12.3: Pre-Cambrian Animal Life - Biology

12.3: Pre-Cambrian Animal Life - Biology

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Learning Outcomes

  • Describe the features that characterized the earliest animals and when they appeared on earth

The time before the Cambrian period is known as the Ediacaran period (from about 635 million years ago to 543 million years ago), the final period of the late Proterozoic Neoproterozoic Era (Figure 1). In addition to their morphological similarity, molecular analyses have revealed similar sequence homologies in their DNA.

The earliest life comprising Ediacaran biota was long believed to include only tiny, sessile, soft-bodied sea creatures. However, recently there has been increasing scientific evidence suggesting that more varied and complex animal species lived during this time, and possibly even before the Ediacaran period.

Fossils believed to represent the oldest animals with hard body parts were recently discovered in South Australia. These sponge-like fossils, named Coronacollina acula, date back as far as 560 million years, and are believed to show the existence of hard body parts and spicules that extended 20–40 cm from the main body (estimated about 5 cm long). Other fossils from the Ediacaran period are shown in Figure 2.

Another recent fossil discovery may represent the earliest animal species ever found. While the validity of this claim is still under investigation, these primitive fossils appear to be small, one-centimeter long, sponge-like creatures. These fossils from South Australia date back 650 million years, actually placing the putative animal before the great ice age extinction event that marked the transition between the Cryogenian period and the Ediacaran period. Until this discovery, most scientists believed that there was no animal life prior to the Ediacaran period. Many scientists now believe that animals may in fact have evolved during the Cryogenian period.

12.3: Pre-Cambrian Animal Life - Biology

By the end of this section, you will be able to:

  • Describe the features that characterized the earliest animals and when they appeared on earth
  • Explain the significance of the Cambrian period for animal evolution and the changes in animal diversity that took place during that time
  • Describe some of the unresolved questions surrounding the Cambrian explosion
  • Discuss the implications of mass animal extinctions that have occurred in evolutionary history

Many questions regarding the origins and evolutionary history of the animal kingdom continue to be researched and debated, as new fossil and molecular evidence change prevailing theories. Some of these questions include the following: How long have animals existed on Earth? What were the earliest members of the animal kingdom, and what organism was their common ancestor? While animal diversity increased during the Cambrian period of the Paleozoic era, 530 million years ago, modern fossil evidence suggests that primitive animal species existed much earlier.


The seemingly rapid appearance of fossils in the "Primordial Strata" was noted by William Buckland in the 1840s, [14] 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. [15] The long-running puzzlement about the appearance of the Cambrian fauna, seemingly abruptly, without precursor, centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period of time 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. [16] 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. [14] 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. [17] 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, Charles 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. [15] In the sixth edition of his book, he stressed his problem further as: [18]

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. [19]

Earlier fossil evidence has since been found. The earliest claim is that the history of life on earth goes back 3,850 million years : [20] 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. [21] In 1948, Preston Cloud argued that a period of "eruptive" evolution occurred in the Early Cambrian, [22] but as recently as the 1970s, no sign was seen of how the 'relatively' modern-looking organisms of the Middle and Late Cambrian arose. [21]

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. [23] [24] 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, [25] 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. [26]

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. [27] [28] [29]

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. [30]

Therefore, dates or descriptions of sequences of events should be regarded with some caution until better data become available.

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. [31] Also, biases exist in the fossil record: different environments are more favourable to the preservation of different types of organism or parts of organisms. [32] 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. [21]

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) [33] the middle Cambrian Burgess Shale (British Columbia, Canada) [34] 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. [35] 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. [36]

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. [37] [38] [39]

Trace fossils Edit

Trace fossils consist mainly of tracks and burrows, but also include coprolites (fossil feces) and marks left by feeding. [40] [41] 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. [42] 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). [41]

Geochemical observations Edit

Several chemical markers indicate a drastic change in the environment around the start of the Cambrian. The markers are consistent with a mass extinction, [43] [44] or with a massive warming resulting from the release of methane ice. [45] 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. [45] 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, [46] and estimates produced by different techniques vary by a factor of two. [47] 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. [48]

  • = 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. [50] Despite the seemingly different external appearances of organisms, they are classified into phyla based on their internal and developmental organizations. [51] 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. [21] [52]

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. [49] [53]

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. [54] Porifera and Cnidaria are radially symmetrical, not bilaterian, and not triploblastic.

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.

Phylogenetic analysis has been used to support the view that during the Cambrian explosion, metazoans (multi-celled animals) evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates. [ citation needed ]

Evidence of animals around 1 billion years ago Edit

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. [27] [28] [55]

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. [55] A consideration of taxon longevity appears to support an increase in predation pressure around this time. [56] 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. [57]

Fossils of the Doushantuo formation Edit

The layers of the Doushantuo formation from around 580 million year old [58] 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. [59] Another fossil, Vernanimalcula, has been interpreted as a coelomate bilaterian, [60] but may simply be an infilled bubble. [61]

These fossils form the earliest hard-and-fast evidence of animals, as opposed to other predators. [59] [62]

Burrows Edit

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. [65] They fed above the sediment surface, but were forced to burrow to avoid predators. [66]

Around the start of the Cambrian (about 542 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. [64]

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 artefact. [67]

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. [68] 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. [67]

Ediacaran organisms Edit

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. [57] This radiation, the first in the fossil record, [57] is followed soon after by an array of unfamiliar, large fossils dubbed the Ediacara biota, [69] which flourished for 40 million years until the start of the Cambrian. [70] 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. [71]

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. [72]

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), [29] [73] echinoderms (Arkarua) [74] and arthropods (Spriggina, [75] Parvancorina, [76] 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. [77] However, there seems little doubt that Kimberella was at least a triploblastic bilaterian animal. [77] 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. [78]

The first Ediacaran and lowest Cambrian (Nemakit-Daldynian) skeletal fossils represent tubes and problematic sponge spicules. [79] 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. [80] 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) [81] [82] 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. [83] Ediacaran mineralized tubes are often found in carbonates of the stromatolite reefs and thrombolites, [84] [85] 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). [85] [86] [87] 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. [88] A possible "evolutionary arms race" between predators and prey is one of the hypotheses that attempt to explain the Cambrian explosion. [55]

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. [89] [90] [91] [92] Also soft-bodied extant phyla such as comb jellies, scalidophorans, entoproctans, horseshoe worms and lobopodians had armored forms. [93] 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. [94]

