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Cambrian explosion

January 31, 2023

For the 20th century coal mine accidents, see Cambrian Colliery.

The Cambrian explosion, Cambrian radiation,[1] Cambrian diversification, or the Biological Big Bang[2] refers to an interval of time approximately 538.8 million years ago in the Cambrian Period when practically all major animal phyla started appearing in the fossil record.[3][4][5] It lasted for about 13[6][7][8] – 25[9][10] million years and resulted in the divergence of most modern metazoan phyla.[11] The event was accompanied by major diversification in other groups of organisms as well.[a]

Before early Cambrian diversification,[b] most organisms were relatively simple, composed of individual cells, or small multicellular organisms, occasionally organized into colonies. As the rate of diversification subsequently accelerated, the variety of life became much more complex, and began to resemble that of today.[13] Almost all present-day animal phyla appeared during this period,[14][15] including the earliest chordates.[16]

A 2019 paper suggests that the timing should be expanded back to include the late Ediacaran, rather than just the narrower timeframe of the "Cambrian Explosion" event visible in the fossil record, based on analysis of chemicals that would have laid the building blocks for a progression of transitional radiations starting with the Ediacaran period and continuing at a similar rate into the Cambrian.[17]

History and significance

Main article: History of life

The seemingly rapid appearance of fossils in the "Primordial Strata" was noted by William Buckland in the 1840s,[18] and in his 1859 book On the Origin of Species, Charles Darwin discussed the then-inexplicable lack of earlier fossils as one of the main difficulties for his theory of descent with slow modification through natural selection.[19] The long-running puzzlement about the seemingly-sudden appearance of the Cambrian fauna without evident precursor(s) centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period during the early Cambrian, what might have caused such rapid change, and what it would imply about the origin of animal life. Interpretation is difficult, owing to a limited supply of evidence based mainly on an incomplete fossil record and chemical signatures remaining in Cambrian rocks.

The first discovered Cambrian fossils were trilobites, described by Edward Lhuyd, the curator of Oxford Museum, in 1698.[20] Although their evolutionary importance was not known, on the basis of their old age, William Buckland (1784–1856) realized that a dramatic step-change in the fossil record had occurred around the base of what we now call the Cambrian.[18] Nineteenth-century geologists such as Adam Sedgwick and Roderick Murchison used the fossils for dating rock strata, specifically for establishing the Cambrian and Silurian periods.[21] By 1859, leading geologists including Roderick Murchison were convinced that what was then called the lowest Silurian stratum showed the origin of life on Earth, though others, including Charles Lyell, differed. In On the Origin of Species, Darwin considered this sudden appearance of a solitary group of trilobites, with no apparent antecedents, and absence of other fossils, to be "undoubtedly of the gravest nature" among the difficulties in his theory of natural selection. He reasoned that earlier seas had swarmed with living creatures, but that their fossils had not been found because of the imperfections of the fossil record.[19] In the sixth edition of his book, he stressed his problem further as:[22]

To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, I can give no satisfactory answer.

American paleontologist Charles Walcott, who studied the Burgess Shale fauna, proposed that an interval of time, the "Lipalian", was not represented in the fossil record or did not preserve fossils, and that the ancestors of the Cambrian animals evolved during this time.[23]

Earlier fossil evidence has since been found. The earliest claim is that the history of life on earth goes back 3,850 million years:[24] Rocks of that age at Warrawoona, Australia, were claimed to contain fossil stromatolites, stubby pillars formed by colonies of microorganisms. Fossils (Grypania) of more complex eukaryotic cells, from which all animals, plants, and fungi are built, have been found in rocks from 1,400 million years ago, in China and Montana. Rocks dating from 580 to 543 million years ago contain fossils of the Ediacara biota, organisms so large that they are likely multicelled, but very unlike any modern organism.[25] In 1948, Preston Cloud argued that a period of "eruptive" evolution occurred in the Early Cambrian,[26] but as recently as the 1970s, no sign was seen of how the 'relatively' modern-looking organisms of the Middle and Late Cambrian arose.[25]

 
Opabinia made the largest single contribution to modern interest in the Cambrian explosion.

The intense modern interest in this "Cambrian explosion" was sparked by the work of Harry B. Whittington and colleagues, who, in the 1970s, reanalysed many fossils from the Burgess Shale and concluded that several were as complex as, but different from, any living animals.[27][28] The most common organism, Marrella, was clearly an arthropod, but not a member of any known arthropod class. Organisms such as the five-eyed Opabinia and spiny slug-like Wiwaxia were so different from anything else known that Whittington's team assumed they must represent different phyla, seemingly unrelated to anything known today. Stephen Jay Gould's popular 1989 account of this work, Wonderful Life,[29] brought the matter into the public eye and raised questions about what the explosion represented. While differing significantly in details, both Whittington and Gould proposed that all modern animal phyla had appeared almost simultaneously in a rather short span of geological period. This view led to the modernization of Darwin's tree of life and the theory of punctuated equilibrium, which Eldredge and Gould developed in the early 1970s and which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change.[30]

Other analyses, some more recent and some dating back to the 1970s, argue that complex animals similar to modern types evolved well before the start of the Cambrian.[31][32][33]

Dating the Cambrian

Radiometric dates for much of the Cambrian, obtained by analysis of radioactive elements contained within rocks, have only recently become available, and for only a few regions.

Relative dating (A was before B) is often assumed sufficient for studying processes of evolution, but this, too, has been difficult, because of the problems involved in matching up rocks of the same age across different continents.[34]

Therefore, dates or descriptions of sequences of events should be regarded with some caution until better data become available. In 2004, the start of the Cambrian was dated to 542 Ma.[35] In 2012, it was revised to 541 Ma[36] then in 2022 it was changed again to 538.8 Ma.[3]

Body fossils

Fossils of organisms' bodies are usually the most informative type of evidence. Fossilization is a rare event, and most fossils are destroyed by erosion or metamorphism before they can be observed. Hence, the fossil record is very incomplete, increasingly so as earlier times are considered. Despite this, they are often adequate to illustrate the broader patterns of life's history.[37] Also, biases exist in the fossil record: different environments are more favourable to the preservation of different types of organism or parts of organisms.[38] Further, only the parts of organisms that were already mineralised are usually preserved, such as the shells of molluscs. Since most animal species are soft-bodied, they decay before they can become fossilised. As a result, although 30-plus phyla of living animals are known, two-thirds have never been found as fossils.[25]

 
This Marrella specimen illustrates how clear and detailed the fossils from the Burgess Shale Lagerstätte actually are as well as the oldest evidence for liquid blood in an animal.

The Cambrian fossil record includes an unusually high number of lagerstätten, which preserve soft tissues. These allow paleontologists to examine the internal anatomy of animals, which in other sediments are only represented by shells, spines, claws, etc. – if they are preserved at all. The most significant Cambrian lagerstätten are the early Cambrian Maotianshan shale beds of Chengjiang (Yunnan, China) and Sirius Passet (Greenland);[39] the middle Cambrian Burgess Shale (British Columbia, Canada);[40] and the late Cambrian Orsten (Sweden) fossil beds.

While lagerstätten preserve far more than the conventional fossil record, they are far from complete. Because lagerstätten are restricted to a narrow range of environments (where soft-bodied organisms can be preserved very quickly, e.g. by mudslides), most animals are probably not represented; further, the exceptional conditions that create lagerstätten probably do not represent normal living conditions.[41] In addition, the known Cambrian lagerstätten are rare and difficult to date, while Precambrian lagerstätten have yet to be studied in detail.

The sparseness of the fossil record means that organisms usually exist long before they are found in the fossil record – this is known as the Signor–Lipps effect.[42]

In 2019, a "stunning" find of lagerstätten, known as the Qingjiang biota, was reported from the Danshui river in Hubei province, China. More than 20,000 fossil specimens were collected, including many soft bodied animals such as jellyfish, sea anemones and worms, as well as sponges, arthropods and algae. In some specimens the internal body structures were sufficiently preserved that soft tissues, including muscles, gills, mouths, guts and eyes, can be seen. The remains were dated to around 518 Mya and around half of the species identified at the time of reporting were previously unknown.[43][44][45]

Trace fossils

 
Rusophycus and other trace fossils from the Gog Group, Middle Cambrian, Lake Louise, Alberta, Canada

Trace fossils consist mainly of tracks and burrows, but also include coprolites (fossil feces) and marks left by feeding.[46][47] Trace fossils are particularly significant because they represent a data source that is not limited to animals with easily fossilized hard parts, and reflects organisms' behaviour. Also, many traces date from significantly earlier than the body fossils of animals that are thought to have been capable of making them.[48] While exact assignment of trace fossils to their makers is generally impossible, traces may, for example, provide the earliest physical evidence of the appearance of moderately complex animals (comparable to earthworms).[47]

Geochemical observations

Main article: Early Cambrian geochemical fluctuations

Several chemical markers indicate a drastic change in the environment around the start of the Cambrian. The markers are consistent with a mass extinction,[49][50] or with a massive warming resulting from the release of methane ice.[51] Such changes may reflect a cause of the Cambrian explosion, although they may also have resulted from an increased level of biological activity – a possible result of the explosion.[51] Despite these uncertainties, the geochemical evidence helps by making scientists focus on theories that are consistent with at least one of the likely environmental changes.

Phylogenetic techniques

Cladistics is a technique for working out the "family tree" of a set of organisms. It works by the logic that, if groups B and C have more similarities to each other than either has to group A, then B and C are more closely related to each other than either is to A. Characteristics that are compared may be anatomical, such as the presence of a notochord, or molecular, by comparing sequences of DNA or protein. The result of a successful analysis is a hierarchy of clades – groups whose members are believed to share a common ancestor. The cladistic technique is sometimes problematic, as some features, such as wings or camera eyes, evolved more than once, convergently – this must be taken into account in analyses.

From the relationships, it may be possible to constrain the date that lineages first appeared. For instance, if fossils of B or C date to X million years ago and the calculated "family tree" says A was an ancestor of B and C, then A must have evolved more than X million years ago.

It is also possible to estimate how long ago two living clades diverged – i.e. about how long ago their last common ancestor must have lived  – by assuming that DNA mutations accumulate at a constant rate. These "molecular clocks", however, are fallible, and provide only a very approximate timing: they are not sufficiently precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved,[52] and estimates produced by different techniques vary by a factor of two.[53] However, the clocks can give an indication of branching rate, and when combined with the constraints of the fossil record, recent clocks suggest a sustained period of diversification through the Ediacaran and Cambrian.[54]

Explanation of key scientific terms

 
Stem groups[55]
  • — = Lines of descent
  •   = Basal node
  •   = Crown node
  •   = Total group
  •   = Crown group
  •   = Stem group

Phylum

A phylum is the highest level in the Linnaean system for classifying organisms. Phyla can be thought of as groupings of animals based on general body plan.[56] Despite the seemingly different external appearances of organisms, they are classified into phyla based on their internal and developmental organizations.[57] For example, despite their obvious differences, spiders and barnacles both belong to the phylum Arthropoda, but earthworms and tapeworms, although similar in shape, belong to different phyla. As chemical and genetic testing becomes more accurate, previously hypothesised phyla are often entirely reworked.

A phylum is not a fundamental division of nature, such as the difference between electrons and protons. It is simply a very high-level grouping in a classification system created to describe all currently living organisms. This system is imperfect, even for modern animals: different books quote different numbers of phyla, mainly because they disagree about the classification of a huge number of worm-like species. As it is based on living organisms, it accommodates extinct organisms poorly, if at all.[25][58]

Stem group

The concept of stem groups was introduced to cover evolutionary "aunts" and "cousins" of living groups, and have been hypothesized based on this scientific theory. A crown group is a group of closely related living animals plus their last common ancestor plus all its descendants. A stem group is a set of offshoots from the lineage at a point earlier than the last common ancestor of the crown group; it is a relative concept, for example tardigrades are living animals that form a crown group in their own right, but Budd (1996) regarded them as also being a stem group relative to the arthropods.[55][59]

 
 
A coelomate animal is basically a set of concentric tubes, with a gap between the gut and the outer tubes.

Triploblastic

The term Triploblastic means consisting of three layers, which are formed in the embryo, quite early in the animal's development from a single-celled egg to a larva or juvenile form. The innermost layer forms the digestive tract (gut); the outermost forms skin; and the middle one forms muscles and all the internal organs except the digestive system. Most types of living animal are triploblastic – the best-known exceptions are Porifera (sponges) and Cnidaria (jellyfish, sea anemones, etc.).

