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44-529: The Yanjiahe Formation is an Ediacaran to Cambrian fossiliferous geologic formation found in South China . Among others, the following fossils were found in the formation: Ediacaran The Ediacaran ( / ˌ iː d i ˈ æ k ər ə n , ˌ ɛ d i -/ EE -dee- AK -ər-ən, ED -ee- ) is a geological period of the Neoproterozoic Era that spans 96 million years from

88-486: A calcified exoskeleton, but mineralized skeletons did not become common until the beginning of the Cambrian period, with the rise of the " small shelly fauna ". Just after the base of the Cambrian, these miniature fossils become diverse and abundant – this abruptness may be an illusion since the chemical conditions which preserved the small shells appeared at the same time. Most other shell-forming organisms appeared during

132-432: A calcified exoskeleton. Some Cloudina shells even show evidence of predation, in the form of borings. The fossil record primarily contains mineralized exoskeletons, since these are by far the most durable. Since most lineages with exoskeletons are thought to have started with a non-mineralized exoskeleton which they later mineralized, it is difficult to comment on the very early evolution of each lineage's exoskeleton. It

176-521: A chemically distinctive carbonate layer that is referred to as a " cap carbonate ", because it caps glacial deposits. This bed is characterized by an unusual depletion of C that indicates a sudden climatic change at the end of the Marinoan ice age . The lower global boundary stratotype section (GSSP) of the Ediacaran is at the base of the cap carbonate (Nuccaleena Formation), immediately above

220-406: A common misconception, echinoderms do not possess an exoskeleton and their test is always contained within a layer of living tissue. Exoskeletons have evolved independently many times; 18 lineages evolved calcified exoskeletons alone. Further, other lineages have produced tough outer coatings, such as some mammals, that are analogous to an exoskeleton. This coating is constructed from bone in

264-615: A name that was earlier, in 1952, proposed by Russian geologist and paleontologist Boris Sokolov . The Vendian concept was formed stratigraphically top-down, and the lower boundary of the Cambrian became the upper boundary of the Vendian. Paleontological substantiation of this boundary was worked out separately for the siliciclastic basin (base of the Baltic Stage of the Eastern European Platform ) and for

308-488: Is eventually hardened. In contrast, moulting reptiles shed only the outer layer of skin and often exhibit indeterminate growth. These animals produce new skin and integuments throughout their life, replacing them according to growth. Arthropod growth, however, is limited by the space within its current exoskeleton. Failure to shed the exoskeleton once outgrown can result in the animal's death or prevent subadults from reaching maturity, thus preventing them from reproducing. This

352-544: Is in creating the proteins and polysaccharides required for the shell's composite structure , not in the precipitation of the mineral components. Skeletonization also appeared at almost the same time that animals started burrowing to avoid predation, and one of the earliest exoskeletons was made of glued-together mineral flakes, suggesting that skeletonization was likewise a response to increased pressure from predators. Ocean chemistry may also control which mineral shells are constructed of. Calcium carbonate has two forms,

396-602: Is known, however, that in a very short course of time, just before the Cambrian period, exoskeletons made of various materials – silica, calcium phosphate , calcite , aragonite , and even glued-together mineral flakes – sprang up in a range of different environments. Most lineages adopted the form of calcium carbonate which was stable in the ocean at the time they first mineralized, and did not change from this mineral morph - even when it became less favourable. Some Precambrian (Ediacaran) organisms produced tough but non-mineralized outer shells, while others, such as Cloudina , had

440-624: Is named after the Ediacara Hills of South Australia , where trace fossils of a diverse community of previously unrecognized lifeforms (later named the Ediacaran biota ) were first discovered by geologist Reg Sprigg in 1946. Its status as an official geological period was ratified in 2004 by the International Union of Geological Sciences (IUGS), making it the first new geological period declared in 120 years. Although

484-402: Is the mechanism behind some insect pesticides, such as Azadirachtin . Exoskeletons, as hard parts of organisms, are greatly useful in assisting the preservation of organisms, whose soft parts usually rot before they can be fossilized. Mineralized exoskeletons can be preserved as shell fragments. The possession of an exoskeleton permits a couple of other routes to fossilization . For instance,