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. [95] Older (

750 Ma) fossils indicate that mineralization long preceded the Cambrian, probably defending small photosynthetic algae from single-celled eukaryotic predators. [96] [97]

Trace 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. [98]

Small shelly fauna Edit

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. [99]

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 Edit

The earliest trilobite fossils are about 530 million years old, but the class was already quite diverse and worldwide, suggesting they had been around for quite some time. [100] 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. [101]

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

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. [102] 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. [103]

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. [104]

Early Cambrian crustaceans Edit

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". [105] 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) [106] 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. [105]

Early Ordovician radiation Edit

After an extinction at the Cambrian–Ordovician boundary, another radiation occurred, which established the taxa that would dominate the Palaeozoic. [107]

During this radiation, the total number of orders doubled, and families tripled, [107] increasing marine diversity to levels typical of the Palaeozoic, [45] and disparity to levels approximately equivalent to today's. [11]

The event lasted for about the next 20 [5] [108] –25 [109] [110] 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. [111] Different authors break the explosion down into stages in 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. [112]

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. [113]

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. [113]

There is strong evidence for species of Cnidaria and Porifera existing in the Ediacaran [114] and possible members of Porifera even before that during the Cryogenian. [115] Bryozoans do not appear in the fossil record until after the Cambrian, in the Lower Ordovician. [116]

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. [24] [25]

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), [29] [73] the protostome and deuterostome lineages must have split significantly before 550 million years ago (deuterostomes are the other main group of coelomates). [117] Even if it is not a protostome, it is widely accepted as a bilaterian. [77] [117] 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 . [117]

Trace fossils [71] and predatory borings in Cloudina shells provide further evidence of Ediacaran animals. [118] 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. [60] [61] [119] Earlier still, predatory pressure has acted on stromatolites and acritarchs since around 1,250 million years ago . [55]

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. [121] 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. [12] 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. [12]

Ecological complexity among marine animals increased in the Cambrian, as well later in the Ordovician. [11] However, recent research has overthrown the once-popular idea that disparity was exceptionally high throughout the Cambrian, before subsequently decreasing. [122] In fact, disparity remains relatively low throughout the Cambrian, with modern levels of disparity only attained after the early Ordovician radiation. [11]

The diversity of many Cambrian assemblages is similar to today's, [123] [105] 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. [124] 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. [125] Thus Harry Blackmore Whittington's questions regarding the abrupt nature of the Cambrian explosion remain, and have yet to be satisfactorily answered. [126]

The Cambrian explosion as survivorship bias Edit

Budd and Mann [127] 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.

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 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. [128] Initially, oxygen levels did not increase substantially in the atmosphere. [129] 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. [130] As a general trend, the concentration of oxygen in the atmosphere has risen gradually over about the last 2.5 billion years. [21]

Oxygen levels seem to have a positive correlation with diversity in eukaryotes well before the Cambrian period. [131] 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. [131] 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. [131]

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) but, 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. [43] Other metabolic functions may have been inhibited by lack of oxygen, for example the construction of tissue such as collagen, required for the construction of complex structures, [132] or to form molecules for the construction of a hard exoskeleton. [133] However, animals were not affected when similar oceanographic conditions occurred in the Phanerozoic there is no convincing correlation between oxygen levels and evolution, so oxygen may have been no more a prerequisite to complex life than liquid water or primary productivity. [134]

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. [135] 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

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. [136] 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 [45] the cold periods may even have delayed the evolution of large size organisms. [55]

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. [137] Alternatively a high influx of ions could have been provided by the widespread erosion that produced Powell's Great Unconformity. [138]

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. [139]

Developmental explanations Edit

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 [45] combines with molecular data [140] 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. [141]

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. [142]

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. [45]

End-Ediacaran mass extinction Edit

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 δ 13 C record. It is suspected that several global anoxic events were responsible for the extinction. [143] [144]

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. [45]

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 able to seek out more oxygen-rich environments an advantage over sessile forms of life. [145]

Evolution of eyes Edit

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. [146] Nevertheless, many scientists doubt that vision could have caused the explosion. Eyes may well have evolved long before the start of the Cambrian. [147] 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. [45]

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. [148]

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. [55] However, the intensity of predation does appear to have increased dramatically during the Cambrian [149] as new predatory "tactics" (such as shell-crushing) emerged. [150] 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. [151] 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". [151]

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. [35]

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). [35]

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. [10] [152]

Metazoans have an amazing ability to increase diversity through coevolution. [57] 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. [57]

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. [153]

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. [154] 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. [129] These proteins translate into larger, more complex structures that allow organisms better to adapt to their environments. [155] 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. [155]

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. [57] 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. [21] 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. [156]

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. [157]

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 . [21] Furthermore, angiosperms (flowering plants) originated and rapidly diversified during the Cretaceous period.

2. From Antiquity to the Renaissance

Humans have been using other vertebrate animal species (referred to henceforth as animals) as models of their anatomy and physiology since the dawn of medicine. Because of the taboos regarding the dissection of humans, physicians in ancient Greece dissected animals for anatomical studies [1]. Prominent physicians from this period who performed “vivisections” (stricto sensu the exploratory surgery of live animals, and historically used lato sensu as a depreciative way of referring to animal experiments) include Alcmaeon of Croton (6th𠄵th century BCE) [2,3], Aristotle, Diocles, Praxagoras (4th century BCE), Erasistratus, and Herophilus (4th𠄳rd century BCE) [1,3,4]. The latter two were Hellenic Alexandrians who disregarded the established taboos and went on to perform dissection and vivisection on convicted criminals, benefiting from the favorable intellectual and scientific environment in Alexandria at the time [1]. All of these authors had a great influence on Galen of Pergamon (2nd𠄳rd century CE), the prolific Roman physician of Greek ethnicity who developed, to an unprecedented level, the techniques for dissection and vivisection of animals [3,5] and on which he based his many treatises of medicine. These remained canonical, authoritative, and undisputed until the Renaissance [1,6].