Bilaterian

The bilaterians are animals that have right and left sides at some point in their life histories. This implies that they have top and bottom surfaces and, importantly, distinct front and back ends. All known bilaterian animals are triploblastic, and all known triploblastic animals are bilaterian. Living echinoderms (sea stars, sea urchins, sea cucumbers, etc.) 'look' radially symmetrical (like wheels) rather than bilaterian, but their larvae exhibit bilateral symmetry and some of the earliest echinoderms may have been bilaterally symmetrical.[60] Porifera and Cnidaria are radially symmetrical, not bilaterian, and not triploblastic (but the common Bilateria-Cnidaria ancestor's planula larva is suspected to be bilaterally symmetric).

Coelomate

The term Coelomate means having a body cavity (coelom) containing the internal organs. Most of the phyla featured in the debate about the Cambrian explosion[clarification needed] are coelomates: arthropods, annelid worms, molluscs, echinoderms, and chordates – the noncoelomate priapulids are an important exception. All known coelomate animals are triploblastic bilaterians, but some triploblastic bilaterian animals do not have a coelom – for example flatworms, whose organs are surrounded by unspecialized tissues.

Precambrian life

Evidence of animals around 1 billion years ago

Further information: Acritarch and Stromatolite
 
Stromatolites (Pika Formation, Middle Cambrian) near Helen Lake, Banff National Park, Canada
 
Modern stromatolites in Hamelin Pool Marine Nature Reserve, Western Australia

Changes in the abundance and diversity of some types of fossil have been interpreted as evidence for "attacks" by animals or other organisms. Stromatolites, stubby pillars built by colonies of microorganisms, are a major constituent of the fossil record from about 2,700 million years ago, but their abundance and diversity declined steeply after about 1,250 million years ago. This decline has been attributed to disruption by grazing and burrowing animals.[31][32][61]

Precambrian marine diversity was dominated by small fossils known as acritarchs. This term describes almost any small organic walled fossil – from the egg cases of small metazoans to resting cysts of many different kinds of green algae. After appearing around 2,000 million years ago, acritarchs underwent a boom around 1,000 million years ago, increasing in abundance, diversity, size, complexity of shape, and especially size and number of spines. Their increasingly spiny forms in the last 1 billion years may indicate an increased need for defence against predation. Other groups of small organisms from the Neoproterozoic era also show signs of antipredator defenses.[61] A consideration of taxon longevity appears to support an increase in predation pressure around this time.[62] In general, the fossil record shows a very slow appearance of these lifeforms in the Precambrian, with many cyanobacterial species making up much of the underlying sediment.[63]

Fossils of the Doushantuo formation

Main article: Doushantuo Formation

The layers of the Doushantuo Formation from around 580 million year old[64] harbour microscopic fossils that may represent early bilaterians. Some have been described as animal embryos and eggs, although some may represent the remains of giant bacteria.[65] Another fossil, Vernanimalcula, has been interpreted as a coelomate bilaterian,[66] but may simply be an infilled bubble.[67]

These fossils form the earliest hard-and-fast evidence of animals, as opposed to other predators.[65][68]

Burrows

Main article: Cambrian substrate revolution
 
An Ediacaran trace fossil, made when an organism burrowed below a microbial mat.

The traces of organisms moving on and directly underneath the microbial mats that covered the Ediacaran sea floor are preserved from the Ediacaran period, about 565 million years ago.[c] They were probably made by organisms resembling earthworms in shape, size, and how they moved. The burrow-makers have never been found preserved, but, because they would need a head and a tail, the burrowers probably had bilateral symmetry – which would in all probability make them bilaterian animals.[71] They fed above the sediment surface, but were forced to burrow to avoid predators.[72]

Around the start of the Cambrian (about 539 million years ago), many new types of traces first appear, including well-known vertical burrows such as Diplocraterion and Skolithos, and traces normally attributed to arthropods, such as Cruziana and Rusophycus. The vertical burrows indicate that worm-like animals acquired new behaviours, and possibly new physical capabilities. Some Cambrian trace fossils indicate that their makers possessed hard exoskeletons, although they were not necessarily mineralised.[70]

Burrows provide firm evidence of complex organisms; they are also much more readily preserved than body fossils, to the extent that the absence of trace fossils has been used to imply the genuine absence of large, motile, bottom-dwelling organisms.[citation needed] They provide a further line of evidence to show that the Cambrian explosion represents a real diversification, and is not a preservational artifact.[73]

This new habit changed the seafloor's geochemistry, and led to decreased oxygen in the ocean and increased CO2 levels in the seas and the atmosphere, resulting in global warming for tens of millions years, and could be responsible for mass extinctions.[74] But as burrowing became established, it allowed an explosion of its own, for as burrowers disturbed the sea floor, they aerated it, mixing oxygen into the toxic muds. This made the bottom sediments more hospitable, and allowed a wider range of organisms to inhabit them – creating new niches and the scope for higher diversity.[73]

Ediacaran organisms

 
Dickinsonia costata, an Ediacaran organism of unknown affinity, with a quilted appearance
Main articles: Ediacaran biota, Cloudinidae, Kimberella, and Spriggina

At the start of the Ediacaran period, much of the acritarch fauna, which had remained relatively unchanged for hundreds of millions of years, became extinct, to be replaced with a range of new, larger species, which would prove far more ephemeral.[63] This radiation, the first in the fossil record,[63] is followed soon after by an array of unfamiliar, large fossils dubbed the Ediacara biota,[75] which flourished for 40 million years until the start of the Cambrian.[76] Most of this "Ediacara biota" were at least a few centimeters long, significantly larger than any earlier fossils. The organisms form three distinct assemblages, increasing in size and complexity as time progressed.[77]

Many of these organisms were quite unlike anything that appeared before or since, resembling discs, mud-filled bags, or quilted mattresses – one palæontologist proposed that the strangest organisms should be classified as a separate kingdom, Vendozoa.[78]

 
Fossil of Kimberella, a triploblastic bilaterian, and possibly a mollusc

At least some may have been early forms of the phyla at the heart of the "Cambrian explosion" debate,[clarification needed] having been interpreted as early molluscs (Kimberella),[33][79] echinoderms (Arkarua);[80] and arthropods (Spriggina,[81] Parvancorina,[82] Yilingia). Still, debate exists about the classification of these specimens, mainly because the diagnostic features that allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the ediacarans.[83] However, there seems little doubt that Kimberella was at least a triploblastic bilaterian animal.[83] These organisms are central to the debate about how abrupt the Cambrian explosion was.[citation needed] If some were early members of the animal phyla seen today, the "explosion" looks a lot less sudden than if all these organisms represent an unrelated "experiment", and were replaced by the animal kingdom fairly soon thereafter (40M years is "soon" by evolutionary and geological standards).

Beck Spring Dolomite

Paul Knauth, a geologist at Arizona State University, maintains that photosynthesizing organisms such as algae may have grown over a 750- to 800-million-year-old formation in Death Valley known as the Beck Spring Dolomite. In the early 1990s, samples from this 1,000-foot thick layer of dolomite revealed that the region housed flourishing mats of photosynthesizing, unicellular life forms which antedated the Cambrian explosion.

Microfossils have been unearthed from holes riddling the otherwise barren surface of the dolomite. These geochemical and microfossil findings support the idea that during the Precambrian period, complex life evolved both in the oceans and on land. Knauth contends that animals may well have had their origins in freshwater lakes and streams, and not in the oceans.

Some 30 years later, a number of studies have documented an abundance of geochemical and microfossil evidence showing that life covered the continents as far back as 2.2 billion years ago. Many paleobiologists now accept the idea that simple life forms existed on land during the Precambrian, but are opposed to the more radical idea that multicellular life thrived on land more than 600 million years ago.[84]

Ediacaran–Early Cambrian skeletonisation

The first Ediacaran and lowest Cambrian (Nemakit-Daldynian) skeletal fossils represent tubes and problematic sponge spicules.[85] The oldest sponge spicules are monaxon siliceous, aged around 580 million years ago, known from the Doushantou Formation in China and from deposits of the same age in Mongolia, although the interpretation of these fossils as spicules has been challenged.[86] In the late Ediacaran-lowest Cambrian, numerous tube dwellings of enigmatic organisms appeared. It was organic-walled tubes (e.g. Saarina) and chitinous tubes of the sabelliditids (e.g. Sokoloviina, Sabellidites, Paleolina)[87][88] that prospered up to the beginning of the Tommotian. The mineralized tubes of Cloudina, Namacalathus, Sinotubulites, and a dozen more of the other organisms from carbonate rocks formed near the end of the Ediacaran period from 549 to 542 million years ago, as well as the triradially symmetrical mineralized tubes of anabaritids (e.g. Anabarites, Cambrotubulus) from uppermost Ediacaran and lower Cambrian.[89] Ediacaran mineralized tubes are often found in carbonates of the stromatolite reefs and thrombolites,[90][91] i.e. they could live in an environment adverse to the majority of animals.

Although they are as hard to classify as most other Ediacaran organisms, they are important in two other ways. First, they are the earliest known calcifying organisms (organisms that built shells from calcium carbonate).[91][92][93] Secondly, these tubes are a device to rise over a substrate and competitors for effective feeding and, to a lesser degree, they serve as armor for protection against predators and adverse conditions of environment. Some Cloudina fossils show small holes in shells. The holes possibly are evidence of boring by predators sufficiently advanced to penetrate shells.[94] A possible "evolutionary arms race" between predators and prey is one of the hypotheses that attempt to explain the Cambrian explosion.[61]

In the lowest Cambrian, the stromatolites were decimated. This allowed animals to begin colonization of warm-water pools with carbonate sedimentation. At first, it was anabaritids and Protohertzina (the fossilized grasping spines of chaetognaths) fossils. Such mineral skeletons as shells, sclerites, thorns, and plates appeared in uppermost Nemakit-Daldynian; they were the earliest species of halkierids, gastropods, hyoliths and other rare organisms. The beginning of the Tommotian has historically been understood to mark an explosive increase of the number and variety of fossils of molluscs, hyoliths, and sponges, along with a rich complex of skeletal elements of unknown animals, the first archaeocyathids, brachiopods, tommotiids, and others.[95][96][97][98] Also soft-bodied extant phyla such as comb jellies, scalidophorans, entoproctans, horseshoe worms and lobopodians had armored forms.[99] This sudden increase is partially an artefact of missing strata at the Tommotian type section, and most of this fauna in fact began to diversify in a series of pulses through the Nemakit-Daldynian and into the Tommotian.[100]

Some animals may already have had sclerites, thorns, and plates in the Ediacaran (e.g. Kimberella had hard sclerites, probably of carbonate), but thin carbonate skeletons cannot be fossilized in siliciclastic deposits.[101] Older (~750 Ma) fossils indicate that mineralization long preceded the Cambrian, probably defending small photosynthetic algae from single-celled eukaryotic predators.[102][103]

Cambrian life

Trace fossils

Trace fossils (burrows, etc.) are a reliable indicator of what life was around, and indicate a diversification of life around the start of the Cambrian, with the freshwater realm colonized by animals almost as quickly as the oceans.[104]

Small shelly fauna

Main article: Small shelly fauna

Fossils known as "small shelly fauna" have been found in many parts on the world, and date from just before the Cambrian to about 10 million years after the start of the Cambrian (the Nemakit-Daldynian and Tommotian ages; see timeline). These are a very mixed collection of fossils: spines, sclerites (armor plates), tubes, archeocyathids (sponge-like animals), and small shells very like those of brachiopods and snail-like molluscs – but all tiny, mostly 1 to 2 mm long.[105]

While small, these fossils are far more common than complete fossils of the organisms that produced them; crucially, they cover the window from the start of the Cambrian to the first lagerstätten: a period of time otherwise lacking in fossils. Hence, they supplement the conventional fossil record and allow the fossil ranges of many groups to be extended.

Early Cambrian trilobites and echinoderms

 
A fossilized trilobite, an ancient type of arthropod: This specimen, from the Burgess Shale, preserves "soft parts" – the antennae and legs.

The earliest trilobite fossils are about 530 million years old, but the class was already quite diverse and cosmopolitan, suggesting they had been around for quite some time.[106] The fossil record of trilobites began with the appearance of trilobites with mineral exoskeletons – not from the time of their origin.

The earliest generally accepted echinoderm fossils appeared a little bit later, in the Late Atdabanian; unlike modern echinoderms, these early Cambrian echinoderms were not all radially symmetrical.[107]

These provide firm data points for the "end" of the explosion, or at least indications that the crown groups of modern phyla were represented.

