The Ediacaran Period ( /iːdiˈækərən/), spans 94 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 541 Mya. It marks the end of the Proterozoic Eon, and the beginning of the Phanerozoic Eon. It is named after the Ediacara Hills of South Australia.
The Ediacaran Period's 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 the period takes its name from the Ediacara Hills where geologist Reg Sprigg first discovered fossils of the eponymous Ediacara biota in 1946, the type section is located in the bed of the Enorama Creek within Brachina Gorge in the Flinders Ranges of South Australia, at .
635–541 million years ago
|Mean atmospheric O
2 content over period duration
|c. 8 vol %|
(40 % of modern level)
|Mean atmospheric CO
2 content over period duration
|c. 4500 ppm|
(16 times pre-industrial level)
|Mean surface temperature over period duration||c. 17 °C|
(3 °C above modern level)
The Ediacaran Period overlaps, but is shorter than the Vendian Period, 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 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 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. 635–541 Mya) represents 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 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 13C that indicates a sudden climatic change at the end of the Marinoan ice age. The lower boundary global boundary stratotype section (GSSP) of the Ediacaran is at the base of the cap carbonate (Nuccaleena Formation), immediately above 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 the history of stratigraphy it was the first case of usage of bioturbations for the System boundary definition.
Cap carbonates generally have a restricted geographic distribution (due to specific conditions of their precipitation) and usually siliciclastic sediments laterally replace 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 the C-isotope data are concerned, due to primary lateral variations of δ l3Ccarb 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 l3Ccarb excursion and glacial events. Also, 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 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 , a preceding stage beginning around 557 Ma with 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's 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. Therefore, the age range of 635 to 542 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) isochron dating from Namibia and China.
Applying this age to the base of the Ediacaran assumes that cap carbonates are laid down synchronously around the world and that the correct cap carbonate layers have been selected in such diverse locales as Australian and Namibia. This is controversial because an age of about 580 million years has been obtained for glacial rocks in Tasmania which some scientists tentatively assign to those just beneath the Ediacaran rocks of the Flinders Ranges. The age of the top is the same as the widely recognised age for the base of the Cambrian Period 542± 0.3 Mya, producing a misalignment, as the end of the Edicarian Period should mark the start of the Cambrian Period.
The fossil record from the Ediacaran Period is sparse, as more easily fossilized hard-shelled animals had yet to evolve. The Ediacaran biota include the oldest definite multicellular organisms (with specialized tissues), the most common types of which resemble segmented worms, fronds, disks, or immobile bags.
Ediacara 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.
There is evidence that Earth's first mass extinction happened during this period when early animals changed the environment.
The relative proximity of the Moon at this time meant that tides were stronger and more rapid than they are now. The day 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:
Ambulacraria or Coelomopora is a Superphylum of invertebrate phyla which includes echinoderms and hemichordates; a member of this group is called an ambulacrarian. Phylogenetic analysis suggest the echinoderms and hemichordates separated around 533 million years ago. The Ambulacraria are part of the deuterostomes, a larger clade that also includes the Chordata, Vetulicolia and Saccorhytus.
The two living clades with representative organisms are:
Echinodermata (sea stars, sea urchins, brittle stars, sea cucumbers, feather stars, sea lilies, etc.)
Hemichordata (acorn worms, Pterobranchia, and possibly graptolites)The group Xenoturbellida (two species of worm-like animals) has previously been considered to be in this clade, but is now considered to be placed more basally among metazoans.Fossil taxa that may lie on the stem lineage:
unranked clade Cambroernida
† unranked clade = Eldoniida
† Stellostomites Caron, Conway Morris & Shu, 2010
† Herpetogaster Caron, Conway Morris & Shu, 2010 - with one species: † Herpetogaster collinsi Caron, Conway Morris & Shu, 2010
† Seputus? Murray, J & MacGabhannBilateria
The bilateria , bilaterians, or triploblasts, are animals with bilateral symmetry, i.e., they have a head (anterior) and a tail (posterior) as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.The bilateria are a major group of animals, including the majority of phyla but not sponges, ctenophores, placozoans, and cnidarians. For the most part, bilateral embryos are triploblastic, having three germ layers: endoderm, mesoderm, and ectoderm. Nearly all are bilaterally symmetrical, or approximately so; the most notable exception is the echinoderms, which achieve near-radial symmetry as adults, but are bilaterally symmetrical as larvae.