For most ancient Greeks, using live animals in experiments did not raise any relevant moral questions. The supposed likeliness of humans to their anthropomorphic deities granted them a higher ranking in the scala naturae (“the chain of being”), a strict hierarchy where all living and non-living natural things𠅏rom minerals to the gods—were ranked according to their proximity to the divine. This view of humans as superior would later influence and underline the Judeo-Christian perspective of human dominion over all nature, as represented by texts by Augustine of Hippo (IV century) and Thomas Aquinas (XIII Century), the most influential Christian theologians of the Middle Ages. For Augustine, animals were part of a natural world created to serve humans (as much as the �rth, water and sky”) and humankind did not have any obligations to them. For Thomas Aquinas, the mistreatment of another person’s animal would be sinful, not for the sake of the animal in itself, but because it is someone else’s property. Cruelty to animals was nevertheless condemned by Aquinas, as it could lead humans to develop feelings and actions of cruelty towards other humans. Also, for this theologian, one could love irrational creatures for the sake of charity, the love of God and the benefit of fellow humans (for selected texts, see reference [7]).

The belief amongst ancient Greek physicians that nature could be understood by means of exploration and experiment𠅊nd the medical knowledge thus obtained to be of clinical relevance in practice—would be replaced by other schools of medical thought. Most notably, the Empiric school (3rd century BCE𠄴th century) would reject the study of anatomy and physiology by dissection of cadavers or by vivisection, not only on the grounds of cruelty and the established taboos, but also for its uselessness. Empiricists believed pain and death would distort the normal appearance of internal organs and criticized the speculative nature of the conclusions drawn from experiments. Indeed, and despite taking an experimental approach to understand the human body and illness, the interpretations of physiological processes made by ancient Greeks who performed vivisections were often inaccurate. The theoretical frameworks by which physicians interpreted their experiments more often than not led them to misguided conclusions. Observations would be understood in light of such paradigms as the Hippocratic theory of the four humors or the Pythagorean theory of the four elements, along with others of natural or supernatural basis, and to which they added their own theoretical conceptions and observational errors [1,4,6,8,9]. The study of human or animal anatomy and physiology was hence deemed irrelevant for clinical practice. Beginning with the decline of the Roman Empire and continuing throughout the Middle Ages, physiological experiments𠅊long with scientific activity in general—would fall almost entirely into disuse and medical knowledge would become dogmatic. In an increasingly Christianized Europe, there was little motivation to pursue scientific advancement of medical knowledge, as people became more concerned with eternal life than with worldly life, and returned to Pre-Hippocratic beliefs in supernatural causes for disease and in the healing power of faith and superstition. Therefore, and despite medieval physicians’ reverence for Galen and his predecessors, the experimental approach used by these classical authors had been sentenced to oblivion [3,8,9,10,11].

The use of animal experiments to satisfy scientific enquiry would only re-emerge in the Renaissance. Flemish anatomist Vesalius (1514�), through the course of his work as a physician and surgeon, realized that many anatomical structures thought to exist in humans—on account of them being present in other animals—were in fact absent [6]. This led him to break the established civil and religious rules and dissect illegally obtained human cadavers, and publish very accurate descriptions of the human anatomy, which challenged the authority of the classical authors. As Herophilus did centuries before (but not carried on by his successors) [1] Vesalius would also examine the similarities and differences between the internal structure of humans and other animals, thus setting the foundations of modern comparative anatomy.

Alongside the progress in anatomical knowledge made possible by experimenters defying the Catholic Church’s opposition to the dissection of human bodies, the Renaissance period also witnessed the resurgence of vivisection as a heuristic method for the understanding of animal physiology. Vesalius would again recognize the value of physiological experiments on animals as both a learning and teaching resource—he would vivisect animals for medical students as the finishing touch at the end of his courses𠅊 view shared by his contemporary, and presumable student and rival, Realdo Colombo (1516�) [3]. Later, Francis Bacon (1561�), considered by many the founder of modern scientific methodology, would also approve of the scientific relevance of vivisection, stating that “the inhumanity of anatomia vivorum was by Celsus justly reproved yet in regard of the great use of this observation, the inquiry needed (…) might have been well diverted upon the dissection of beasts alive, which notwithstanding the dissimilitude of their parts may sufficiently satisfy this inquiry” [12].

Public Forums

One of the striking features of de-extinction is that, even at this early phase, public outreach has been proactive and extensive. The far-reaching coverage of this effort in National Geographic, through the TEDx programme and other TEDx talks and related media coverage of these, and academic conferences attest to this. This public outreach has indeed helped to establish and define de-extinction as a scientific field and topic of public interest. Meanwhile, discussions on cloning endangered animals have, in contrast, been more conventional, largely occurring in the context of professional conferences, within zoological organizations, and in journal commentaries.

Despite the seemingly more ‘public’ nature of de-extinction, we argue that cloning endangered animals has, at times, engaged with ‘public debate’ in a manner that de-extinction could usefully learn from as it moves forward. At least some cloning experiments involving endangered animals have taken up and addressed the concerns of their critics by changing their scientific practices. For example, different kinds of cells and animals were used in different cloning experiments so that the resulting animal did not simply show that it was possible to clone, but also how cloning could be of value to species preservation efforts. After the gaur died, the San Diego Zoo decided to clone a banteng instead because he was considered more genetically valuable within contemporary ex situ preservation practices [9],[13]. In other words, the concerns of conservationists have been the basis for reformulating the experimental practice of cloning endangered animals. This raises the question: how might future de-extinction experiments be designed in order to address the concerns that have been raised over the past year? As Marris and Rose noted [5], ‘upstream’ public engagement seeks “to enable a range of actors, including lay publics, but also the widest possible range of people who might be interested or affected, to help shape the trajectory of innovation.” The TEDx programme and Stanford conference show that a range of actors have been brought together in order to discuss de-extinction. This is a laudable opening, which can now take on the challenge of bringing such diverse groups together in the conduct of de-extinction research itself.

Developing effective animal-assisted intervention programs involving visiting dogs for institutionalized geriatric patients: a pilot study

Aim: An ever increasing interest in the therapeutic aspects of the human-animal bond has led to a proliferation of animal-assisted interventions (AAI) involving dogs. However, most of these programs lack a solid methodological structure, and basic evaluative research is needed. The purpose of this study was to test the value of dog-assisted interventions as an innovative tool to increase quality of life in the geriatric population.

Methods: Nineteen patients (men and women) with a mean age of 85 years participated in the study. Interactions between patients and visiting dogs occurred either in a social situation (socialization sessions) or in a therapeutic context (physical therapy sessions). We derived and characterized a specific ethogram of elderly-dog interactions aimed at evaluating the effectiveness of visiting dogs in improving mood, catalyzing social interactions and reducing their everyday apathetic state. Cortisol levels were also measured in the saliva, and depressive state was evaluated.