Burgess Shale type faunas

Main article: Burgess Shale-type preservation

The Burgess Shale and similar lagerstätten preserve the soft parts of organisms, which provide a wealth of data to aid in the classification of enigmatic fossils. It often preserved complete specimens of organisms only otherwise known from dispersed parts, such as loose scales or isolated mouthparts. Further, the majority of organisms and taxa in these horizons are entirely soft-bodied, hence absent from the rest of the fossil record.[108] Since a large part of the ecosystem is preserved, the ecology of the community can also be tentatively reconstructed.[verification needed] However, the assemblages may represent a "museum": a deep-water ecosystem that is evolutionarily "behind" the rapidly diversifying fauna of shallower waters.[109]

Because the lagerstätten provide a mode and quality of preservation that is virtually absent outside of the Cambrian, many organisms appear completely different from anything known from the conventional fossil record. This led early workers in the field to attempt to shoehorn the organisms into extant phyla; the shortcomings of this approach led later workers to erect a multitude of new phyla to accommodate all the oddballs. It has since been realised that most oddballs diverged from lineages before they established the phyla known today[clarification needed] – slightly different designs, which were fated to perish rather than flourish into phyla, as their cousin lineages did.

The preservational mode is rare in the preceding Ediacaran period, but those assemblages known show no trace of animal life – perhaps implying a genuine absence of macroscopic metazoans.[110]

Early Cambrian crustaceans

Further information: Orsten

Crustaceans, one of the four great modern groups of arthropods, are very rare throughout the Cambrian. Convincing crustaceans were once thought to be common in Burgess Shale-type biotas, but none of these individuals can be shown to fall into the crown group of "true crustaceans".[111] The Cambrian record of crown-group crustaceans comes from microfossils. The Swedish Orsten horizons contain later Cambrian crustaceans, but only organisms smaller than 2 mm are preserved. This restricts the data set to juveniles and miniaturised adults.

A more informative data source is the organic microfossils of the Mount Cap formation, Mackenzie Mountains, Canada. This late Early Cambrian assemblage (510 to 515 million years ago) consists of microscopic fragments of arthropods' cuticle, which is left behind when the rock is dissolved with hydrofluoric acid. The diversity of this assemblage is similar to that of modern crustacean faunas. Analysis of fragments of feeding machinery found in the formation shows that it was adapted to feed in a very precise and refined fashion. This contrasts with most other early Cambrian arthropods, which fed messily by shovelling anything they could get their feeding appendages on into their mouths. This sophisticated and specialised feeding machinery belonged to a large (about 30 cm)[112] organism, and would have provided great potential for diversification: Specialised feeding apparatus allows a number of different approaches to feeding and development, and creates a number of different approaches to avoid being eaten.[111]

Early Ordovician radiation

Main article: Great Ordovician Biodiversification Event

After an extinction at the Cambrian–Ordovician boundary, another radiation occurred, which established the taxa that would dominate the Palaeozoic.[113]

During this radiation, the total number of orders doubled, and families tripled,[113] increasing marine diversity to levels typical of the Palaeozoic,[51] and disparity to levels approximately equivalent to today's.[13]

Stages

The early Cambrian interval of diversification lasted for about the next 20[7][114]–25[9][115] million years, and its elevated rates of evolution had ended by the base of Cambrian Series 2, 521 million years ago, coincident with the first trilobites in the fossil record.[116] Different authors define intervals of diversification during the early Cambrian different ways:

Ed Landing recognizes three stages: Stage 1, spanning the Ediacaran-Cambrian boundary, corresponds to a diversification of biomineralizing animals and of deep and complex burrows; Stage 2, corresponding to the radiation of molluscs and stem-group Brachiopods (hyoliths and tommotiids), which apparently arose in intertidal waters; and Stage 3, seeing the Atdabanian diversification of trilobites in deeper waters, but little change in the intertidal realm.[117]

Graham Budd synthesises various schemes to produce a compatible view of the SSF record of the Cambrian explosion, divided slightly differently into four intervals: a "Tube world", lasting from 550 to 536 million years ago, spanning the Ediacaran-Cambrian boundary, dominated by Cloudina, Namacalathus and pseudoconodont-type elements; a "Sclerite world", seeing the rise of halkieriids, tommotiids, and hyoliths, lasting to the end of the Fortunian (c. 525 Ma); a brachiopod world, perhaps corresponding to the as yet unratified Cambrian Stage 2; and Trilobite World, kicking off in Stage 3.[118]

Complementary to the shelly fossil record, trace fossils can be divided into five subdivisions: "Flat world" (late Ediacaran), with traces restricted to the sediment surface; Protreozoic III (after Jensen), with increasing complexity; pedum world, initiated at the base of the Cambrian with the base of the T.pedum zone (see Cambrian#Dating the Cambrian); Rusophycus world, spanning 536 to 521 million years ago and thus corresponding exactly to the periods of Sclerite World and Brachiopod World under the SSF paradigm; and Cruziana world, with an obvious correspondence to Trilobite World. [118]

Validity

There is strong evidence for species of Cnidaria and Porifera existing in the Ediacaran[119] and possible members of Porifera even before that during the Cryogenian.[120] Bryozoans, once thought to not appear in the fossil record until after the Cambrian, are now known from strata of Cambrian Age 3 from Australia and South China.[121]

The fossil record as Darwin knew it seemed to suggest that the major metazoan groups appeared in a few million years of the early to mid-Cambrian, and even in the 1980s, this still appeared to be the case.[28][29]

However, evidence of Precambrian Metazoa is gradually accumulating. If the Ediacaran Kimberella was a mollusc-like protostome (one of the two main groups of coelomates),[33][79] the protostome and deuterostome lineages must have split significantly before 550 million years ago (deuterostomes are the other main group of coelomates).[122] Even if it is not a protostome, it is widely accepted as a bilaterian.[83][122] Since fossils of rather modern-looking cnidarians (jellyfish-like organisms) have been found in the Doushantuo lagerstätte, the cnidarian and bilaterian lineages must have diverged well over 580 million years ago.[122]

Trace fossils[77] and predatory borings in Cloudina shells provide further evidence of Ediacaran animals.[123] Some fossils from the Doushantuo formation have been interpreted as embryos and one (Vernanimalcula) as a bilaterian coelomate, although these interpretations are not universally accepted.[66][67][124] Earlier still, predatory pressure has acted on stromatolites and acritarchs since around 1,250 million years ago.[61]

Some say that the evolutionary change was accelerated by an order of magnitude,[d] but the presence of Precambrian animals somewhat dampens the "bang" of the explosion; not only was the appearance of animals gradual, but their evolutionary radiation ("diversification") may also not have been as rapid as once thought. Indeed, statistical analysis shows that the Cambrian explosion was no faster than any of the other radiations in animals' history.[e] However, it does seem that some innovations linked to the explosion – such as resistant armour – only evolved once in the animal lineage; this makes a lengthy Precambrian animal lineage harder to defend.[126] Further, the conventional view that all the phyla arose in the Cambrian is flawed; while the phyla may have diversified in this time period, representatives of the crown groups of many phyla do not appear until much later in the Phanerozoic.[14] Further, the mineralised phyla that form the basis of the fossil record may not be representative of other phyla, since most mineralised phyla originated in a benthic setting. The fossil record is consistent with a Cambrian explosion that was limited to the benthos, with pelagic phyla evolving much later.[14]

Ecological complexity among marine animals increased in the Cambrian, as well later in the Ordovician.[13] However, recent research has overthrown the once-popular idea that disparity was exceptionally high throughout the Cambrian, before subsequently decreasing.[127] In fact, disparity remains relatively low throughout the Cambrian, with modern levels of disparity only attained after the early Ordovician radiation.[13]

The diversity of many Cambrian assemblages is similar to today's,[128][111] and at a high (class/phylum) level, diversity is thought by some to have risen relatively smoothly through the Cambrian, stabilizing somewhat in the Ordovician.[129] This interpretation, however, glosses over the astonishing and fundamental pattern of basal polytomy and phylogenetic telescoping at or near the Cambrian boundary, as seen in most major animal lineages.[130] Thus Harry Blackmore Whittington's questions regarding the abrupt nature of the Cambrian explosion remain, and have yet to be satisfactorily answered.[131]

The Cambrian explosion as survivorship bias

Budd and Mann[132] suggested that the Cambrian explosion was the result of a type of survivorship bias called the "Push of the past". As groups at their origin tend to go extinct, it follows that any long-lived group would have experienced an unusually rapid rate of diversification early on, creating the illusion of a general speed-up in diversification rates. However, rates of diversification could remain at background levels and still generate this sort of effect in the surviving lineages.

Possible causes

Despite the evidence that moderately complex animals (triploblastic bilaterians) existed before and possibly long before the start of the Cambrian, it seems that the pace of evolution was exceptionally fast in the early Cambrian. Possible explanations for this fall into three broad categories: environmental, developmental, and ecological changes. Any explanation must explain both the timing and magnitude of the explosion.

Changes in the environment

Increase in oxygen levels

Earth's earliest atmosphere contained no free oxygen (O2); the oxygen that animals breathe today, both in the air and dissolved in water, is the product of billions of years of photosynthesis. Cyanobacteria were the first organisms to evolve the ability to photosynthesize, introducing a steady supply of oxygen into the environment.[133] Initially, oxygen levels did not increase substantially in the atmosphere.[134] The oxygen quickly reacted with iron and other minerals in the surrounding rock and ocean water. Once a saturation point was reached for the reactions in rock and water, oxygen was able to exist as a gas in its diatomic form. Oxygen levels in the atmosphere increased substantially afterward.[135] As a general trend, the concentration of oxygen in the atmosphere has risen gradually over about the last 2.5 billion years.[25]

Oxygen levels seem to have a positive correlation with diversity in eukaryotes well before the Cambrian period.[136] The last common ancestor of all extant eukaryotes is thought to have lived around 1.8 billion years ago. Around 800 million years ago, there was a notable increase in the complexity and number of eukaryotes species in the fossil record.[136] Before the spike in diversity, eukaryotes are thought to have lived in highly sulfuric environments. Sulfide interferes with mitochondrial function in aerobic organisms, limiting the amount of oxygen that could be used to drive metabolism. Oceanic sulfide levels decreased around 800 million years ago, which supports the importance of oxygen in eukaryotic diversity.[136] The increased ventilation of the oceans by sponges, which had already evolved and diversified during the Neoproterozoic, has been proposed to have increased the availability of oxygen and powered the Cambrian's rapid diversification of multicellular life.[137][138]

The shortage of oxygen might well have prevented the rise of large, complex animals. The amount of oxygen an animal can absorb is largely determined by the area of its oxygen-absorbing surfaces (lungs and gills in the most complex animals; the skin in less complex ones), while the amount needed is determined by its volume, which grows faster than the oxygen-absorbing area if an animal's size increases equally in all directions. An increase in the concentration of oxygen in air or water would increase the size to which an organism could grow without its tissues becoming starved of oxygen. However, members of the Ediacara biota reached metres in length tens of millions of years before the Cambrian explosion.[49] Other metabolic functions may have been inhibited by lack of oxygen, for example the construction of tissue such as collagen, which is required for the construction of complex structures,[139] or the biosynthesis of molecules for the construction of a hard exoskeleton.[140] However, animals were not affected when similar oceanographic conditions occurred in the Phanerozoic; therefore, some see no forcing role of the oxygen level on evolution.[141]

Ozone formation

The amount of ozone (O3) required to shield Earth from biologically lethal UV radiation, wavelengths from 200 to 300 nanometers (nm), is believed to have been in existence around the Cambrian explosion.[142] The presence of the ozone layer may have enabled the development of complex life and life on land, as opposed to life being restricted to the water.

Snowball Earth

Main article: Snowball Earth

In the late Neoproterozoic (extending into the early Ediacaran period), the Earth suffered massive glaciations in which most of its surface was covered by ice. This may have caused a mass extinction, creating a genetic bottleneck; the resulting diversification may have given rise to the Ediacara biota, which appears soon after the last "Snowball Earth" episode.[143] However, the snowball episodes occurred a long time before the start of the Cambrian, and it is difficult to see how so much diversity could have been caused by even a series of bottlenecks;[51] the cold periods may even have delayed the evolution of large size organisms.[61] Massive rock erosion caused by glaciers during the "Snowball Earth" may have deposited nutrient-rich sediments into the oceans, setting the stage for the Cambrian explosion.[144]

Increase in the calcium concentration of the Cambrian seawater

Newer research suggests that volcanically active midocean ridges caused a massive and sudden surge of the calcium concentration in the oceans, making it possible for marine organisms to build skeletons and hard body parts.[145] Alternatively a high influx of ions could have been provided by the widespread erosion that produced Powell's Great Unconformity.[146]

An increase of calcium may also have been caused by erosion of the Transgondwanan Supermountain that existed at the time of the explosion. The roots of the mountain are preserved in present-day East Africa as an orogen.[147]

Developmental explanations

Further information: Evolutionary developmental biology

A range of theories are based on the concept that minor modifications to animals' development as they grow from embryo to adult may have been able to cause very large changes in the final adult form. The Hox genes, for example, control which organs individual regions of an embryo will develop into. For instance, if a certain Hox gene is expressed, a region will develop into a limb; if a different Hox gene is expressed in that region (a minor change), it could develop into an eye instead (a phenotypically major change).