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528-661: The Shuram excursion is prolonged and is estimated to last for ~9.0 Myrs. As to the Treptichnus pedum , a reference ichnofossil for the lower boundary of the Cambrian, its usage for the stratigraphic detection of this boundary is always risky, because of the occurrence of very similar trace fossils belonging to the Treptichnids group well below the level of T. pedum in Namibia , Spain and Newfoundland , and possibly, in

572-515: The armadillo , and hair in the pangolin . The armour of reptiles like turtles and dinosaurs like Ankylosaurs is constructed of bone; crocodiles have bony scutes and horny scales. Since exoskeletons are rigid, they present some limits to growth. Organisms with open shells can grow by adding new material to the aperture of their shell, as is the case in gastropods , bivalves , and other molluscans . A true exoskeleton, like that found in panarthropods, must be shed via moulting ( ecdysis ) when

616-674: The carbonate basin (base of the Tommotian stage of the Siberian Platform ). The lower boundary of the Vendian was suggested to be defined at the base of the Varanger ( Laplandian ) tillites . The Vendian in its type area consists of large subdivisions such as Laplandian, Redkino , Kotlin and Rovno regional stages with the globally traceable subdivisions and their boundaries, including its lower one. The Redkino, Kotlin and Rovno regional stages have been substantiated in

660-736: The test /tunic of sea squirts and sea urchins , and the prominent mollusc shell shared by snails , clams , tusk shells , chitons and nautilus . Some vertebrate animals, such as the turtle , have both an endoskeleton and a protective exoskeleton . Exoskeletons contain rigid and resistant components that fulfil a set of functional roles in addition to structural support in many animals, including protection, respiration, excretion, sensation, feeding and courtship display , and as an osmotic barrier against desiccation in terrestrial organisms. Exoskeletons have roles in defence from parasites and predators and in providing attachment points for musculature . Arthropod exoskeletons contain chitin ;

704-594: The western United States . The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, and probably in Spain. The Ediacaran Period is not yet formally subdivided, but a proposed scheme recognises an Upper Ediacaran whose base corresponds with the Gaskiers glaciation , a Terminal Ediacaran Stage starting around 550  million years ago , a preceding stage beginning around 575 Ma with

748-515: The Avalon explosion 575 million years ago and died out during the End-Ediacaran extinction event 539 million years ago. Forerunners of some modern animal phyla also appeared during this period, including cnidarians and early bilaterians , as well as mollusc -like Kimberella . Hard-bodied organisms with mineralized shells also began their fossil record in the last few million years of

792-527: The C-isotope data are concerned, due to primary lateral variations of δ C carb in the upper layer of the ocean. Furthermore, Oman presents in its stratigraphic record a large negative carbon isotope excursion, within the Shuram Formation that is clearly away from any glacial evidence strongly questioning systematic association of negative δ C carb excursion and glacial events. Also,

836-515: The Cambrian period, with the Bryozoans being the only calcifying phylum to appear later, in the Ordovician . The sudden appearance of shells has been linked to a change in ocean chemistry which made the calcium compounds of which the shells are constructed stable enough to be precipitated into a shell. However, this is unlikely to be a sufficient cause, as the main construction cost of shells

880-978: The Ediacara Hills fossil site. The Ediacaran marks the first widespread appearance of complex multicellular fauna following the end of the Cryogenian global glaciation known as the Snowball Earth . The relatively sudden evolutionary radiation event, known as the Avalon Explosion , is represented by now-extinct, relatively simple soft-bodied animal phyla such as Proarticulata ( bilaterians with simple articulation , e.g. Dickinsonia and Spriggina ), Petalonamae ( sea pen -like animals, e.g. Charnia ), Aspidella (radial-shaped animals, e.g. Cyclomedusa ) and Trilobozoa (animals with tri-radial symmetry , e.g. Tribrachidium ). Most of these organisms appeared during or after

924-469: The Ediacaran. The supercontinent Pannotia formed and broke apart by the end of the period. The Ediacaran also witnessed several glaciation events , such as the Gaskiers and Baykonurian glaciations . The Shuram excursion also occurred during this period, but its glacial origin is unlikely. The Ediacaran Period overlaps but is shorter than the Vendian Period (650 to 543 million years ago),

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968-816: The Elatina diamictite in the Enorama Creek section, Brachina Gorge, Flinders Ranges, South Australia. The GSSP of the upper boundary of the Ediacaran is the lower boundary of the Cambrian on the SE coast of Newfoundland approved by the International Commission on Stratigraphy as a preferred alternative to the base of the Tommotian Stage in Siberia which was selected on the basis of the ichnofossil Treptichnus pedum (Seilacher, 1955). In