Results: Overall, results show a time-dependent increase in social behaviour and spontaneous interactions with the dogs. Dog-mediated interactions affected the daily increase in cortisol levels, thus having an 'activational effect', in contrast to the apathetic state of institutionalized elderly.

Conclusions: Dog-mediated intervention programs appear to be promising tools to improve the social skills and enrich the daily activities of the institutionalized elderly.

© 2012 The Authors. Psychogeriatrics © 2012 Japanese Psychogeriatric Society.


The first Ediacaran fossils discovered were the disc-shaped Aspidella terranovica in 1868. Their discoverer, Scottish geologist Alexander Murray, found them useful aids for correlating the age of rocks around Newfoundland. [18] However, since they lay below the "Primordial Strata" of the Cambrian that was then thought to contain the very first signs of animal life, a proposal four years after their discovery by Elkanah Billings that these simple forms represented fauna was dismissed by his peers. Instead, they were interpreted as gas escape structures or inorganic concretions. [18] No similar structures elsewhere in the world were then known and the one-sided debate soon fell into obscurity. [18] In 1933, Georg Gürich discovered specimens in Namibia [19] but the firm belief that complex life originated in the Cambrian led to them being assigned to the Cambrian Period and no link to Aspidella was made. In 1946, Reg Sprigg noticed "jellyfishes" in the Ediacara Hills of Australia's Flinders Ranges [20] but these rocks were believed to be Early Cambrian so, while the discovery sparked some interest, little serious attention was garnered.

It was not until the British discovery of the iconic Charnia that the pre-Cambrian was seriously considered as containing life. This frond-shaped fossil was found in England's Charnwood Forest first by a 15-year-old girl in 1956 (Tina Negus, who was not believed [21] [22] ) and then the next year by a group of three schoolboys including 15-year-old Roger Mason. [23] [24] [25] Due to the detailed geological mapping of the British Geological Survey, there was no doubt these fossils sat in Precambrian rocks. Palaeontologist Martin Glaessner finally, in 1959, made the connection between this and the earlier finds [26] [27] and with a combination of improved dating of existing specimens and an injection of vigour into the search many more instances were recognised. [28]

All specimens discovered until 1967 were in coarse-grained sandstone that prevented preservation of fine details, making interpretation difficult. S.B. Misra's discovery of fossiliferous ash-beds at the Mistaken Point assemblage in Newfoundland changed all this as the delicate detail preserved by the fine ash allowed the description of features that were previously undiscernible. [29] [30] It was also the first discovery of Ediacarans in deep water sediments. [31]

Poor communication, combined with the difficulty in correlating globally distinct formations, led to a plethora of different names for the biota. In 1960 the French name "Ediacarien" – after the Ediacara Hills – was added to the competing terms "Sinian" and "Vendian" [32] for terminal-Precambrian rocks, and these names were also applied to the life-forms. "Ediacaran" and "Ediacarian" were subsequently applied to the epoch or period of geological time and its corresponding rocks. In March 2004, the International Union of Geological Sciences ended the inconsistency by formally naming the terminal period of the Neoproterozoic after the Australian locality. [33]

The term "Ediacaran biota" and similar ("Ediacara"/"Ediacaran"/"Ediacarian"/"Vendian", "fauna"/"biota") has, at various times, been used in a geographic, stratigraphic, taphonomic, or biological sense, with the latter the most common in modern literature. [34]

Microbial mats Edit

Microbial mats are areas of sediment stabilised by the presence of colonies of microbes that secrete sticky fluids or otherwise bind the sediment particles. They appear to migrate upwards when covered by a thin layer of sediment but this is an illusion caused by the colony's growth individuals do not, themselves, move. If too thick a layer of sediment is deposited before they can grow or reproduce through it, parts of the colony will die leaving behind fossils with a characteristically wrinkled ("elephant skin") and tubercular texture. [35]

Some Ediacaran strata with the texture characteristics of microbial mats contain fossils, and Ediacaran fossils are almost always found in beds that contain these microbial mats. Although microbial mats were once widespread, the evolution of grazing organisms in the Cambrian vastly reduced their numbers. [36] These communities are now limited to inhospitable refugia, such as the stromatolites found in Hamelin Pool Marine Nature Reserve in Shark Bay, Western Australia, where the salt levels can be twice those of the surrounding sea. [37]

Fossilization Edit

The preservation of these fossils is one of their great fascinations to science. As soft-bodied organisms, they would normally not fossilize and, unlike later soft-bodied fossil biota such as the Burgess Shale or Solnhofen Limestone, the Ediacaran biota is not found in a restricted environment subject to unusual local conditions: they were a global phenomenon. The processes that were operating must have been systemic and worldwide. There was something very different about the Ediacaran Period that permitted these delicate creatures to be left behind and it is thought the fossils were preserved by virtue of rapid covering by ash or sand, trapping them against the mud or microbial mats on which they lived. [38] Their preservation was possibly enhanced by the high concentration of silica in the oceans before silica-secreting organisms such as sponges and diatoms became prevalent. [39] Ash beds provide more detail and can readily be dated to the nearest million years or better using radiometric dating. [40] However, it is more common to find Ediacaran fossils under sandy beds deposited by storms or high-energy bottom-scraping ocean currents known as turbidites. [38] Soft-bodied organisms today rarely fossilize during such events, but the presence of widespread microbial mats probably aided preservation by stabilising their impressions in the sediment below. [41]

Scale of preservation Edit

The rate of cementation of the overlying substrate relative to the rate of decomposition of the organism determines whether the top or bottom surface of an organism is preserved. Most disc-shaped fossils decomposed before the overlying sediment was cemented, whereupon ash or sand slumped in to fill the void, leaving a cast of the organism's underside.