Except for a few phyla (i.e. flatworms and gnathostomulids), bilaterians have complete digestive tracts with a separate mouth and anus. Some bilaterians lack body cavities (acoelomates, i.e. Platyhelminthes, Gastrotricha and Gnathostomulida), while others display primary body cavities (deriving from the blastocoel, as pseudocoeloms) or secondary cavities (that appear de novo, for example the coelom).Cambrian
The Cambrian Period ( or ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago (mya) to the beginning of the Ordovician Period 485.4 mya. Its subdivisions, and its base, are somewhat in flux. The period was established (as “Cambrian series”) by Adam Sedgwick, who named it after Cambria, the Latin name of Wales, where Britain's Cambrian rocks are best exposed. The Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells. As a result, our understanding of the Cambrian biology surpasses that of some later periods.The Cambrian marked a profound change in life on Earth; prior to the Cambrian, the majority of living organisms on the whole were small, unicellular and simple; the Precambrian Charnia being exceptional. Complex, multicellular organisms gradually became more common in the millions of years immediately preceding the Cambrian, but it was not until this period that mineralized—hence readily fossilized—organisms became common. The rapid diversification of life forms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla. Phylogenetic analysis has supported the view that during the Cambrian radiation, metazoa (animals) evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates.
Although diverse life forms prospered in the oceans, the land is thought to have been comparatively barren—with nothing more complex than a microbial soil crust and a few molluscs that emerged to browse on the microbial biofilm. Most of the continents were probably dry and rocky due to a lack of vegetation. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia. The seas were relatively warm, and polar ice was absent for much of the period.Cambrian explosion
The Cambrian explosion or Cambrian radiation was an event approximately 541 million years ago in the Cambrian period when most major animal phyla appeared in the fossil record. It lasted for about 13 – 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification of other organisms.Before the Cambrian explosion, most organisms were simple, composed of individual cells occasionally organized into colonies. As the rate of diversification subsequently accelerated, the variety of life began to resemble that of today. Almost all present animal phyla appeared during this period.Charnia
Charnia is a genus of frond-like Ediacaran lifeforms with segmented, leaf-like ridges branching alternately to the right and left from a zig-zag medial suture (thus exhibiting glide reflection, or opposite isometry). The genus Charnia was named after Charnwood Forest in Leicestershire, England, where the first fossilised specimen was found. Charnia is significant because it was the first Precambrian fossil to be recognized as such.
The living organism was a type of life form that grew on the sea floor and is believed to have fed on nutrients in the water. Despite Charnia's fern-like appearance, it is not a photosynthetic plant or alga because the nature of the fossilbeds where specimens have been found implies that it originally lived in deep water, well below the photic zone where photosynthesis can occur.Cloudinidae
The cloudinids, an early metazoan family containing the genera Acuticocloudina, Cloudina and Conotubus, 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 remains unknown. The name Cloudina honors the 20th-century geologist and paleontologist Preston Cloud.Cloudinids comprise two genera: Cloudina itself is mineralized, whereas Conotubus is at best weakly mineralized, whilst sharing the same "funnel-in-funnel" construction.Cloudinids had a wide geographic range, reflected in the present distribution of localities in which their fossils are found, and are an abundant component of some deposits. They never appear in the same layers as soft-bodied Ediacaran biota, but the fact that some sequences contain cloudinids and Ediacaran biota in alternating layers suggests that these groups had different environmental preferences. It has been suggested that cloudinids lived embedded in microbial mats, growing new cones to avoid being buried by silt. However no specimens have been found embedded in mats, and their mode of life is still an unresolved question.
The classification of the cloudinids has proved difficult: they were initially regarded as polychaete worms, and then as coral-like cnidarians on the basis of what look like buds on some specimens. Current scientific opinion is divided between classifying them as polychaetes and regarding it as unsafe to classify them as members of any broader grouping.