Such a system allows a large range of disparity to appear from a limited set of genes, but such theories linking this with the explosion struggle to explain why the origin of such a development system should by itself lead to increased diversity or disparity. Evidence of Precambrian metazoans[51] combines with molecular data[148] to show that much of the genetic architecture that could feasibly have played a role in the explosion was already well established by the Cambrian.

This apparent paradox is addressed in a theory that focuses on the physics of development. It is proposed that the emergence of simple multicellular forms provided a changed context and spatial scale in which novel physical processes and effects were mobilized by the products of genes that had previously evolved to serve unicellular functions. Morphological complexity (layers, segments, lumens, appendages) arose, in this view, by self-organization.[149]

Horizontal gene transfer has also been identified as a possible factor in the rapid acquisition of the biochemical capability of biomineralization among organisms during this period, based on evidence that the gene for a critical protein in the process was originally transferred from a bacterium into sponges.[150]

Ecological explanations

These focus on the interactions between different types of organism. Some of these hypotheses deal with changes in the food chain; some suggest arms races between predators and prey, and others focus on the more general mechanisms of coevolution. Such theories are well suited to explaining why there was a rapid increase in both disparity and diversity, but they do not explain why the "explosion" happened when it did.[51]

End-Ediacaran mass extinction

Main article: End-Ediacaran extinction

Evidence for such an extinction includes the disappearance from the fossil record of the Ediacara biota and shelly fossils such as Cloudina, and the accompanying perturbation in the δ13C record. It is suspected that several global anoxic events were responsible for the extinction.[151][152]

Mass extinctions are often followed by adaptive radiations as existing clades expand to occupy the ecospace emptied by the extinction. However, once the dust had settled, overall disparity and diversity returned to the pre-extinction level in each of the Phanerozoic extinctions.[51]

Anoxia

The late Ediacaran oceans appears to have suffered from an anoxia that covered much of the seafloor, which would have given mobile animals with the ability to seek out more oxygen-rich environments an advantage over sessile forms of life.[153]

Evolution of eyes

Main article: Evolution of the eye

Andrew Parker has proposed that predator-prey relationships changed dramatically after eyesight evolved. Prior to that time, hunting and evading were both close-range affairs – smell, vibration, and touch were the only senses used. When predators could see their prey from a distance, new defensive strategies were needed. Armor, spines, and similar defenses may also have evolved in response to vision. He further observed that, where animals lose vision in unlighted environments such as caves, diversity of animal forms tends to decrease.[154] Nevertheless, many scientists doubt that vision could have caused the explosion. Eyes may well have evolved long before the start of the Cambrian.[155] It is also difficult to understand why the evolution of eyesight would have caused an explosion, since other senses, such as smell and pressure detection, can detect things at a greater distance in the sea than sight can; but the appearance of these other senses apparently did not cause an evolutionary explosion.[51]

Arms races between predators and prey

The ability to avoid or recover from predation often makes the difference between life and death, and is therefore one of the strongest components of natural selection. The pressure to adapt is stronger on the prey than on the predator: if the predator fails to win a contest, it loses a meal; if the prey is the loser, it loses its life.[156]

But, there is evidence that predation was rife long before the start of the Cambrian, for example in the increasingly spiny forms of acritarchs, the holes drilled in Cloudina shells, and traces of burrowing to avoid predators. Hence, it is unlikely that the appearance of predation was the trigger for the Cambrian "explosion", although it may well have exhibited a strong influence on the body forms that the "explosion" produced.[61] However, the intensity of predation does appear to have increased dramatically during the Cambrian[157] as new predatory "tactics" (such as shell-crushing) emerged.[158] This rise of predation during the Cambrian was confirmed by the temporal pattern of the median predator ratio at the scale of genus, in fossil communities covering the Cambrian and Ordovician periods, but this pattern is not correlated to diversification rate.[159] This lack of correlation between predator ratio and diversification over the Cambrian and Ordovician suggests that predators did not trigger the large evolutionary radiation of animals during this interval. Thus the role of predators as triggerers of diversification may have been limited to the very beginning of the "Cambrian explosion".[159]

Increase in size and diversity of planktonic animals

Geochemical evidence strongly indicates that the total mass of plankton has been similar to modern levels since early in the Proterozoic. Before the start of the Cambrian, their corpses and droppings were too small to fall quickly towards the seabed, since their drag was about the same as their weight. This meant they were destroyed by scavengers or by chemical processes before they reached the sea floor.[41]

Mesozooplankton are plankton of a larger size. Early Cambrian specimens filtered microscopic plankton from the seawater. These larger organisms would have produced droppings and ultimately corpses large enough to fall fairly quickly. This provided a new supply of energy and nutrients to the mid-levels and bottoms of the seas, which opened up a new range of possible ways of life. If any of these remains sank uneaten to the sea floor they could be buried; this would have taken some carbon out of circulation, resulting in an increase in the concentration of breathable oxygen in the seas (carbon readily combines with oxygen).[41]

The initial herbivorous mesozooplankton were probably larvae of benthic (seafloor) animals. A larval stage was probably an evolutionary innovation driven by the increasing level of predation at the seafloor during the Ediacaran period.[12][160]

Metazoans have an amazing ability to increase diversity through coevolution.[63] This means that an organism's traits can lead to traits evolving in other organisms; a number of responses are possible, and a different species can potentially emerge from each one. As a simple example, the evolution of predation may have caused one organism to develop a defence, while another developed motion to flee. This would cause the predator lineage to diverge into two species: one that was good at chasing prey, and another that was good at breaking through defences. Actual coevolution is somewhat more subtle, but, in this fashion, great diversity can arise: three quarters of living species are animals, and most of the rest have formed by coevolution with animals.[63]

Ecosystem engineering

Evolving organisms inevitably change the environment they evolve in. The Devonian colonization of land had planet-wide consequences for sediment cycling and ocean nutrients, and was likely linked to the Devonian mass extinction. A similar process may have occurred on smaller scales in the oceans, with, for example, the sponges filtering particles from the water and depositing them in the mud in a more digestible form; or burrowing organisms making previously unavailable resources available for other organisms.[161]

Complexity threshold

The explosion may not have been a significant evolutionary event. It may represent a threshold being crossed: for example a threshold in genetic complexity that allowed a vast range of morphological forms to be employed.[162] This genetic threshold may have a correlation to the amount of oxygen available to organisms. Using oxygen for metabolism produces much more energy than anaerobic processes. Organisms that use more oxygen have the opportunity to produce more complex proteins, providing a template for further evolution.[134] These proteins translate into larger, more complex structures that allow organisms better to adapt to their environments.[163] With the help of oxygen, genes that code for these proteins could contribute to the expression of complex traits more efficiently. Access to a wider range of structures and functions would allow organisms to evolve in different directions, increasing the number of niches that could be inhabited. Furthermore, organisms had the opportunity to become more specialized in their own niches.[163]

Uniqueness of the early Cambrian biodiversification

The "Cambrian explosion" can be viewed as two waves of metazoan expansion into empty niches: first, a coevolutionary rise in diversity as animals explored niches on the Ediacaran sea floor, followed by a second expansion in the early Cambrian as they became established in the water column.[63] The rate of diversification seen in the Cambrian phase of the explosion is unparalleled among marine animals: it affected all metazoan clades of which Cambrian fossils have been found. Later radiations, such as those of fish in the Silurian and Devonian periods, involved fewer taxa, mainly with very similar body plans.[25] Although the recovery from the Permian-Triassic extinction started with about as few animal species as the Cambrian explosion, the recovery produced far fewer significantly new types of animals.[164]

Whatever triggered the early Cambrian diversification opened up an exceptionally wide range of previously unavailable ecological niches. When these were all occupied, limited space existed for such wide-ranging diversifications to occur again, because strong competition existed in all niches and incumbents usually had the advantage. If a wide range of empty niches had continued, clades would be able to continue diversifying and become disparate enough for us to recognise them as different phyla; when niches are filled, lineages will continue to resemble one another long after they diverge, as limited opportunity exists for them to change their life-styles and forms.[165]

There were two similar explosions in the evolution of land plants: after a cryptic history beginning about 450 million years ago, land plants underwent a uniquely rapid adaptive radiation during the Devonian period, about 400 million years ago.[25] Furthermore, angiosperms (flowering plants) originated and rapidly diversified during the Cretaceous period.

Footnotes

  1. ^ This included at least animals, phytoplankton and calcimicrobes.[12]
  2. ^ At 610 million years ago, Aspidella disks appeared, but it is not clear that these represented complex life forms.
  3. ^ Older marks found in billion-year-old rocks[69] have since been recognised as nonbiogenic.[14][70]
  4. ^ As defined in terms of the extinction and origination rate of species.[63]
  5. ^ The analysis considered the bioprovinciality of trilobite lineages, as well as their evolutionary rate.[125]

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Further reading

  • Budd, G. E.; Jensen, J. (2000). "A critical reappraisal of the fossil record of the bilaterian phyla". Biological Reviews. 75 (2): 253–295. doi:10.1111/j.1469-185X.1999.tb00046.x. PMID 10881389. S2CID 39772232.
  • Collins, Allen G. "Metazoa: Fossil record". Retrieved Dec. 14, 2005.
  • Conway Morris, S. (1997). The Crucible of Creation: the Burgess Shale and the rise of animals. Oxford University Press. ISBN 0-19-286202-2.
  • Conway Morris, S. (June 2006). "Darwin's dilemma: the realities of the Cambrian 'explosion'". Philosophical Transactions of the Royal Society B: Biological Sciences. 361 (1470): 1069–1083. doi:10.1098/rstb.2006.1846. ISSN 0962-8436. PMC 1578734. PMID 16754615. An enjoyable account.
  • Gould, S.J. (1989). Wonderful Life: The Burgess Shale and the Nature of History. W.W. Norton & Company. Bibcode:1989wlbs.book.....G.
  • Kennedy, M.; M. Droser; L. Mayer.; D. Pevear & D. Mrofka (2006). "Clay and Atmospheric Oxygen". Science. 311 (5766): 1341. doi:10.1126/science.311.5766.1341c. S2CID 220101640.
  • Knoll, A.H.; Carroll, S.B. (1999-06-25). "Early Animal Evolution: Emerging Views from Comparative Biology and Geology". Science. 284 (5423): 2129–37. doi:10.1126/science.284.5423.2129. PMID 10381872. S2CID 8908451.
  • Markov, Alexander V.; Korotayev, Andrey V. (2007). "Phanerozoic marine biodiversity follows a hyperbolic trend". Palaeoworld. 16 (4): 311–318. doi:10.1016/j.palwor.2007.01.002.
  • Montenari, M.; Leppig, U. (2003). "The Acritarcha: their classification morphology, ultrastructure and palaeoecological/palaeogeographical distribution". Paläontologische Zeitschrift. 77: 173–194. doi:10.1007/bf03004567. S2CID 127238427.
  • Wang, D. Y.-C.; S. Kumar; S. B. Hedges (January 1999). "Divergence time estimates for the early history of animal phyla and the origin of plants, animals and fungi". Proceedings of the Royal Society B. 266 (1415): 163–71. doi:10.1098/rspb.1999.0617. ISSN 0962-8452. PMC 1689654. PMID 10097391.
  • Wood, Rachel A., "The Rise of Animals: New fossils and analyses of ancient ocean chemistry reveal the surprisingly deep roots of the Cambrian explosion", Scientific American, vol. 320, no. 6 (June 2019), pp. 24–31.
  • Xiao, S.; Y. Zhang & A. Knoll (January 1998). "Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite". Nature. 391 (1): 553–58. Bibcode:1998Natur.391..553X. doi:10.1038/35318. ISSN 0090-9556. S2CID 4350507.

Timeline References:

  • Martin, M.W; Grazhdankin, D.V; Bowring, S.A; Evans, D.A.D; Fedonkin, M.A; Kirschvink, J.L (2000). "Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution". Science. 288 (5467): 841–845. Bibcode:2000Sci...288..841M. doi:10.1126/science.288.5467.841. PMID 10797002.