Conversely, quilted fossils tended to decompose after the cementation of the overlying sediment hence their upper surfaces are preserved. Their more resistant nature is reflected in the fact that, in rare occasions, quilted fossils are found within storm beds as the high-energy sedimentation did not destroy them as it would have the less-resistant discs. Further, in some cases, the bacterial precipitation of minerals formed a "death mask", ultimately leaving a positive, cast-like impression of the organism. [42] [43]

Forms of Ediacaran fossil
The earliest discovered potential embryo, preserved within an acanthomorphic acritarch. The term 'acritarch' describes a range of unclassified cell-like fossils.
Tateana inflata (= 'Cyclomedusa' radiata) is the attachment disk of an unknown organism.
A cast of Charnia, the first accepted complex Precambrian organism. Charnia was once interpreted as a relative of the sea pens.
Dickinsonia displays the characteristic quilted appearance of Ediacaran enigmata.
Spriggina was originally interpreted as annelid or arthropod. However, lack of known limbs, and glide reflected isomers instead of true segments, rejects any such classification despite some superficial resemblance.
Late Ediacaran Archaeonassa-type trace fossils are commonly preserved on the top surfaces of sandstone strata.
Epibaion waggoneris, chain of trace platforms and the imprint of the body of Yorgia waggoneri (right), which created these traces on microbial mat.

The Ediacaran biota exhibited a vast range of morphological characteristics. Size ranged from millimetres to metres complexity from "blob-like" to intricate rigidity from sturdy and resistant to jelly-soft. Almost all forms of symmetry were present. [38] These organisms differed from earlier fossils by displaying an organised, differentiated multicellular construction and centimetre-plus sizes.

These disparate morphologies can be broadly grouped into form taxa:

"Embryos" Recent discoveries of Precambrian multicellular life have been dominated by reports of embryos, particularly from the Doushantuo Formation in China. Some finds [44] generated intense media excitement [45] though some have claimed they are instead inorganic structures formed by the precipitation of minerals on the inside of a hole. [46] Other "embryos" have been interpreted as the remains of the giant sulfur-reducing bacteria akin to Thiomargarita, [47] a view that, while it had enjoyed a notable gain of supporters [48] [49] as of 2007, has since suffered following further research comparing the potential Doushantuo embryos' morphologies with those of Thiomargarita specimens, both living and in various stages of decay. [50] A recent discovery of comparable Ediacaran fossil embryos from the Portfjeld Formation in Greenland has significantly expanded the paleogeograpical occurrence of Doushanuto-type fossil "embryos" with similar biotic forms now reported from differing paleolatitudes. [51] Microfossils dating from 632.5 million years ago – just 3 million years after the end of the Cryogenian glaciations – may represent embryonic 'resting stages' in the life cycle of the earliest known animals. [52] An alternative proposal is that these structures represent adult stages of the multicellular organisms of this period. [53] Discs Circular fossils, such as Ediacaria, Cyclomedusa and Rugoconites led to the initial identification of Ediacaran fossils as cnidaria, which include jellyfish and corals. [20] Further examination has provided alternative interpretations of all disc-shaped fossils: not one is now confidently recognised as a jellyfish. Alternate explanations include holdfasts and protists [54] the patterns displayed where two meet have led to many 'individuals' being identified as microbial colonies, [55] [56] and yet others may represent scratch marks formed as stalked organisms spun around their holdfasts. [57] Useful diagnostic characters are often lacking because only the underside of the organism is preserved by fossilisation. Bags Fossils such as Pteridinium preserved within sediment layers resemble "mud-filled bags". The scientific community is a long way from reaching a consensus on their interpretation. [58] Toroids The fossil Vendoglossa tuberculata from the Nama Group, Namibia, has been interpreted as a dorso-ventrally compressed stem-group metazoan, with a large gut cavity and a transversely ridged ectoderm. The organism is in the shape of a flattened torus, with the long axis of its toroidal body running through the approximate center of the presumed gut cavity. [59] Quilted organisms The organisms considered in Seilacher's revised definition of the Vendobionta [11] share a "quilted" appearance and resembled an inflatable mattress. Sometimes these quilts would be torn or ruptured prior to preservation: such damaged specimens provide valuable clues in the reconstruction process. For example, the three (or more) petaloid fronds of Swartpuntia germsi could only be recognised in a posthumously damaged specimen – usually multiple fronds were hidden as burial squashed the organisms flat. [60] These organisms appear to form two groups: the fractal rangeomorphs and the simpler erniettomorphs. [61] Including such fossils as the iconic Charnia and Swartpuntia, the group is both the most iconic of the Ediacaran biota and the most difficult to place within the existing tree of life. Lacking any mouth, gut, reproductive organs, or indeed any evidence of internal anatomy, their lifestyle was somewhat peculiar by modern standards the most widely accepted hypothesis holds that they sucked nutrients out of the surrounding seawater by osmotrophy [62] or osmosis. [63] However, others argue against this. [64] Non-Vendobionts Some Ediacaran organisms have more complex details preserved, which has allowed them to be interpreted as possible early forms of living phyla excluding them from some definitions of the Ediacaran biota. The earliest such fossil is the reputed bilaterian Vernanimalcula claimed by some, however, to represent the infilling of an egg-sac or acritarch. [46] [65] In 2020, Ikaria wariootia was claimed to represent one of the oldest organisms with anterior and posterior differentiation. [66] Later examples are almost universally accepted as bilaterians and include the mollusc-like Kimberella, [67] Spriggina (pictured) [68] and the shield-shaped Parvancorina [69] whose affinities are currently debated. [70] A suite of fossils known as the small shelly fossils are represented in the Ediacaran, most famously by Cloudina [71] a shelly tube-like fossil that often shows evidence of predatory boring, suggesting that, while predation may not have been common in the Ediacaran Period, it was at least present. Organic microfossils known as small carbonaceous fossils are also found in Ediacaran sediments, including the spiral-shaped Cochleatina [72] which has been found to span the Ediacaran–Cambrian boundary. Representatives of modern taxa existed in the Ediacaran, some of which are recognisable today. Sponges, red and green algæ, protists and bacteria are all easily recognisable with some pre-dating the Ediacaran by nearly three billion years. Possible arthropods have also been described. [73] Fossils of the hard-shelled foraminifera Platysolenites are known from the latest Ediacaran of western Siberia, coexisting with Cloudina and Namacalathus. [74] Filaments Filament-shaped structures in Precambrian fossils have been observed on many occasions. Frondose fossils in Newfoundland have been observed to have developed filamentous bedding planes, inferred to be stolonic outgrowths. [75] A study of Brazilian Ediacaran fossils found filamentous microfossils, suggested to be eukaryotes or large sulfur-oxidizing-bacteria (SOBs). [76] Fungus-like filaments found in the Doushantuo Formation have been interpreted as eukaryotes and possibly fungi, providing possible evidence for the evolution and terrestrialization of fungi