Cloudinids are important in the history of animal evolution for two reasons. They are among the earliest and most abundant of the small shelly fossils with mineralized skeletons, and therefore feature in the debate about why such skeletons first appeared in the Late Ediacaran. The most widely supported answer is that their shells are a defense against predators, as some Cloudina specimens from China bear the marks of multiple attacks, which suggests they survived at least a few of them. The holes made by predators are approximately proportional to the size of the Cloudina specimens, and Sinotubulites fossils, which are often found in the same beds, have so far shown no such holes. These two points suggest that predators attacked in a selective manner, and the evolutionary arms race which this indicates is commonly cited as a cause of the Cambrian explosion of animal diversity and complexity.Deuterostome
Deuterostomes (taxonomic term: Deuterostomia; meaning "second mouth" in Greek) constitute a superphylum of animals. It is a sister clade of Protostomia, with which it forms the Nephrozoa clade.
Deuterostomia is a subtaxon of the Bilateria branch of the subkingdom Eumetazoa, within Animalia, and are distinguished from protostomes by their deuterostomic embryonic development; in deuterostomes, the first opening (the blastopore) becomes the anus, while in protostomes, it becomes the mouth. (There are some occurrences of deuterostomy among protostomes.)Deuterostomes are also known as enterocoelomates because their coelom develops through enterocoely.
Many groups of organisms originally thought to have belonged to this group (e.g. Lophophorata, Chaetognatha) have been placed elsewhere, to the point where the possibility of the term deuterostome being deprecated is considered.An undisputed extant group in the deuterostome clade is the Chordata, i.e. the vertebrates and their kin.
The other possible deuterostome group is the Ambulacraria: Echinodermata (starfish, sea urchins, sea cucumbers) + Hemichordata (acorn worms and graptolites).
In 2019, there was cautious support for the assessment that the Ambulacraria are sister to the Xenacoelomorpha together forming the Xenambulacraria, probably as basal Bilateria or deuterostome. Without Ambulacraria, Deuterostomes would then be a junior synonym to the Chordata.Doushantuo Formation
The Doushantuo Formation (Chinese: 陡山沱; pinyin: dǒu shān tuó) is a fossil Lagerstätte in Weng'an County, Guizhou Province, China that is notable for being one of the oldest beds to contain minutely preserved microfossils, phosphatic fossils that are so characteristic they have given their name to "Doushantuo type preservation". The formation is of particular interest because a part of it appears to cover the boundary between the enigmatic organisms of the Ediacaran geological period and the more familiar fauna of the Cambrian explosion where lifeforms recognizable as ancestors of later and recent lifeforms first emerged.
Taken as a whole, the Doushantuo Formation ranges from about 635 Ma (million years ago) at its base to about 551 Ma at its top, predating by perhaps five Ma the earliest of the 'classical' Ediacaran faunas from Mistaken Point on the Avalon peninsula of Newfoundland, and recording conditions up to a good forty to fifty million years before the Cambrian explosion.Ediacaran biota
The Ediacaran (; formerly Vendian) biota consisted of enigmatic tubular and frond-shaped, mostly sessile organisms that lived during the Ediacaran Period (ca. 635–542 Mya). Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms. The Ediacaran biota may have radiated in a proposed event called the Avalon explosion, 575 million years ago, after the Earth had thawed from the Cryogenian period's extensive glaciation. The biota largely disappeared with the rapid increase in biodiversity known as the Cambrian explosion. Most of the currently existing body plans of animals first appeared in the fossil record of the Cambrian rather than the Ediacaran. For macroorganisms, the Cambrian biota appears to have completely replaced the organisms that dominated the Ediacaran fossil record, although relationships are still a matter of debate.