External links

  • by Stephen Jay Gould
  • Conway Morris, S. (April 2000). "The Cambrian "explosion": Slow-fuse or megatonnage?". Proceedings of the National Academy of Sciences. 97 (9): 4426–4429. Bibcode:2000PNAS...97.4426C. doi:10.1073/pnas.97.9.4426. PMC 34314. PMID 10781036.
  • The Cambrian Explosion – In Our Time, BBC Radio 4 broadcast, 17 February 2005
  • "Burgess Shale". Virtual Museum of Canada. 2011., exhaustive details about the Burgess Shale, its fossils, and its significance for the Cambrian explosion
  • Utah's Cambrian life – new (2008) website with good images of a range of Burgess-shale-type and other Cambrian fossils
  • Smithsonian National Museum

January 31, 2023
cambrian, explosion, 20th, century, coal, mine, accidents, cambrian, colliery, cambrian, radiation, cambrian, diversification, biological, bang, refers, interval, time, approximately, million, years, cambrian, period, when, practically, major, animal, phyla, s. For the 20th century coal mine accidents see Cambrian Colliery The Cambrian explosion Cambrian radiation 1 Cambrian diversification or the Biological Big Bang 2 refers to an interval of time approximately 538 8 million years ago in the Cambrian Period when practically all major animal phyla started appearing in the fossil record 3 4 5 It lasted for about 13 6 7 8 25 9 10 million years and resulted in the divergence of most modern metazoan phyla 11 The event was accompanied by major diversification in other groups of organisms as well a Before early Cambrian diversification b most organisms were relatively simple composed of individual cells or small multicellular organisms occasionally organized into colonies As the rate of diversification subsequently accelerated the variety of life became much more complex and began to resemble that of today 13 Almost all present day animal phyla appeared during this period 14 15 including the earliest chordates 16 A 2019 paper suggests that the timing should be expanded back to include the late Ediacaran rather than just the narrower timeframe of the Cambrian Explosion event visible in the fossil record based on analysis of chemicals that would have laid the building blocks for a progression of transitional radiations starting with the Ediacaran period and continuing at a similar rate into the Cambrian 17 Key Cambrian explosion eventsThis box viewtalkedit 590 580 570 560 550 540 530 520 510 500 490 NeoproterozoicPaleozoicEdiacaranCambrianOrdovicianTerreneuvianSeries2Series3FurongianUpperMiddleLowerFortunian Stage 2 Stage 3 Stage 4 Stage 5 DrumianGuzhangianPaibianJiangshanian Stage 10 Ediacaran Biota Baykonur glaciation Orsten Fauna Burgess Shale Kaili biota Archaeocyatha extinction Emu Bay Shale Sirius Passet biota Chengjiang biota First arthropods with mineralized carapace Trilobites SSF diversification first brachiopods amp archaeocyatha First halkieriids mollusss hyoliths SSF Treptichnus pedumLarge negative d13C peak First Cloudina amp Namacalathus mineral tubular fossils Mollusc like Kimberella and its trace fossils Gaskiers glaciationArchaeonassa type trace fossilsContents 1 History and significance 1 1 Dating the Cambrian 1 2 Body fossils 1 3 Trace fossils 1 4 Geochemical observations 1 5 Phylogenetic techniques 2 Explanation of key scientific terms 2 1 Phylum 2 2 Stem group 2 3 Triploblastic 2 4 Bilaterian 2 5 Coelomate 3 Precambrian life 3 1 Evidence of animals around 1 billion years ago 3 2 Fossils of the Doushantuo formation 3 3 Burrows 3 4 Ediacaran organisms 3 5 Beck Spring Dolomite 4 Ediacaran Early Cambrian skeletonisation 5 Cambrian life 5 1 Trace fossils 5 2 Small shelly fauna 5 3 Early Cambrian trilobites and echinoderms 5 4 Burgess Shale type faunas 5 5 Early Cambrian crustaceans 5 6 Early Ordovician radiation 6 Stages 7 Validity 7 1 The Cambrian explosion as survivorship bias 8 Possible causes 8 1 Changes in the environment 8 1 1 Increase in oxygen levels 8 1 2 Ozone formation 8 1 3 Snowball Earth 8 1 4 Increase in the calcium concentration of the Cambrian seawater 8 2 Developmental explanations 8 3 Ecological explanations 8 3 1 End Ediacaran mass extinction 8 3 2 Anoxia 8 3 3 Evolution of eyes 8 3 4 Arms races between predators and prey 8 3 5 Increase in size and diversity of planktonic animals 8 4 Ecosystem engineering 8 5 Complexity threshold 9 Uniqueness of the early Cambrian biodiversification 10 Footnotes 11 References 12 Further reading 13 External linksHistory and significance EditMain article History of life The seemingly rapid appearance of fossils in the Primordial Strata was noted by William Buckland in the 1840s 18 and in his 1859 book On the Origin of Species Charles Darwin discussed the then inexplicable lack of earlier fossils as one of the main difficulties for his theory of descent with slow modification through natural selection 19 The long running puzzlement about the seemingly sudden appearance of the Cambrian fauna without evident precursor s centers on three key points whether there really was a mass diversification of complex organisms over a relatively short period during the early Cambrian what might have caused such rapid change and what it would imply about the origin of animal life Interpretation is difficult owing to a limited supply of evidence based mainly on an incomplete fossil record and chemical signatures remaining in Cambrian rocks The first discovered Cambrian fossils were trilobites described by Edward Lhuyd the curator of Oxford Museum in 1698 20 Although their evolutionary importance was not known on the basis of their old age William Buckland 1784 1856 realized that a dramatic step change in the fossil record had occurred around the base of what we now call the Cambrian 18 Nineteenth century geologists such as Adam Sedgwick and Roderick Murchison used the fossils for dating rock strata specifically for establishing the Cambrian and Silurian periods 21 By 1859 leading geologists including Roderick Murchison were convinced that what was then called the lowest Silurian stratum showed the origin of life on Earth though others including Charles Lyell differed In On the Origin of Species Darwin considered this sudden appearance of a solitary group of trilobites with no apparent antecedents and absence of other fossils to be undoubtedly of the gravest nature among the difficulties in his theory of natural selection He reasoned that earlier seas had swarmed with living creatures but that their fossils had not been found because of the imperfections of the fossil record 19 In the sixth edition of his book he stressed his problem further as 22 To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system I can give no satisfactory answer American paleontologist Charles Walcott who studied the Burgess Shale fauna proposed that an interval of time the Lipalian was not represented in the fossil record or did not preserve fossils and that the ancestors of the Cambrian animals evolved during this time 23 Earlier fossil evidence has since been found The earliest claim is that the history of life on earth goes back 3 850 million years 24 Rocks of that age at Warrawoona Australia were claimed to contain fossil stromatolites stubby pillars formed by colonies of microorganisms Fossils Grypania of more complex eukaryotic cells from which all animals plants and fungi are built have been found in rocks from 1 400 million years ago in China and Montana Rocks dating from 580 to 543 million years ago contain fossils of the Ediacara biota organisms so large that they are likely multicelled but very unlike any modern organism 25 In 1948 Preston Cloud argued that a period of eruptive evolution occurred in the Early Cambrian 26 but as recently as the 1970s no sign was seen of how the relatively modern looking organisms of the Middle and Late Cambrian arose 25 Opabinia made the largest single contribution to modern interest in the Cambrian explosion The intense modern interest in this Cambrian explosion was sparked by the work of Harry B Whittington and colleagues who in the 1970s reanalysed many fossils from the Burgess Shale and concluded that several were as complex as but different from any living animals 27 28 The most common organism Marrella was clearly an arthropod but not a member of any known arthropod class Organisms such as the five eyed Opabinia and spiny slug like Wiwaxia were so different from anything else known that Whittington s team assumed they must represent different phyla seemingly unrelated to anything known today Stephen Jay Gould s popular 1989 account of this work Wonderful Life 29 brought the matter into the public eye and raised questions about what the explosion represented While differing significantly in details both Whittington and Gould proposed that all modern animal phyla had appeared almost simultaneously in a rather short span of geological period This view led to the modernization of Darwin s tree of life and the theory of punctuated equilibrium which Eldredge and Gould developed in the early 1970s and which views evolution as long intervals of near stasis punctuated by short periods of rapid change 30 Other analyses some more recent and some dating back to the 1970s argue that complex animals similar to modern types evolved well before the start of the Cambrian 31 32 33 Dating the Cambrian Edit Radiometric dates for much of the Cambrian obtained by analysis of radioactive elements contained within rocks have only recently become available and for only a few regions Relative dating A was before B is often assumed sufficient for studying processes of evolution but this too has been difficult because of the problems involved in matching up rocks of the same age across different continents 34 Therefore dates or descriptions of sequences of events should be regarded with some caution until better data become available In 2004 the start of the Cambrian was dated to 542 Ma 35 In 2012 it was revised to 541 Ma 36 then in 2022 it was changed again to 538 8 Ma 3 Body fossils Edit Fossils of organisms bodies are usually the most informative type of evidence Fossilization is a rare event and most fossils are destroyed by erosion or metamorphism before they can be observed Hence the fossil record is very incomplete increasingly so as earlier times are considered Despite this they are often adequate to illustrate the broader patterns of life s history 37 Also biases exist in the fossil record different environments are more favourable to the preservation of different types of organism or parts of organisms 38 Further only the parts of organisms that were already mineralised are usually preserved such as the shells of molluscs Since most animal species are soft bodied they decay before they can become fossilised As a result although 30 plus phyla of living animals are known two thirds have never been found as fossils 25 This Marrella specimen illustrates how clear and detailed the fossils from the Burgess Shale Lagerstatte actually are as well as the oldest evidence for liquid blood in an animal The Cambrian fossil record includes an unusually high number of lagerstatten which preserve soft tissues These allow paleontologists to examine the internal anatomy of animals which in other sediments are only represented by shells spines claws etc if they are preserved at all The most significant Cambrian lagerstatten are the early Cambrian Maotianshan shale beds of Chengjiang Yunnan China and Sirius Passet Greenland 39 the middle Cambrian Burgess Shale British Columbia Canada 40 and the late Cambrian Orsten Sweden fossil beds While lagerstatten preserve far more than the conventional fossil record they are far from complete Because lagerstatten are restricted to a narrow range of environments where soft bodied organisms can be preserved very quickly e g by mudslides most animals are probably not represented further the exceptional conditions that create lagerstatten probably do not represent normal living conditions 41 In addition the known Cambrian lagerstatten are rare and difficult to date while Precambrian lagerstatten have yet to be studied in detail The sparseness of the fossil record means that organisms usually exist long before they are found in the fossil record this is known as the Signor Lipps effect 42 In 2019 a stunning find of lagerstatten known as the Qingjiang biota was reported from the Danshui river in Hubei province China More than 20 000 fossil specimens were collected including many soft bodied animals such as jellyfish sea anemones and worms as well as sponges arthropods and algae In some specimens the internal body structures were sufficiently preserved that soft tissues including muscles gills mouths guts and eyes can be seen The remains were dated to around 518 Mya and around half of the species identified at the time of reporting were previously unknown 43 44 45 Trace fossils Edit Rusophycus and other trace fossils from the Gog Group Middle Cambrian Lake Louise Alberta Canada Trace fossils consist mainly of tracks and burrows but also include coprolites fossil feces and marks left by feeding 46 47 Trace fossils are particularly significant because they represent a data source that is not limited to animals with easily fossilized hard parts and reflects organisms behaviour Also many traces date from significantly earlier than the body fossils of animals that are thought to have been capable of making them 48 While exact assignment of trace fossils to their makers is generally impossible traces may for example provide the earliest physical evidence of the appearance of moderately complex animals comparable to earthworms 47 Geochemical observations Edit Main article Early Cambrian geochemical fluctuations Several chemical markers indicate a drastic change in the environment around the start of the Cambrian The markers are consistent with a mass extinction 49 50 or with a massive warming resulting from the release of methane ice 51 Such changes may reflect a cause of the Cambrian explosion although they may also have resulted from an increased level of biological activity a possible result of the explosion 51 Despite these uncertainties the geochemical evidence helps by making scientists focus on theories that are consistent with at least one of the likely environmental changes Phylogenetic techniques Edit Cladistics is a technique for working out the family tree of a set of organisms It works by the logic that if groups B and C have more similarities to each other than either has to group A then B and C are more closely related to each other than either is to A Characteristics that are compared may be anatomical such as the presence of a notochord or molecular by comparing sequences of DNA or protein The result of a successful analysis is a hierarchy of clades groups whose members are believed to share a common ancestor The cladistic technique is sometimes problematic as some features such as wings or camera eyes evolved more than once convergently this must be taken into account in analyses From the relationships it may be possible to constrain the date that lineages first appeared For instance if fossils of B or C date to X million years ago and the calculated family tree says A was an ancestor of B and C then A must have evolved more than X million years ago It is also possible to estimate how long ago two living clades diverged i e about how long ago their last common ancestor must have lived by assuming that DNA mutations accumulate at a constant rate These molecular clocks however are fallible and provide only a very approximate timing they are not sufficiently precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved 52 and estimates produced by different techniques vary by a factor of two 53 However the clocks can give an indication of branching rate and when combined with the constraints of the fossil record recent clocks suggest a sustained period of diversification through the Ediacaran and Cambrian 