635 Ma. [77] Trace fossils With the exception of some very simple vertical burrows [78] the only Ediacaran burrows are horizontal, lying on or just below the surface of the seafloor. Such burrows have been taken to imply the presence of motile organisms with heads, which would probably have had a bilateral symmetry. This could place them in the bilateral clade of animals [79] but they could also have been made by simpler organisms feeding as they slowly rolled along the sea floor. [80] Putative "burrows" dating as far back as 1,100 million years may have been made by animals that fed on the undersides of microbial mats, which would have shielded them from a chemically unpleasant ocean [81] however their uneven width and tapering ends make a biological origin so difficult to defend [82] that even the original proponent no longer believes they are authentic. [83] The burrows observed imply simple behaviour, and the complex efficient feeding traces common from the start of the Cambrian are absent. Some Ediacaran fossils, especially discs, have been interpreted tentatively as trace fossils but this hypothesis has not gained widespread acceptance. As well as burrows, some trace fossils have been found directly associated with an Ediacaran fossil. Yorgia and Dickinsonia are often found at the end of long pathways of trace fossils matching their shape [84] these fossils are thought to be associated with ciliary feeding but the precise method of formation of these disconnected and overlapping fossils largely remains a mystery. [85] The potential mollusc Kimberella is associated with scratch marks, perhaps formed by a radula. [86]

Classification of the Ediacarans is difficult, and hence a variety of theories exist as to their placement on the tree of life.

Martin Glaessner proposed in The Dawn of Animal Life (1984) that the Ediacaran biota were recognizable crown group members of modern phyla, but were unfamiliar because they had yet to evolve the characteristic features we use in modern classification. [87]

In 1998 Mark McMenamin claimed Ediacarans did not possess an embryonic stage, and thus could not be animals. He believed that they independently evolved a nervous system and brains, meaning that "the path toward intelligent life was embarked upon more than once on this planet". [54]

In 2018 analysis of ancient sterols was taken as evidence that one of the period's most-prominent and iconic fossils, Dickinsonia, was an early animal. [13]

Cnidarians Edit

Since the most primitive eumetazoans—multi-cellular animals with tissues—are cnidarians, the first attempt to categorise these fossils designated them as jellyfish and sea pens. [88] However, more recent discoveries have established that many of the circular forms formerly considered "cnidarian medusa" are actually holdfasts – sand-filled vesicles occurring at the base of the stem of upright frond-like Ediacarans. A notable example is the form known as Charniodiscus, a circular impression later found to be attached to the long 'stem' of a frond-like organism that now bears the name. [89] [90]

The link between certain frond-like Ediacarans and sea pens has been thrown into doubt by multiple lines of evidence chiefly the derived nature of the most frond-like pennatulacean octocorals, their absence from the fossil record before the Tertiary, and the apparent cohesion between segments in Ediacaran frond-like organisms. [91] Some researchers have suggested that an analysis of "growth poles" discredits the pennatulacean nature of Ediacaran fronds. [92] [93]

Protozoans Edit

Adolf Seilacher has suggested the Ediacaran sees animals usurping giant protists as the dominant life form. [94] The modern xenophyophores are giant single-celled protozoans found throughout the world's oceans, largely on the abyssal plain. A recent genetic study suggested that the xenophyophores are a specialised group of Foraminifera. There are approximately 42 recognised species in 13 genera and 2 orders one of which, Syringammina fragilissima, is among the largest known protozoans at up to 20 centimetres in diameter.

New phylum Edit

Seilacher has suggested that the Ediacaran organisms represented a unique and extinct grouping of related forms descended from a common ancestor (clade) and created the kingdom Vendozoa, [95] [96] named after the now-obsolete Vendian era. He later excluded fossils identified as metazoans and relaunched the phylum "Vendobionta".

He described the Vendobionta as quilted cnidarians lacking stinging cells. This absence precludes the current cnidarian method of feeding, so Seilacher suggested that the organisms may have survived by symbiosis with photosynthetic or chemoautotrophic organisms. [97] Mark McMenamin saw such feeding strategies as characteristic for the entire biota, and referred to the marine biota of this period as a "Garden of Ediacara". [98]

Lichen hypothesis Edit

Greg Retallack's hypothesis that Ediacaran organisms were lichens [100] [101] has been controversial. [102] [103] [104] He argues that the fossils are not as squashed as known fossil jellyfish, and their relief is closer to compressed woody branches whose compaction can be estimated as compressed cylinders. He points out the chitinous walls of lichen colonies would provide a similar resistance to compaction, and claims the large size of the organisms (up to 1.5 metres long, far larger than any of the preserved burrows) also hints against classification with animals. Thin sections of Ediacaran fossils show lichen-like compartments and hypha-like wisps of ferruginized clay. [99] Finally, Ediacaran fossils from classic localities of the Flinders Ranges have been found in strata that Rettalack interprets to be both growth position within strata that he controversially interprets to be red calcareous and gypsiferous paleosols and possibly well-drained temperate desert soils. [101] [105] According to Retallack's interpretations, such habitats limit interpretive options for fractal Ediacaran fossils such as Dickinsonia to lichenised or unlichenised fungi, but other Ediacaran fossils could have been slime moulds or microbial colonies.

Other interpretations Edit

Several classifications have been used to accommodate the Ediacaran biota at some point, [106] from algae, [107] to protozoans, [108] to fungi [109] to bacterial or microbial colonies, [55] to hypothetical intermediates between plants and animals. [9]

A new extant genus discovered in 2014, Dendrogramma, which appears to be a basal metazoan but of unknown taxonomic placement, has been noted to have similarities with the Ediacaran fauna. [110] It has since been found to be a siphonophore, possibly even sections of a more complex species, [111] though this in turn has raised suspicions for a similar status for at least some Ediacaran organisms.

It took almost 4 billion years from the formation of the Earth for the Ediacaran fossils to first appear, 655 million years ago. While putative fossils are reported from 3,460 million years ago , [112] [113] the first uncontroversial evidence for life is found 2,700 million years ago , [114] and cells with nuclei certainly existed by 1,200 million years ago : [115] The reason why it took so long for forms with an Ediacaran grade of organisation to appear is uncertain.