The organisms of the Ediacaran Period first appeared around 600 million years ago and flourished until the cusp of the Cambrian 542 million years ago, when the characteristic communities of fossils vanished. A diverse Ediacaran community was discovered in 1995 in Sonora, Mexico, and is approximately 555 million years in age, roughly coeval with Ediacaran fossils of the Ediacara Hills, South Australia and the White Sea, Russia. 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 ecosystems. Multiple hypotheses exist to explain the disappearance of this biota, including preservation bias, a changing environment, the advent of predators and competition from other life-forms. Recent (2018) sampling of late Ediacaran strata across Baltica (<560 mya) suggests the flourishing of the organisms coincided with conditions of low overall productivity with a very high percentage produced by bacteria, which may have led to high concentrations of dissolved organic material in the oceans.Determining where Ediacaran organisms fit in the tree of life has proven challenging; it is not even established that they were animals, with suggestions that they were lichens (fungus-alga symbionts), algae, protists known as foraminifera, fungi or microbial colonies, or hypothetical intermediates between plants and animals. The morphology and habit of some taxa (e.g. Funisia dorothea) suggest relationships to Porifera or Cnidaria. Kimberella may show a similarity to molluscs, and other organisms have been thought to possess bilateral symmetry, although this is controversial. Most macroscopic fossils are morphologically distinct from later life-forms: they resemble discs, tubes, mud-filled bags or quilted mattresses. Due to the difficulty of deducing evolutionary relationships among these organisms, some palaeontologists have suggested that these represent completely extinct lineages that do not resemble any living organism. One palaeontologist 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 evolving independently from unrelated single-celled organisms. However, a 2018 study confirmed that one of the period's most-prominent and iconic fossils, Dickinsonia, included cholesterol, limiting its affinities to that of animals, fungi, or red algae .
The concept of "Ediacaran Biota" is somewhat artificial as it can not be defined geographically, stratigraphically, taphonomically, or biologically.End-Ediacaran extinction
Evidence suggesting that a mass extinction occurred at the end of the Ediacaran period, 542 million years ago, includes:
A mass extinction of acritarchs
The sudden disappearance of the Ediacara biota and calcifying organisms;
The time gap before Cambrian organisms "replaced" them.Eumetazoa
Eumetazoa (Greek: εὖ [eu], well + μετά [metá], after + ζῷον [zóon], animal) or Diploblasts, or Epitheliozoa, or Histozoa are a proposed basal animal clade as a sister group of the Porifera. The basal Eumetazoan clades are the Ctenophora and the ParaHoxozoa. Placozoa is now also seen as a Eumetazoan in the Parahoxozoa.
Several other extinct or obscure life forms, such as Iotuba and Thectardis appear to have emerged in the group. Characteristics of eumetazoans include true tissues organized into germ layers, the presence of neurons, and an embryo that goes through a gastrula stage.
Some phylogenists have speculated the sponges and eumetazoans evolved separately from single-celled organisms, which would mean that the animal kingdom does not form a clade (a complete grouping of all organisms descended from a common ancestor). However, genetic studies and some morphological characteristics, like the common presence of choanocytes, support a common origin.Traditionally, Eumetazoans are a major group of animals in the Five Kingdoms classification of Lynn Margulis and K. V. Schwartz, comprising the Radiata and Bilateria — all animals except the sponges. When treated as a formal taxon Eumetazoa is typically ranked as a subkingdom. The name Metazoa has also been used to refer to this group, but more often refers to the Animalia as a whole. Many classification schemes do not include a subkingdom Eumetazoa.Holozoa
Holozoa is a group of organisms that includes animals and their closest single-celled relatives, but excludes fungi. Holozoa is also an old name for the tunicate genus Distaplia.
Because Holozoa is a clade including all organisms more closely related to animals than to fungi, some authors prefer it to recognizing paraphyletic groups such as Choanozoa, which mostly consists of Holozoa minus animals.Perhaps the best-known holozoans, apart from animals, are the choanoflagellates, which strongly resemble the collar cells of sponges, and so were theorized to be related to sponges even in the 19th century. Proterospongia is an example of a colonial choanoflagellate that may shed light on the origin of sponges.
The affinities of the other single-celled holozoans only began to be recognized in the 1990s. The sub-classification Icthyosporea or Mesomycetozoea contains a number of mostly parasitic species. The amoeboid genera Ministeria and Capsaspora may be united in a group called Filasterea by the structure of their thread-like pseudopods. Along with choanoflagellates, filastereans may be closely related to animals, and one analysis grouped them together as the clade Filozoa.List of Ediacaran genera
This is a list of all described Ediacaran genera, including the Ediacaran biota. 227 genera are contained in this list.Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 541 million years ago.It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran Periods. It is preceded by the Mesoproterozoic era and succeeded by the Paleozoic era.