54 Explanation of key scientific terms Edit Stem groups 55 Lines of descent Basal node Crown node Total group Crown group Stem group Phylum Edit A phylum is the highest level in the Linnaean system for classifying organisms Phyla can be thought of as groupings of animals based on general body plan 56 Despite the seemingly different external appearances of organisms they are classified into phyla based on their internal and developmental organizations 57 For example despite their obvious differences spiders and barnacles both belong to the phylum Arthropoda but earthworms and tapeworms although similar in shape belong to different phyla As chemical and genetic testing becomes more accurate previously hypothesised phyla are often entirely reworked A phylum is not a fundamental division of nature such as the difference between electrons and protons It is simply a very high level grouping in a classification system created to describe all currently living organisms This system is imperfect even for modern animals different books quote different numbers of phyla mainly because they disagree about the classification of a huge number of worm like species As it is based on living organisms it accommodates extinct organisms poorly if at all 25 58 Stem group Edit The concept of stem groups was introduced to cover evolutionary aunts and cousins of living groups and have been hypothesized based on this scientific theory A crown group is a group of closely related living animals plus their last common ancestor plus all its descendants A stem group is a set of offshoots from the lineage at a point earlier than the last common ancestor of the crown group it is a relative concept for example tardigrades are living animals that form a crown group in their own right but Budd 1996 regarded them as also being a stem group relative to the arthropods 55 59 Skin ectoderm Muscle mesoderm Coelom Internalorgan Membrane peritoneum Gut endoderm A coelomate animal is basically a set of concentric tubes with a gap between the gut and the outer tubes Triploblastic Edit The term Triploblastic means consisting of three layers which are formed in the embryo quite early in the animal s development from a single celled egg to a larva or juvenile form The innermost layer forms the digestive tract gut the outermost forms skin and the middle one forms muscles and all the internal organs except the digestive system Most types of living animal are triploblastic the best known exceptions are Porifera sponges and Cnidaria jellyfish sea anemones etc Bilaterian Edit The bilaterians are animals that have right and left sides at some point in their life histories This implies that they have top and bottom surfaces and importantly distinct front and back ends All known bilaterian animals are triploblastic and all known triploblastic animals are bilaterian Living echinoderms sea stars sea urchins sea cucumbers etc look radially symmetrical like wheels rather than bilaterian but their larvae exhibit bilateral symmetry and some of the earliest echinoderms may have been bilaterally symmetrical 60 Porifera and Cnidaria are radially symmetrical not bilaterian and not triploblastic but the common Bilateria Cnidaria ancestor s planula larva is suspected to be bilaterally symmetric Coelomate Edit The term Coelomate means having a body cavity coelom containing the internal organs Most of the phyla featured in the debate about the Cambrian explosion clarification needed are coelomates arthropods annelid worms molluscs echinoderms and chordates the noncoelomate priapulids are an important exception All known coelomate animals are triploblastic bilaterians but some triploblastic bilaterian animals do not have a coelom for example flatworms whose organs are surrounded by unspecialized tissues Precambrian life EditEvidence of animals around 1 billion years ago Edit Further information Acritarch and Stromatolite Stromatolites Pika Formation Middle Cambrian near Helen Lake Banff National Park Canada Modern stromatolites in Hamelin Pool Marine Nature Reserve Western Australia Changes in the abundance and diversity of some types of fossil have been interpreted as evidence for attacks by animals or other organisms Stromatolites stubby pillars built by colonies of microorganisms are a major constituent of the fossil record from about 2 700 million years ago but their abundance and diversity declined steeply after about 1 250 million years ago This decline has been attributed to disruption by grazing and burrowing animals 31 32 61 Precambrian marine diversity was dominated by small fossils known as acritarchs This term describes almost any small organic walled fossil from the egg cases of small metazoans to resting cysts of many different kinds of green algae After appearing around 2 000 million years ago acritarchs underwent a boom around 1 000 million years ago increasing in abundance diversity size complexity of shape and especially size and number of spines Their increasingly spiny forms in the last 1 billion years may indicate an increased need for defence against predation Other groups of small organisms from the Neoproterozoic era also show signs of antipredator defenses 61 A consideration of taxon longevity appears to support an increase in predation pressure around this time 62 In general the fossil record shows a very slow appearance of these lifeforms in the Precambrian with many cyanobacterial species making up much of the underlying sediment 63 Fossils of the Doushantuo formation Edit Main article Doushantuo Formation The layers of the Doushantuo Formation from around 580 million year old 64 harbour microscopic fossils that may represent early bilaterians Some have been described as animal embryos and eggs although some may represent the remains of giant bacteria 65 Another fossil Vernanimalcula has been interpreted as a coelomate bilaterian 66 but may simply be an infilled bubble 67 These fossils form the earliest hard and fast evidence of animals as opposed to other predators 65 68 Burrows Edit Main article Cambrian substrate revolution An Ediacaran trace fossil made when an organism burrowed below a microbial mat The traces of organisms moving on and directly underneath the microbial mats that covered the Ediacaran sea floor are preserved from the Ediacaran period about 565 million years ago c They were probably made by organisms resembling earthworms in shape size and how they moved The burrow makers have never been found preserved but because they would need a head and a tail the burrowers probably had bilateral symmetry which would in all probability make them bilaterian animals 71 They fed above the sediment surface but were forced to burrow to avoid predators 72 Around the start of the Cambrian about 539 million years ago many new types of traces first appear including well known vertical burrows such as Diplocraterion and Skolithos and traces normally attributed to arthropods such as Cruziana and Rusophycus The vertical burrows indicate that worm like animals acquired new behaviours and possibly new physical capabilities Some Cambrian trace fossils indicate that their makers possessed hard exoskeletons although they were not necessarily mineralised 70 Burrows provide firm evidence of complex organisms they are also much more readily preserved than body fossils to the extent that the absence of trace fossils has been used to imply the genuine absence of large motile bottom dwelling organisms citation needed They provide a further line of evidence to show that the Cambrian explosion represents a real diversification and is not a preservational artifact 73 This new habit changed the seafloor s geochemistry and led to decreased oxygen in the ocean and increased CO2 levels in the seas and the atmosphere resulting in global warming for tens of millions years and could be responsible for mass extinctions 74 But as burrowing became established it allowed an explosion of its own for as burrowers disturbed the sea floor they aerated it mixing oxygen into the toxic muds This made the bottom sediments more hospitable and allowed a wider range of organisms to inhabit them creating new niches and the scope for higher diversity 73 Ediacaran organisms Edit Dickinsonia costata an Ediacaran organism of unknown affinity with a quilted appearance Main articles Ediacaran biota Cloudinidae Kimberella and Spriggina At the start of the Ediacaran period much of the acritarch fauna which had remained relatively unchanged for hundreds of millions of years became extinct to be replaced with a range of new larger species which would prove far more ephemeral 63 This radiation the first in the fossil record 63 is followed soon after by an array of unfamiliar large fossils dubbed the Ediacara biota 75 which flourished for 40 million years until the start of the Cambrian 76 Most of this Ediacara biota were at least a few centimeters long significantly larger than any earlier fossils The organisms form three distinct assemblages increasing in size and complexity as time progressed 77 Many of these organisms were quite unlike anything that appeared before or since resembling discs mud filled bags or quilted mattresses one palaeontologist proposed that the strangest organisms should be classified as a separate kingdom Vendozoa 78 Fossil of Kimberella a triploblastic bilaterian and possibly a mollusc At least some may have been early forms of the phyla at the heart of the Cambrian explosion debate clarification needed having been interpreted as early molluscs Kimberella 33 79 echinoderms Arkarua 80 and arthropods Spriggina 81 Parvancorina 82 Yilingia Still debate exists about the classification of these specimens mainly because the diagnostic features that allow taxonomists to classify more recent organisms such as similarities to living organisms are generally absent in the ediacarans 83 However there seems little doubt that Kimberella was at least a triploblastic bilaterian animal 83 These organisms are central to the debate about how abrupt the Cambrian explosion was citation needed If some were early members of the animal phyla seen today the explosion looks a lot less sudden than if all these organisms represent an unrelated experiment and were replaced by the animal kingdom fairly soon thereafter 40M years is soon by evolutionary and geological standards Beck Spring Dolomite Edit Paul Knauth a geologist at Arizona State University maintains that photosynthesizing organisms such as algae may have grown over a 750 to 800 million year old formation in Death Valley known as the Beck Spring Dolomite In the early 1990s samples from this 1 000 foot thick layer of dolomite revealed that the region housed flourishing mats of photosynthesizing unicellular life forms which antedated the Cambrian explosion Microfossils have been unearthed from holes riddling the otherwise barren surface of the dolomite These geochemical and microfossil findings support the idea that during the Precambrian period complex life evolved both in the oceans and on land Knauth contends that animals may well have had their origins in freshwater lakes and streams and not in the oceans Some 30 years later a number of studies have documented an abundance of geochemical and microfossil evidence showing that life covered the continents as far back as 2 2 billion years ago Many paleobiologists now accept the idea that simple life forms existed on land during the Precambrian but are opposed to the more radical idea that multicellular life thrived on land more than 600 million years ago 84 Ediacaran Early Cambrian skeletonisation EditThe first Ediacaran and lowest Cambrian Nemakit Daldynian skeletal fossils represent tubes and problematic sponge spicules 85 The oldest sponge spicules are monaxon siliceous aged around 580 million years ago known from the Doushantou Formation in China and from deposits of the same age in Mongolia although the interpretation of these fossils as spicules has been challenged 86 In the late Ediacaran lowest Cambrian numerous tube dwellings of enigmatic organisms appeared It was organic walled tubes e g Saarina and chitinous tubes of the sabelliditids e g Sokoloviina Sabellidites Paleolina 87 88 that prospered up to the beginning of the Tommotian The mineralized tubes of Cloudina Namacalathus Sinotubulites and a dozen more of the other organisms from carbonate rocks formed near the end of the Ediacaran period from 549 to 542 million years ago as well as the triradially symmetrical mineralized tubes of anabaritids e g Anabarites Cambrotubulus from uppermost Ediacaran and lower Cambrian 89 Ediacaran mineralized tubes are often found in carbonates of the stromatolite reefs and thrombolites 90 91 i e they could live in an environment adverse to the majority of animals Although they are as hard to classify as most other Ediacaran organisms they are important in two other ways First they are the earliest known calcifying organisms organisms that built shells from calcium carbonate 91 92 93 Secondly these tubes are a device to rise over a substrate and competitors for effective feeding and to a lesser degree they serve as armor for protection against predators and adverse conditions of environment Some Cloudina fossils show small holes in shells The holes possibly are evidence of boring by predators sufficiently advanced to penetrate shells 94 A possible evolutionary arms race between predators and prey is one of the hypotheses that attempt to explain the Cambrian explosion 61 In the lowest Cambrian the stromatolites were decimated This allowed animals to begin colonization of warm water pools with carbonate sedimentation At first it was anabaritids and Protohertzina the fossilized grasping spines of chaetognaths fossils Such mineral skeletons as shells sclerites thorns and plates appeared in uppermost Nemakit Daldynian they were the earliest species of halkierids gastropods hyoliths and other rare organisms The beginning of the Tommotian has historically been understood to mark an explosive increase of the number and variety of fossils of molluscs hyoliths and sponges along with a rich complex of skeletal elements of unknown animals the first archaeocyathids brachiopods tommotiids and others 95 96 97 98 Also soft bodied extant phyla such as comb jellies scalidophorans entoproctans horseshoe worms and lobopodians had armored forms 99 This sudden increase is partially an artefact of missing strata at the Tommotian type section and most of this fauna in fact began to diversify in a series of pulses through the Nemakit Daldynian and into the Tommotian 100 Some animals may already have had sclerites thorns and plates in the Ediacaran e g Kimberella had hard sclerites probably of carbonate but thin carbonate skeletons cannot be fossilized in siliciclastic deposits 101 Older 750 Ma fossils indicate that mineralization long preceded the Cambrian probably defending small photosynthetic algae from single celled eukaryotic predators 102 103 Cambrian life EditTrace fossils Edit Trace fossils burrows etc are a reliable indicator of what life was around and indicate a diversification of life around the start of the Cambrian with the freshwater realm colonized by animals almost as quickly as the oceans 104 Small shelly fauna Edit Main article Small shelly fauna Fossils known as small shelly fauna have been found in many parts on the world and date from just before the Cambrian to about 10 million years after the start of the Cambrian the Nemakit Daldynian and Tommotian ages see timeline These are a very mixed collection of fossils spines sclerites armor plates tubes archeocyathids sponge like animals and small shells very like those of brachiopods and snail like molluscs but all tiny mostly 1 to 2 mm long 105 While small these fossils are far more common than complete fossils of the organisms that produced them crucially they cover the window from the start of the Cambrian to