It could be that no special explanation is required: the slow process of evolution simply required 4 billion years to accumulate the necessary adaptations. Indeed, there does seem to be a slow increase in the maximum level of complexity seen over this time, with more and more complex forms of life evolving as time progresses, with traces of earlier semi-complex life such as Nimbia, found in the 610 million year old Twitya formation, [116] and older rocks dating to 770 million years ago in Kazakhstan, [117] possibly displaying the most complex morphology of the time.

The alternative train of thought is that it was simply not advantageous to be large until the appearance of the Ediacarans: the environment favoured the small over the large. Examples of such scenarios today include plankton, whose small size allows them to reproduce rapidly to take advantage of ephemerally abundant nutrients in algal blooms. But for large size never to be favourable, the environment would have to be very different indeed.

A primary size-limiting factor is the amount of atmospheric oxygen. Without a complex circulatory system, low concentrations of oxygen cannot reach the centre of an organism quickly enough to supply its metabolic demand.

On the early Earth, reactive elements, such as iron and uranium, existed in a reduced form that would react with any free oxygen produced by photosynthesising organisms. Oxygen would not be able to build up in the atmosphere until all the iron had rusted (producing banded iron formations), and all the other reactive elements had been oxidised. Donald Canfield detected records of the first significant quantities of atmospheric oxygen just before the first Ediacaran fossils appeared [118] – and the presence of atmospheric oxygen was soon heralded as a possible trigger for the Ediacaran radiation. [119] Oxygen seems to have accumulated in two pulses the rise of small, sessile (stationary) organisms seems to correlate with an early oxygenation event, with larger and mobile organisms appearing around the second pulse of oxygenation. [120] However, the assumptions underlying the reconstruction of atmospheric composition have attracted some criticism, with widespread anoxia having little effect on life where it occurs in the Early Cambrian and the Cretaceous. [121]

Periods of intense cold have also been suggested as a barrier to the evolution of multicellular life. The earliest known embryos, from China's Doushantuo Formation, appear just a million years after the Earth emerged from a global glaciation, suggesting that ice cover and cold oceans may have prevented the emergence of multicellular life. [122] Potentially, complex life may have evolved before these glaciations, and been wiped out. However, the diversity of life in modern Antarctica has sparked disagreement over whether cold temperatures increase or decrease the rate of evolution.

In early 2008 a team analysed the range of basic body structures ("disparity") of Ediacaran organisms from three different fossil beds: Avalon in Canada, 575 to 565 million years ago White Sea in Russia, 560 to 550 million years ago and Nama in Namibia, 550 to 542 million years ago , immediately before the start of the Cambrian. They found that, while the White Sea assemblage had the most species, there was no significant difference in disparity between the three groups, and concluded that before the beginning of the Avalon timespan these organisms must have gone through their own evolutionary "explosion", which may have been similar to the famous Cambrian explosion . [123]

Preservation bias Edit

The paucity of Ediacaran fossils after the Cambrian could simply be due to conditions that no longer favoured the fossilisation of Ediacaran organisms, which may have continued to thrive unpreserved. [35] However, if they were common, more than the occasional specimen [7] [124] might be expected in exceptionally preserved fossil assemblages (Konservat-Lagerstätten) such as the Burgess Shale and Chengjiang. [125] There are at present no widely accepted reports of Ediacara-type organisms in the Cambrian period, though there are a few disputed reports, as well as unpublished observations of 'vendobiont' fossils from 535 Ma Orsten-type deposits in China. [126]

Predation and grazing Edit

It is suggested that by the Early Cambrian, organisms higher in the food chain caused the microbial mats to largely disappear. If these grazers first appeared as the Ediacaran biota started to decline, then it may suggest that they destabilised the microbial substrate, leading to displacement or detachment of the biota or that the destruction of the mat destabilised the ecosystem, causing extinctions.

Alternatively, skeletonised animals could have fed directly on the relatively undefended Ediacaran biota. [54] However, if the interpretation of the Ediacaran age Kimberella as a grazer is correct then this suggests that the biota had already had limited exposure to "predation". [67]

There is however little evidence for any trace fossils in the Ediacaran Period, which may speak against the active grazing theory. Further, the onset of the Cambrian Period is defined by the appearance of a worldwide trace fossil assemblage, quite distinct from the activity-barren Ediacaran Period.

Competition Edit

It is possible that increased competition due to the evolution of key innovations among other groups, perhaps as a response to predation, [127] drove the Ediacaran biota from their niches. However, this argument has not successfully explained similar phenomena. For instance, the bivalve molluscs' "competitive exclusion" of brachiopods was eventually deemed to be a coincidental result of two unrelated trends. [128]

Change in environmental conditions Edit

While it is difficult to infer the effect of changing planetary conditions on organisms, communities and ecosystems, great changes were happening at the end of the Precambrian and the start of the Early Cambrian. The breakup of the supercontinents, [129] rising sea levels (creating shallow, "life-friendly" seas), [130] a nutrient crisis, [131] fluctuations in atmospheric composition, including oxygen and carbon dioxide levels, [132] and changes in ocean chemistry [133] (promoting biomineralisation) [134] could all have played a part.

Ediacaran-type fossils are recognised globally in 25 localities [33] and a variety of depositional conditions, and are commonly grouped into three main types, known as assemblages and named after typical localities. Each assemblage tends to occupy its own region of morphospace, and after an initial burst of diversification changes little for the rest of its existence. [135]

Avalon-type assemblage Edit

The Avalon-type assemblage is defined at Mistaken Point in Canada, the oldest locality with a large quantity of Ediacaran fossils. [137] The assemblage is easily dated because it contains many fine ash-beds, which are a good source of zircons used in the uranium-lead method of radiometric dating. These fine-grained ash beds also preserve exquisite detail. Constituents of this biota appear to survive through until the extinction of all Ediacarans at the base of the Cambrian. [135]

One interpretation of the biota is as deep-sea-dwelling rangeomorphs [138] such as Charnia, all of which share a fractal growth pattern. They were probably preserved in situ (without post-mortem transportation), although this point is not universally accepted. The assemblage, while less diverse than the Ediacara- or Nama-types, resembles Carboniferous suspension-feeding communities, which may suggest filter feeding [139] – by most interpretations, the assemblage is found in water too deep for photosynthesis. The low diversity may reflect the depth of water – which would restrict speciation opportunities – or it may just be too young for a rich biota to have evolved. Opinion is currently divided between these conflicting hypotheses. [140]

An alternative explanation for the distinct composition of the Avalon-type assemblage is that it was a terrestrial assemblage of volcaniclastic coastal soils near a continental volcanic arc. [136] This view is based on geochemical studies of the substrates of Mistaken Point fossils and associated matrix supported tuffs and volcanic bombs that could only form on land. [141] Some of these fossils such as Fractofusus and Charniodiscus were found in strata that Retallack interprets to be red well drained paleosols of coastal plains, but others such as Aspidella were found in strata that Retallack interprets to be intertidal paleosols.