The most severe glaciation known in the geologic record occurred during the Cryogenian, when ice sheets reached the equator and formed a possible "Snowball Earth".
The earliest fossils of multicellular life are found in the Ediacaran, including the Ediacarans, which were the earliest animals.
According to Rino and co-workers, the sum of the continental crust formed in the Pan-African orogeny and the Grenville orogeny makes the Neoproterozoic the period of Earth's history that has produced most continental crust.Nephrozoa
Nephrozoa is a major clade of bilaterians, divided into the protostomes and the deuterostomes, containing almost all animal phyla and over a million extant species. Its sister clade is the Xenacoelomorpha. The Ambulacraria (conventionally deuterostomes) may actually be sister to the Xenacoelomorpha, forming the Xenambulacraria as basal Deuterostomes, or basal Bilateria invalidating Nephrozoa and Deuterostomes. The coelom, the excretory organs, and nerve cords developed in the Nephrozoa.Chordates (which include all the vertebrates) are deuterostomes. It seems very likely that the 555 million year old Kimberella was a member of the protostomes. If so, this means that the protostome and deuterostome lineages must have split some time before Kimberella appeared — at least 558 million years ago, and hence well before the start of the Cambrian 541 million years ago.Planolites
Planolites is an ichnogenus found throughout the Phanerozoic that is made during the feeding process of worm-like animals. The traces are generally small, 1–5 mm (0.039–0.197 in), unlined, and rarely branched, with fill that differs from the host rock.Protostome
Protostomia (from Greek πρωτο- proto- "first" and στόμα stoma "mouth") is a clade of animals. Together with the deuterostomes and xenacoelomorpha, its members make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers. The major distinctions between deuterostomes and protostomes are found in embryonic development and is based on the embryological origins of the mouth and anus.
In most, but not all protostomes, the mouth forms first, then the anus, whereas the reverse is true in deuterostomes.Small shelly fauna
The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambrian Period. They are very diverse, and there is no formal definition of "small shelly fauna" or "small shelly fossils". Almost all are from earlier rocks than more familiar fossils such as trilobites. Since most SSFs were preserved by being covered quickly with phosphate and this method of preservation is mainly limited to the Late Ediacaran and Early Cambrian periods, the animals that made them may actually have arisen earlier and persisted after this time span.
Some of the fossils represent the entire skeletons of small organisms, including the mysterious Cloudina and some snail-like molluscs. However, the bulk of the fossils are fragments or disarticulated remains of larger organisms, including sponges, molluscs, slug-like halkieriids, brachiopods, echinoderms, and onychophoran-like organisms that may have been close to the ancestors of arthropods.
One of the early explanations for the appearance of the SSFs – and therefore the evolution of mineralized skeletons – suggested a sudden increase in the ocean's concentration of calcium. However, many SSFs are constructed of other minerals, such as silica. Because the first SSFs appear around the same time as organisms first started burrowing to avoid predation, it is more likely that they represent early steps in an evolutionary arms race between predators and increasingly well-defended prey. On the other hand, mineralized skeletons may have evolved simply because they are stronger and cheaper to produce than all-organic skeletons like those of insects. Nevertheless, it is still true that the animals used minerals that were most easily accessible.
Although the small size and often fragmentary nature of SSFs makes it difficult to identify and classify them, they provide very important evidence for how the main groups of marine invertebrates evolved, and particularly for the pace and pattern of evolution in the Cambrian explosion. Besides including the earliest known representatives of some modern phyla, they have the great advantage of presenting a nearly continuous record of Early Cambrian organisms whose bodies include hard parts.Terra Australis Orogen
The Terra Australis Orogen (TAO) was the oceanic southern margin of Gondwana which stretched from South America to Eastern Australia and encompassed South Africa, West Antarctica, New Zealand and Victoria Land in East Antarctica.