1012-677: The Nama biotic assemblage. There is evidence for a mass extinction during this period from early animals changing the environment, dating to the same time as the transition between the White Sea and the Nama-type biotas. Alternatively, this mass extinction has also been theorised to have been the result of an anoxic event . The relative proximity of the Moon at this time meant that tides were stronger and more rapid than they are now. The day

1056-465: The addition of calcium carbonate makes them harder and stronger, at the price of increased weight. Ingrowths of the arthropod exoskeleton known as apodemes serve as attachment sites for muscles. These structures are composed of chitin and are approximately six times stronger and twice the stiffness of vertebrate tendons . Similar to tendons, apodemes can stretch to store elastic energy for jumping, notably in locusts . Calcium carbonates constitute

1100-519: The age range of 635 to 538.8 million years is based on correlations to other countries where dating has been possible. The base age of approximately 635 million years is based on U–Pb ( uranium – lead ) and Re–Os ( rhenium – osmium ) dating from Africa, China, North America, and Tasmania. The fossil record from the Ediacaran Period is sparse, as more easily fossilized hard-shelled animals had yet to evolve. The Ediacaran biota include

1144-449: The animal starts to outgrow it. A new exoskeleton is produced beneath the old one, and the new skeleton is soft and pliable before shedding the old one. The animal will typically stay in a den or burrow during moulting, as it is quite vulnerable to trauma during this period. Once at least partially set, the organism will plump itself up to try to expand the exoskeleton. The new exoskeleton is still capable of growing to some degree before it

1188-431: The cap carbonates in a rather short distance but cap carbonates do not occur above every tillite elsewhere in the world. The C-isotope chemostratigraphic characteristics obtained for contemporaneous cap carbonates in different parts of the world may be variable in a wide range owing to different degrees of secondary alteration of carbonates, dissimilar criteria used for selection of the least altered samples, and, as far as

1232-727: The earliest widespread Ediacaran biota fossils; two proposed schemes differ on whether the lower strata should be divided into an Early and Middle Ediacaran or not, because it is not clear whether the Shuram excursion (which would divide the Early and Middle) is a separate event from the Gaskiers, or whether the two events are correlated. The dating of the rock type section of the Ediacaran Period in South Australia has proven uncertain due to lack of overlying igneous material. Therefore,

1276-658: The end of the Cryogenian Period at 635 Mya to the beginning of the Cambrian Period at 538.8 Mya. It is the last period of the Proterozoic Eon as well as the last of the so-called " Precambrian supereon", before the beginning of the subsequent Cambrian Period marks the start of the Phanerozoic Eon, where recognizable fossil evidence of life becomes common. The Ediacaran Period

1320-468: The fossil record shortly before the base of the Cambrian period , 550  million years ago . The evolution of a mineralised exoskeleton is considered a possible driving force of the Cambrian explosion of animal life, resulting in a diversification of predatory and defensive tactics. However, some Precambrian ( Ediacaran ) organisms produced tough outer shells while others, such as Cloudina , had

1364-494: The history of stratigraphy it was the first case of usage of bioturbations for the System boundary definition. Nevertheless, the definitions of the lower and upper boundaries of the Ediacaran on the basis of chemostratigraphy and ichnofossils are disputable. Cap carbonates generally have a restricted geographic distribution (due to specific conditions of their precipitation) and usually siliciclastic sediments laterally replace

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1408-421: The magnesium/calcium ratio of the oceans appears to have a negligible impact on organisms' success, which is instead controlled mainly by how well they recover from mass extinctions. A recently discovered modern gastropod Chrysomallon squamiferum that lives near deep-sea hydrothermal vents illustrates the influence of both ancient and modern local chemical environments: its shell is made of aragonite, which

1452-426: The molluscs, whose shells often comprise both forms, most lineages use just one form of the mineral. The form used appears to reflect the seawater chemistry – thus which form was more easily precipitated – at the time that the lineage first evolved a calcified skeleton, and does not change thereafter. However, the relative abundance of calcite- and aragonite-using lineages does not reflect subsequent seawater chemistry –