Ediacara-type assemblage Edit

The Ediacara-type assemblage is named after Australia's Ediacara Hills, and consists of fossils preserved in facies of coastal lagoons and rivers. They are typically found in red gypsiferous and calcareous paleosols formed on loess and flood deposits in an arid cool temperate paleoclimate. [101] Most fossils are preserved as imprints in microbial earths, [142] but a few are preserved within sandy units. [140] [135]

Nama-type assemblage Edit

The Nama assemblage is best represented in Namibia. Three-dimensional preservation is most common, with organisms preserved in sandy beds containing internal bedding. Dima Grazhdankin believes that these fossils represent burrowing organisms, [58] while Guy Narbonne maintains they were surface dwellers. [143] These beds are sandwiched between units comprising interbedded sandstones, siltstones and shales – with microbial mats, where present, usually containing the fossils. The environment is interpreted as sand bars formed at the mouth of a delta's distributaries. [140] Mattress-like vendobionts (Ernietta, Pteridinium, Rangea) in these sandstones form a very different assemblage from vermiform fossils (Cloudina, Namacalathus) of Ediacaran "wormworld" in marine dolomite of Namibia. [144]

Significance of assemblages Edit

In the White Sea region of Russia, all three assemblage types have been found in close proximity. This, and the faunas' considerable temporal overlap, makes it unlikely that they represent evolutionary stages or temporally distinct communities. Since they are globally distributed – described on all continents except Antarctica – geographical boundaries do not appear to be a factor [145] the same fossils are found at all palaeolatitudes (the latitude where the fossil was created, accounting for continental drift) and in separate sedimentary basins. [140]

It is most likely that the three assemblages mark organisms adapted to survival in different environments, and that any apparent patterns in diversity or age are in fact an artefact of the few samples that have been discovered – the timeline (right) demonstrates the paucity of Ediacaran fossil-bearing assemblages. An analysis of one of the White Sea fossil beds, where the layers cycle from continental seabed to inter-tidal to estuarine and back again a few times, found that a specific set of Ediacaran organisms was associated with each environment. [140]

As the Ediacaran biota represent an early stage in multicellular life's history, it is unsurprising that not all possible modes of life are occupied. It has been estimated that of 92 potentially possible modes of life – combinations of feeding style, tiering and motility — no more than a dozen are occupied by the end of the Ediacaran. Just four are represented in the Avalon assemblage. [146] The lack of large-scale predation and vertical burrowing are perhaps the most significant factors limiting the ecological diversity the emergence of these during the Early Cambrian allowed the number of lifestyles occupied to rise to 30.


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Gene therapy applied to optic nerve regeneration

In the current study, the researchers targeted cells in the central nervous system because it is the first part of the body affected by aging. After birth, the ability of the central nervous system to regenerate declines rapidly.

To test whether the regenerative capacity of young animals could be imparted to adult mice, the researchers delivered the modified three-gene combination via an AAV into retinal ganglion cells of adult mice with optic nerve injury.

For the work, Lu and Sinclair partnered with Zhigang He , HMS professor of neurology and of ophthalmology at Boston Children’s Hospital, who studies optic nerve and spinal cord neuro-regeneration.

The treatment resulted in a two-fold increase in the number of surviving retinal ganglion cells after the injury and a five-fold increase in nerve regrowth.

“At the beginning of this project, many of our colleagues said our approach would fail or would be too dangerous to ever be used,” said Lu. “Our results suggest this method is safe and could potentially revolutionize the treatment of the eye and many other organs affected by aging.”

Lessons from the Hunt

The Gunflint and Bitter Springs articles of 1965 charted a new course, showing for the first time that a search strategy centered on the peculiarities of the Precambrian fossil record would pay off. The four keys of the strategy, as valid today as they were three decades ago, are to search for (i) microscopic fossils in (ii) black cherts that are (iii) fine-grained and (iv) associated with Cryptozoon-like structures. Each part plays a role.

(i) Megascopic eukaryotes, the large organisms of the Phanerozoic, are now known not to have appeared until shortly before the beginning of the Cambrian—except in immediately sub-Cambrian strata, the hunt for large body fossils in Precambrian rocks was doomed from the outset.

(ii) The blackness of a chert commonly gives a good indication of its organic carbon content—like fossil-bearing coal deposits, cherts rich in petrified organic-walled microfossils are usually a deep jet black color.

(iii) The fineness of the quartz grains making up a chert provides another hint of its fossil-bearing potential—cherts subjected to the heat and pressure of geologic metamorphism are often composed of recrystallized large grains that give them a sugary appearance whereas cherts that have escaped fossil-destroying processes are made up of cryptocrystalline quartz and have a waxy glasslike luster.

(iv) Cryptozoon-like structures (stromatolites) are now known to have been produced by flourishing microbial communities, layer upon layer of microscopic organisms that make up localized biocoenoses. Stromatolites permineralized by fine-grained chert early during diagenesis represent promising hunting grounds for the fossilized remnants of the microorganisms that built them.

Measured by virtually any criterion one might propose (Fig. 5), studies of Precambrian life have burst forth since the mid-1960s to culminate in recent years in discovery of the oldest fossils known, petrified cellular microbes nearly 3,500 million years old, more than three-quarters the age of the Earth (36). Precambrian paleobiology is thriving—the vast majority of all scientists who have ever investigated the early fossil record are alive and working today new discoveries are being made at an ever quickening clip—progress set in motion by the few bold scientists who blazed this trail in the 1950s and 1960s, just as their course was charted by the Dawsons, Walcotts, and Sewards, the pioneering pathfinders of the field. And the collective legacy of all who have played a role dates to Darwin and the dilemma of the missing Precambrian fossil record he first posed. After more than a century of trial and error, of search and final discovery, those of us who wonder about life's early history can be thankful that what was once “inexplicable” to Darwin is no longer so to us.


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