleShale is a fine-grained, clastic sedimentary rock composed of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable...
s, with microbial mats, where present, usually containing fossils. The environment is interpreted as sand bars formed at the mouth of a
deltaA delta is a landform that is created 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
distributariesA 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 SeaThe White Sea is an inlet of the Barents Sea 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....
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
communitiesIn ecology, a community is an assemblage of two or more populations of different species occupying the same geographical area. This term is used only to describe biotic factors....
. Since they are globally distributed — described on all continents except
Antarctica| style="border-top:solid 1px #ccd2d9; padding:0.4em 1em 0.4em 0; vertical-align:top;" | 14,000,000 km
2 280,000 km
2 13,720,000 km
2 |-! style="border-top: solid 1px #ccd2d9; padding: 0.4em 1em 0.4em 0; vertical-align: top;...
— 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 driftContinental 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 basinThe 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 lifeGuilds are groups of species that exploit the same resources in the same way, therefore sharing a similar ecological niche.Some example guilds: forbs, geophytes, graminoids, shrubs, trees, vines, and arthropods....
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
CambrianThe Cambrian is the first geological period of the Paleozoic era, lasting from ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux...
allowed the number of lifestyles occupied to rise to 30.
Further reading
An outdated popular science account of these fossils, with a narrowed focus on only the Namibian Fossils.
Excellent further reading for the keen - includes many interesting chapters with macroevolutionary theme.
External links