1496-632: The oldest definite multicellular organisms (with specialized tissues), the most common types of which resemble segmented worms, fronds, disks, or immobile bags. Auroralumina was a cnidarian . Most members of the Ediacaran biota bear little resemblance to modern lifeforms, and their relationship even with the immediately following lifeforms of the Cambrian explosion is rather difficult to interpret. More than 100 genera have been described, and well known forms include Arkarua , Charnia , Dickinsonia , Ediacaria , Marywadea , Cephalonega , Pteridinium , and Yorgia . However, despite

1540-486: The overall enigmaticness of most Ediacaran organisms, some fossils identifiable as hard-shelled agglutinated foraminifera (which are not classified as animals) are known from latest Ediacaran sediments of western Siberia. Sponges recognisable as such also lived during the Ediacaran. Four different biotic intervals are known in the Ediacaran, each being characterised by the prominence of a unique ecology and faunal assemblage. The first spanned from 635 to around 575 Ma and

1584-614: The parts of organisms that were already mineralised are usually preserved, such as the shells of molluscs. It helps that exoskeletons often contain "muscle scars", marks where muscles have been attached to the exoskeleton, which may allow the reconstruction of much of an organism's internal parts from its exoskeleton alone. The most significant limitation is that, although there are 30-plus phyla of living animals, two-thirds of these phyla have never been found as fossils, because most animal species are soft-bodied and decay before they can become fossilised. Mineralized skeletons first appear in

1628-654: The period took namesake from the Ediacara Hills in the Nilpena Ediacara National Park , the type section is actually located in the bed of the Enorama Creek within the Brachina Gorge in the Ikara-Flinders Ranges National Park , at 31°19′53.8″S 138°38′0.1″E  /  31.331611°S 138.633361°E  / -31.331611; 138.633361 , approximately 55 km (34 mi) southeast of

1672-404: The shells of molluscs, brachiopods , and some tube-building polychaete worms. Silica forms the exoskeleton in the microscopic diatoms and radiolaria . One mollusc species, the scaly-foot gastropod , even uses the iron sulfides greigite and pyrite . Some organisms, such as some foraminifera , agglutinate exoskeletons by sticking grains of sand and shell to their exterior. Contrary to

1716-437: The stable calcite and the metastable aragonite, which is stable within a reasonable range of chemical environments but rapidly becomes unstable outside this range. When the oceans contain a relatively high proportion of magnesium compared to calcium, aragonite is more stable, but as the magnesium concentration drops, it becomes less stable, hence harder to incorporate into an exoskeleton, as it will tend to dissolve. Except for

1760-511: The strong layer can resist compaction, allowing a mould of the organism to be formed underneath the skeleton, which may later decay. Alternatively, exceptional preservation may result in chitin being mineralised, as in the Burgess Shale , or transformed to the resistant polymer keratin , which can resist decay and be recovered. However, our dependence on fossilised skeletons also significantly limits our understanding of evolution. Only

1804-401: The time from the end of global Marinoan glaciation to the first appearance worldwide of somewhat complicated trace fossils ( Treptichnus pedum (Seilacher, 1955)). Although the Ediacaran Period does contain soft-bodied fossils , it is unusual in comparison to later periods because its beginning is not defined by a change in the fossil record. Rather, the beginning is defined at the base of

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1848-474: The type area of the Vendian on the basis of the abundant organic-walled microfossils , megascopic algae, metazoan body fossils and ichnofossils . The lower boundary of the Vendian could have a biostratigraphic substantiation as well taking into consideration the worldwide occurrence of the Pertatataka assemblage of giant acanthomorph acritarchs . The Ediacaran Period (c. 635–538.8 Mya) represents

1892-453: Was 21.9 ± 0.4 hours, and there were 13.1 ± 0.1 synodic months/year and 400 ± 7 solar days/year. A few English language documentaries have featured the Ediacaran Period and biota: Exoskeleton Examples of exoskeletons in animals include the cuticle skeletons shared by arthropods ( insects , chelicerates , myriapods and crustaceans ) and tardigrades , as well as the skeletal cups formed by hardened secretion of stony corals ,

1936-461: Was dominated by acritarchs known as large ornamented Ediacaran microfossils . The second spanned from around 575 to 560 Ma and was characterised by the Avalon biota. The third spanned from 560 to 550 Ma; its biota has been dubbed the White Sea biota due to many fossils from this time being found along the coasts of the White Sea . The fourth lasted from 550 to 539 Ma and is known as the interval of

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