the first lagerstatten a period of time otherwise lacking in fossils Hence they supplement the conventional fossil record and allow the fossil ranges of many groups to be extended Early Cambrian trilobites and echinoderms Edit A fossilized trilobite an ancient type of arthropod This specimen from the Burgess Shale preserves soft parts the antennae and legs The earliest trilobite fossils are about 530 million years old but the class was already quite diverse and cosmopolitan suggesting they had been around for quite some time 106 The fossil record of trilobites began with the appearance of trilobites with mineral exoskeletons not from the time of their origin The earliest generally accepted echinoderm fossils appeared a little bit later in the Late Atdabanian unlike modern echinoderms these early Cambrian echinoderms were not all radially symmetrical 107 These provide firm data points for the end of the explosion or at least indications that the crown groups of modern phyla were represented Burgess Shale type faunas Edit Main article Burgess Shale type preservation The Burgess Shale and similar lagerstatten preserve the soft parts of organisms which provide a wealth of data to aid in the classification of enigmatic fossils It often preserved complete specimens of organisms only otherwise known from dispersed parts such as loose scales or isolated mouthparts Further the majority of organisms and taxa in these horizons are entirely soft bodied hence absent from the rest of the fossil record 108 Since a large part of the ecosystem is preserved the ecology of the community can also be tentatively reconstructed verification needed However the assemblages may represent a museum a deep water ecosystem that is evolutionarily behind the rapidly diversifying fauna of shallower waters 109 Because the lagerstatten provide a mode and quality of preservation that is virtually absent outside of the Cambrian many organisms appear completely different from anything known from the conventional fossil record This led early workers in the field to attempt to shoehorn the organisms into extant phyla the shortcomings of this approach led later workers to erect a multitude of new phyla to accommodate all the oddballs It has since been realised that most oddballs diverged from lineages before they established the phyla known today clarification needed slightly different designs which were fated to perish rather than flourish into phyla as their cousin lineages did The preservational mode is rare in the preceding Ediacaran period but those assemblages known show no trace of animal life perhaps implying a genuine absence of macroscopic metazoans 110 Early Cambrian crustaceans Edit Further information Orsten Crustaceans one of the four great modern groups of arthropods are very rare throughout the Cambrian Convincing crustaceans were once thought to be common in Burgess Shale type biotas but none of these individuals can be shown to fall into the crown group of true crustaceans 111 The Cambrian record of crown group crustaceans comes from microfossils The Swedish Orsten horizons contain later Cambrian crustaceans but only organisms smaller than 2 mm are preserved This restricts the data set to juveniles and miniaturised adults A more informative data source is the organic microfossils of the Mount Cap formation Mackenzie Mountains Canada This late Early Cambrian assemblage 510 to 515 million years ago consists of microscopic fragments of arthropods cuticle which is left behind when the rock is dissolved with hydrofluoric acid The diversity of this assemblage is similar to that of modern crustacean faunas Analysis of fragments of feeding machinery found in the formation shows that it was adapted to feed in a very precise and refined fashion This contrasts with most other early Cambrian arthropods which fed messily by shovelling anything they could get their feeding appendages on into their mouths This sophisticated and specialised feeding machinery belonged to a large about 30 cm 112 organism and would have provided great potential for diversification Specialised feeding apparatus allows a number of different approaches to feeding and development and creates a number of different approaches to avoid being eaten 111 Early Ordovician radiation Edit Main article Great Ordovician Biodiversification Event After an extinction at the Cambrian Ordovician boundary another radiation occurred which established the taxa that would dominate the Palaeozoic 113 During this radiation the total number of orders doubled and families tripled 113 increasing marine diversity to levels typical of the Palaeozoic 51 and disparity to levels approximately equivalent to today s 13 Stages EditThe early Cambrian interval of diversification lasted for about the next 20 7 114 25 9 115 million years and its elevated rates of evolution had ended by the base of Cambrian Series 2 521 million years ago coincident with the first trilobites in the fossil record 116 Different authors define intervals of diversification during the early Cambrian different ways Ed Landing recognizes three stages Stage 1 spanning the Ediacaran Cambrian boundary corresponds to a diversification of biomineralizing animals and of deep and complex burrows Stage 2 corresponding to the radiation of molluscs and stem group Brachiopods hyoliths and tommotiids which apparently arose in intertidal waters and Stage 3 seeing the Atdabanian diversification of trilobites in deeper waters but little change in the intertidal realm 117 Graham Budd synthesises various schemes to produce a compatible view of the SSF record of the Cambrian explosion divided slightly differently into four intervals a Tube world lasting from 550 to 536 million years ago spanning the Ediacaran Cambrian boundary dominated by Cloudina Namacalathus and pseudoconodont type elements a Sclerite world seeing the rise of halkieriids tommotiids and hyoliths lasting to the end of the Fortunian c 525 Ma a brachiopod world perhaps corresponding to the as yet unratified Cambrian Stage 2 and Trilobite World kicking off in Stage 3 118 Complementary to the shelly fossil record trace fossils can be divided into five subdivisions Flat world late Ediacaran with traces restricted to the sediment surface Protreozoic III after Jensen with increasing complexity pedum world initiated at the base of the Cambrian with the base of the T pedum zone see Cambrian Dating the Cambrian Rusophycus world spanning 536 to 521 million years ago and thus corresponding exactly to the periods of Sclerite World and Brachiopod World under the SSF paradigm and Cruziana world with an obvious correspondence to Trilobite World 118 Validity EditThere is strong evidence for species of Cnidaria and Porifera existing in the Ediacaran 119 and possible members of Porifera even before that during the Cryogenian 120 Bryozoans once thought to not appear in the fossil record until after the Cambrian are now known from strata of Cambrian Age 3 from Australia and South China 121 The fossil record as Darwin knew it seemed to suggest that the major metazoan groups appeared in a few million years of the early to mid Cambrian and even in the 1980s this still appeared to be the case 28 29 However evidence of Precambrian Metazoa is gradually accumulating If the Ediacaran Kimberella was a mollusc like protostome one of the two main groups of coelomates 33 79 the protostome and deuterostome lineages must have split significantly before 550 million years ago deuterostomes are the other main group of coelomates 122 Even if it is not a protostome it is widely accepted as a bilaterian 83 122 Since fossils of rather modern looking cnidarians jellyfish like organisms have been found in the Doushantuo lagerstatte the cnidarian and bilaterian lineages must have diverged well over 580 million years ago 122 Trace fossils 77 and predatory borings in Cloudina shells provide further evidence of Ediacaran animals 123 Some fossils from the Doushantuo formation have been interpreted as embryos and one Vernanimalcula as a bilaterian coelomate although these interpretations are not universally accepted 66 67 124 Earlier still predatory pressure has acted on stromatolites and acritarchs since around 1 250 million years ago 61 Some say that the evolutionary change was accelerated by an order of magnitude d but the presence of Precambrian animals somewhat dampens the bang of the explosion not only was the appearance of animals gradual but their evolutionary radiation diversification may also not have been as rapid as once thought Indeed statistical analysis shows that the Cambrian explosion was no faster than any of the other radiations in animals history e However it does seem that some innovations linked to the explosion such as resistant armour only evolved once in the animal lineage this makes a lengthy Precambrian animal lineage harder to defend 126 Further the conventional view that all the phyla arose in the Cambrian is flawed while the phyla may have diversified in this time period representatives of the crown groups of many phyla do not appear until much later in the Phanerozoic 14 Further the mineralised phyla that form the basis of the fossil record may not be representative of other phyla since most mineralised phyla originated in a benthic setting The fossil record is consistent with a Cambrian explosion that was limited to the benthos with pelagic phyla evolving much later 14 Ecological complexity among marine animals increased in the Cambrian as well later in the Ordovician 13 However recent research has overthrown the once popular idea that disparity was exceptionally high throughout the Cambrian before subsequently decreasing 127 In fact disparity remains relatively low throughout the Cambrian with modern levels of disparity only attained after the early Ordovician radiation 13 The diversity of many Cambrian assemblages is similar to today s 128 111 and at a high class phylum level diversity is thought by some to have risen relatively smoothly through the Cambrian stabilizing somewhat in the Ordovician 129 This interpretation however glosses over the astonishing and fundamental pattern of basal polytomy and phylogenetic telescoping at or near the Cambrian boundary as seen in most major animal lineages 130 Thus Harry Blackmore Whittington s questions regarding the abrupt nature of the Cambrian explosion remain and have yet to be satisfactorily answered 131 The Cambrian explosion as survivorship bias Edit Budd and Mann 132 suggested that the Cambrian explosion was the result of a type of survivorship bias called the Push of the past As groups at their origin tend to go extinct it follows that any long lived group would have experienced an unusually rapid rate of diversification early on creating the illusion of a general speed up in diversification rates However rates of diversification could remain at background levels and still generate this sort of effect in the surviving lineages Possible causes EditDespite the evidence that moderately complex animals triploblastic bilaterians existed before and possibly long before the start of the Cambrian it seems that the pace of evolution was exceptionally fast in the early Cambrian Possible explanations for this fall into three broad categories environmental developmental and ecological changes Any explanation must explain both the timing and magnitude of the explosion Changes in the environment Edit Increase in oxygen levels Edit Earth s earliest atmosphere contained no free oxygen O2 the oxygen that animals breathe today both in the air and dissolved in water is the product of billions of years of photosynthesis Cyanobacteria were the first organisms to evolve the ability to photosynthesize introducing a steady supply of oxygen into the environment 133 Initially oxygen levels did not increase substantially in the atmosphere 134 The oxygen quickly reacted with iron and other minerals in the surrounding rock and ocean water Once a saturation point was reached for the reactions in rock and water oxygen was able to exist as a gas in its diatomic form Oxygen levels in the atmosphere increased substantially afterward 135 As a general trend the concentration of oxygen in the atmosphere has risen gradually over about the last 2 5 billion years 25 Oxygen levels seem to have a positive correlation with diversity in eukaryotes well before the Cambrian period 136 The last common ancestor of all extant eukaryotes is thought to have lived around 1 8 billion years ago Around 800 million years ago there was a notable increase in the complexity and number of eukaryotes species in the fossil record 136 Before the spike in diversity eukaryotes are thought to have lived in highly sulfuric environments Sulfide interferes with mitochondrial function in aerobic organisms limiting the amount of oxygen that could be used to drive metabolism Oceanic sulfide levels decreased around 800 million years ago which supports the importance of oxygen in eukaryotic diversity 136 The increased ventilation of the oceans by sponges which had already evolved and diversified during the Neoproterozoic has been proposed to have increased the availability of oxygen and powered the Cambrian s rapid diversification of multicellular life 137 138 The shortage of oxygen might well have prevented the rise of large complex animals The amount of oxygen an animal can absorb is largely determined by the area of its oxygen absorbing surfaces lungs and gills in the most complex animals the skin in less complex ones while the amount needed is determined by its volume which grows faster than the oxygen absorbing area if an animal s size increases equally in all directions An increase in the concentration of oxygen in air or water would increase the size to which an organism could grow without its tissues becoming starved of oxygen However members of the Ediacara biota reached metres in length tens of millions of years before the Cambrian explosion 49 Other metabolic functions may have been inhibited by lack of oxygen for example the construction of tissue such as collagen which is required for the construction of complex structures 139 or the biosynthesis of molecules for the construction of a hard exoskeleton 140 However animals were not affected when similar oceanographic conditions occurred in the Phanerozoic therefore some see no forcing role of the oxygen level on evolution 141 Ozone formation Edit The amount of ozone O3 required to shield Earth from biologically lethal UV radiation wavelengths from 200 to 300 nanometers nm is believed to have been in existence around the Cambrian explosion 142 The presence of the ozone layer may have enabled the development of complex life and life on land as opposed to life being restricted to the water Snowball Earth Edit Main article Snowball Earth In the late Neoproterozoic extending into the early Ediacaran period the Earth suffered massive glaciations in which most of its surface was covered by ice This may have caused a mass extinction creating a genetic bottleneck the resulting diversification may have given rise to the Ediacara biota which appears soon after the last Snowball Earth episode 143 However the snowball episodes occurred a long time before the start of the Cambrian and it is difficult to see how so much diversity could have been caused by even a series of bottlenecks 51 the cold periods may even have delayed the evolution of large size organisms 61 Massive rock erosion caused by glaciers during the Snowball Earth may have deposited nutrient rich sediments into the oceans setting the stage for the Cambrian explosion 144 Increase in the calcium concentration of the Cambrian seawater Edit Newer research suggests that volcanically active midocean ridges caused a massive and sudden surge of the calcium concentration in the oceans making it possible for marine organisms to build skeletons and hard body parts 145 Alternatively a high influx of ions could have been provided by the widespread erosion that produced Powell s Great Unconformity 146 An increase of calcium may also have been caused by erosion of the Transgondwanan Supermountain that existed at the time of the explosion The roots of the mountain are preserved in present day East Africa as an orogen 147 Developmental explanations Edit Further information Evolutionary developmental biology A range of theories are based on the concept that minor modifications to animals development as they grow from embryo to adult may have been able to cause very large changes in the final adult form The Hox genes for example control which organs individual regions of an embryo will develop into For instance if a certain Hox gene is expressed a region will develop into a limb if a different Hox gene is expressed in that region a minor change it could develop into an eye instead a phenotypically major change Such a system allows a large range of disparity to appear from a limited set of genes but such theories linking this with the explosion struggle to explain why the origin of such a development system should by itself lead to increased diversity or disparity Evidence of Precambrian metazoans 51 combines with molecular data 148 to show that much of the genetic architecture that could feasibly have played a role in the explosion was already well established by the Cambrian This apparent paradox is addressed in a theory that focuses on the physics of development It is proposed that the emergence of simple multicellular forms provided a changed context and spatial scale in which novel physical processes and effects were mobilized by the products of genes that had previously evolved to serve unicellular functions Morphological complexity layers segments lumens appendages arose in this view by self organization 149 Horizontal gene transfer has also been identified as a possible factor in the rapid acquisition of the biochemical capability of biomineralization among organisms during this period based on evidence that the gene for a critical protein in the process was originally transferred from a bacterium into sponges 150 Ecological explanations Edit These focus on the interactions between different types of organism Some of these hypotheses deal with changes in the food chain some suggest arms races between predators and prey and others focus on the more general mechanisms of coevolution Such theories are well suited to explaining why there was a rapid increase in both disparity and diversity but they do not explain why the explosion happened when it did 51 End Ediacaran mass extinction Edit Main article End Ediacaran extinction Evidence for such an extinction includes the disappearance from the fossil record of the Ediacara biota and shelly fossils such as Cloudina and the accompanying perturbation in the d 13C record It is suspected that several global anoxic events were responsible for the extinction 151 152 Mass extinctions are often followed by adaptive radiations as existing clades expand to occupy the ecospace emptied by the extinction However once the dust had settled overall disparity and diversity returned to the pre extinction level in each of the Phanerozoic extinctions 51 Anoxia Edit The late Ediacaran oceans appears to have suffered from an anoxia that covered much of the seafloor which would have given mobile animals with the ability to seek out more oxygen rich environments an advantage over sessile forms of life 153 Evolution of eyes Edit Main article Evolution of the eye Andrew Parker has proposed that predator prey relationships changed dramatically after eyesight evolved Prior to that time hunting and evading were both close range affairs smell vibration and touch were the only senses used When predators could see their prey from a distance new defensive strategies were needed Armor spines and similar defenses may also have evolved in response to vision He further observed that where animals lose vision in unlighted environments such as caves diversity of animal forms tends to decrease 154 Nevertheless many scientists doubt that vision could have caused the explosion Eyes may well have evolved long before the start of the Cambrian 155 It is also difficult to understand why the evolution of eyesight would have caused an explosion since other senses such as smell and pressure detection can detect things at a greater distance in the sea than sight can but the appearance of these other senses apparently did not cause an evolutionary explosion 51 Arms races between predators and prey Edit The ability to avoid or recover from predation often makes the difference between life and death and is therefore one of the strongest components of natural selection The pressure to adapt is stronger on the prey than on the predator if the predator fails to win a contest it loses a meal if the prey is the loser it loses its life 156 But there is evidence that predation was rife long before the start of the Cambrian for example in the increasingly spiny forms of acritarchs the holes drilled in Cloudina shells and traces of burrowing to avoid predators Hence it is unlikely that the appearance of predation was the trigger for the Cambrian explosion although it may well have exhibited a strong influence on the body forms that the explosion produced 61 However the intensity of predation does appear to have increased dramatically during the Cambrian 157 as new predatory tactics such as shell crushing emerged 158 This rise of predation during the Cambrian was confirmed by the temporal pattern of the median predator ratio at the scale of genus in fossil communities covering the Cambrian and Ordovician periods but this pattern is not correlated to diversification rate 159 This lack of correlation between predator ratio and diversification over the Cambrian and Ordovician suggests that predators did not trigger the large evolutionary radiation of animals during this interval Thus the role of predators as triggerers of diversification may have been limited to the very beginning of the Cambrian explosion 159 Increase in size and diversity of planktonic animals Edit Geochemical evidence strongly indicates that the total mass of plankton has been similar to modern levels since early in the Proterozoic Before the start of the Cambrian their corpses and droppings were too small to fall quickly towards the seabed since their drag was about the same as their weight This meant they were destroyed by scavengers or by chemical processes before they reached the sea floor 41 Mesozooplankton are plankton of a larger size Early Cambrian specimens filtered microscopic plankton from the seawater These larger organisms would have produced droppings and ultimately corpses large enough to fall fairly quickly This provided a new supply of energy and nutrients to the mid levels and bottoms of the seas which opened up a new range of possible ways of life If any of these remains sank uneaten to the sea floor they could be buried this would have taken some carbon out of circulation resulting in an increase in the concentration of breathable oxygen in the seas carbon readily combines with oxygen 41 The initial herbivorous mesozooplankton were probably larvae of benthic seafloor animals A larval stage was probably an evolutionary innovation driven by the increasing level of predation at the seafloor during the Ediacaran period 12 160 Metazoans have an amazing ability to increase diversity through coevolution 63 This means that an organism s traits can lead to traits evolving in other organisms a number of responses are possible and a different species can potentially emerge from each one As a simple example the evolution of predation may have caused one organism to develop a defence while another developed motion to flee This would cause the predator lineage to diverge into two species one that was good at chasing prey and another that was good at breaking through defences Actual coevolution is somewhat more subtle but in this fashion great diversity can arise three quarters of living species are animals and most of the rest have formed by coevolution with animals 63 Ecosystem engineering Edit Evolving organisms inevitably change the environment they evolve in The Devonian colonization of land had planet wide consequences for sediment cycling and ocean nutrients and was likely linked to the Devonian mass extinction A similar process may have occurred on smaller scales in the oceans with for example the sponges filtering particles from the water and depositing them in the mud in a more digestible form or burrowing organisms making previously unavailable resources available for other organisms 161 Complexity threshold Edit The explosion may not have been a significant evolutionary event It may represent a threshold being crossed for example a threshold in genetic complexity that allowed a vast range of morphological forms to be employed 162 This genetic threshold may have a correlation to the amount of oxygen available to organisms Using oxygen for metabolism produces much more energy than anaerobic processes Organisms that use more oxygen have the opportunity to produce more complex proteins providing a template for further evolution 134 These proteins translate into larger more complex structures that allow organisms better to adapt to their environments 163 With the help of oxygen genes that code for these proteins could contribute to the expression of complex traits more efficiently Access to a wider range of structures and functions would allow organisms to evolve in different directions increasing the number of niches that could be inhabited Furthermore organisms had the opportunity to become more specialized in their own niches 163 Uniqueness of the early Cambrian biodiversification EditThe Cambrian explosion can be viewed as two waves of metazoan expansion into empty niches first a coevolutionary rise in diversity as animals explored niches on the Ediacaran sea floor followed by a second expansion in the early Cambrian as they became established in the water column 63 The rate of diversification seen in the Cambrian phase of the explosion is unparalleled among marine animals it affected all metazoan clades of which Cambrian fossils have been found Later radiations such as those of fish in the Silurian and Devonian periods involved fewer taxa mainly with very similar body plans 25 Although the recovery from the Permian Triassic extinction started with about as few animal species as the Cambrian explosion the recovery produced far fewer significantly new types of animals 164 Whatever triggered the early Cambrian diversification opened up an exceptionally wide range of previously unavailable ecological niches When these were all occupied limited space existed for such wide ranging diversifications to occur again because strong competition existed in all niches and incumbents usually had the advantage If a wide range of empty niches had continued clades would be able to continue diversifying and become disparate enough for us to recognise them as different phyla when niches are filled lineages will continue to resemble one another long after they diverge as limited opportunity exists for them to change their life styles and forms 165 There were two similar explosions in the evolution of land plants after a cryptic history beginning about 450 million years ago land plants underwent a uniquely rapid adaptive radiation during the Devonian period about 400 million years ago 25 Furthermore angiosperms flowering plants originated and rapidly diversified during the Cretaceous period Footnotes Edit This included at least animals phytoplankton and calcimicrobes 12 At 610 million 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Int J Dev Biol 47 7 685 693 arXiv q bio 0311013 Bibcode 2003q bio 11013S PMID 14756344 a b Sperling E A Frieder C A Raman A V Girguis P R Levin L A Knoll A H 2013 Oxygen ecology and the Cambrian radiation of animals PNAS 110 33 13446 13451 Bibcode 2013PNAS 11013446S doi 10 1073 pnas 1312778110 PMC 3746845 PMID 23898193 Erwin D H Valentine J W Sepkoski J J November 1987 A Comparative Study of Diversification Events The Early Paleozoic Versus the Mesozoic Evolution 41 6 1177 1186 doi 10 2307 2409086 JSTOR 2409086 PMID 11542112 Valentine J W April 1995 Why No New Phyla after the Cambrian Genome and Ecospace Hypotheses Revisited PALAIOS abstract 10 2 190 194 Bibcode 1995Palai 10 190V doi 10 2307 3515182 JSTOR 3515182 Further reading EditBudd G E Jensen J 2000 A critical reappraisal of the fossil record of the bilaterian phyla Biological Reviews 75 2 253 295 doi 10 1111 j 1469 185X 1999 tb00046 x PMID 10881389 S2CID 39772232 Collins Allen G Metazoa Fossil record Retrieved Dec 14 2005 Conway Morris S 1997 The Crucible of Creation the Burgess Shale and the rise of animals Oxford University Press ISBN 0 19 286202 2 Conway Morris S June 2006 Darwin s dilemma the realities of the Cambrian explosion Philosophical Transactions of the Royal Society B Biological Sciences 361 1470 1069 1083 doi 10 1098 rstb 2006 1846 ISSN 0962 8436 PMC 1578734 PMID 16754615 An enjoyable account Gould S J 1989 Wonderful Life The Burgess Shale and the Nature of History W W Norton amp Company Bibcode 1989wlbs book G Kennedy M M Droser L Mayer D Pevear amp D Mrofka 2006 Clay and Atmospheric Oxygen Science 311 5766 1341 doi 10 1126 science 311 5766 1341c S2CID 220101640 Knoll A H Carroll S B 1999 06 25 Early Animal Evolution Emerging Views from Comparative Biology and Geology Science 284 5423 2129 37 doi 10 1126 science 284 5423 2129 PMID 10381872 S2CID 8908451 Markov Alexander V Korotayev Andrey V 2007 Phanerozoic marine biodiversity follows a hyperbolic trend Palaeoworld 16 4 311 318 doi 10 1016 j palwor 2007 01 002 Montenari M Leppig U 2003 The Acritarcha their classification morphology ultrastructure and palaeoecological palaeogeographical distribution Palaontologische Zeitschrift 77 173 194 doi 10 1007 bf03004567 S2CID 127238427 Wang D Y C S Kumar S B Hedges January 1999 Divergence time estimates for the early history of animal phyla and the origin of plants animals and fungi Proceedings of the Royal Society B 266 1415 163 71 doi 10 1098 rspb 1999 0617 ISSN 0962 8452 PMC 1689654 PMID 10097391 Wood Rachel A The Rise of Animals New fossils and analyses of ancient ocean chemistry reveal the surprisingly deep roots of the Cambrian explosion Scientific American vol 320 no 6 June 2019 pp 24 31 Xiao S Y Zhang amp A Knoll January 1998 Three dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite Nature 391 1 553 58 Bibcode 1998Natur 391 553X doi 10 1038 35318 ISSN 0090 9556 S2CID 4350507 Timeline References Martin M W Grazhdankin D V Bowring S A Evans D A D Fedonkin M A Kirschvink J L 2000 Age of Neoproterozoic Bilaterian Body and Trace Fossils White Sea Russia Implications for Metazoan Evolution Science 288 5467 841 845 Bibcode 2000Sci 288 841M doi 10 1126 science 288 5467 841 PMID 10797002 External links EditThe Cambrian explosion of metazoans and molecular biology would Darwin be satisfied On embryos and ancestors by Stephen Jay Gould Conway Morris S April 2000 The Cambrian explosion Slow fuse or megatonnage Proceedings of the National Academy of Sciences 97 9 4426 4429 Bibcode 2000PNAS 97 4426C doi 10 1073 pnas 97 9 4426 PMC 34314 PMID 10781036 The Cambrian Explosion In Our Time BBC Radio 4 broadcast 17 February 2005 Burgess Shale Virtual Museum of Canada 2011 exhaustive details about the Burgess Shale its fossils and its significance for the Cambrian explosion Utah s Cambrian life new 2008 website with good images of a range of Burgess shale type and other Cambrian fossils Smithsonian National Museum Retrieved from https en wikipedia org w index php title Cambrian explosion amp oldid 1133693335, wikipedia, wiki, book, books, library,

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