Choanozoa

Choanozoa
Above: A cell colony of the choanoflagellate Salpingoeca rosetta . A marine sponge. A comb jelly of the species Bathocyroe fosteri . Middle: A cnidarians of the genus Chrysaora . A mollusc from the class of snails . An arthropod of the species Apis mellifera . Below: the worm-shaped form of life Xenoturbella japonica . An echinoderm of the species Acanthaster brevispinus . A stringed animal of the species Macaca fascicularis .
Systematics
without rank: Amorphea without rank: Obazoa Super group : Opisthokonta without rank: Holozoa without rank: Filozoa without rank: Choanozoa
Scientific name
Choanozoa
Brunet & King , 2017

The group of Choanozoa includes all multicellular animals ( Metazoa ) and their closest relatives, the frilled flagellates ( Choanoflagellata ). It includes all life forms that develop the cell type of the choanocyte or whose ancestors probably once developed this cell type.

Among today's animals, collar whip cells are only found in the sponges ( Porifera ); they were lost in all other animals. With a few exceptions, these organisms feed on bacteria, which they filter out of the water with the help of the collar whip cells. The other animals have no collar whip cells. Instead, many of them have quite similar cell types.

The collar flagellates are microscopic eukaryotes made of collar flagella cells. Their egg-shaped to rounded cell bodies have a diameter of a few to a few tens of micrometers at most. Collared flagellates form both free-swimming and sessile forms and occur as single cells or in small cell colonies . They thrive in the oceans and brackish water as well as in fresh water including bottom water and aquifers . Collared flagellates feed on bacteria and virus particles as filter feeders.

The tribal history of the Choanozoa probably goes back almost a billion years, but so far no or only controversial fossil evidence has been found for the first few hundred million years.

expression

In 1765 the British naturalist John Ellis published a study on sponges. He noticed that they were actively generating a flow of food and moving the area around their central outflow opening ( osculum ) on their own. Ellis' publication thus provided two pieces of evidence to classify sponges into the group of animals in a well-founded manner. In 1831, the German natural scientist Christian Gottfried Ehrenberg described the “many-headed pillar bell”, a microorganism that is today assigned with some probability to the species Codonosiga botrytis . Ehrenberg had discovered the frilled flagellates. A first clear figure followed more than two decades later by the German botanist Georg Fresenius , but shortly after Ehrenberg's publication, the French naturalist Félix Dujardin recognized the great similarity between these organisms and certain cells of sponges. He baptized those cells choanocytes . A little later, the American biologist Henry James Clark dealt with choanoflagellates and choanocytes . He was the first to speak out in favor of a kinship between frilled flagellates and sponges. Seven years later, the British biologist Thomas Henry Huxley finally placed the Porifera on the one hand at the base of the Metazoa and on the other hand at the same time opposite all other animals.

As early as the last third of the 19th century it was known that the sponges are simply built animals and that they are probably closely related to the frilled flagellates. As a result, the prerequisites were already in place to view animals and frilled flagellates as a common, closed community of descent . However, this community was not given a name of its own for over a hundred and twenty years. With the beginning of phylogenomics , more and more evidence was constantly provided, which spoke in favor of a monophylum from Choanoflagellata and Metazoa.

In order to name the monophylum, it was tentatively suggested to simply regard the collar flagellates as animals as well. This would have resulted in a group called "Animalia", which would have collected Choanoflagellata and Metazoa. This possibility was not widely used. Because the word Animalia had long since established itself with a different meaning, it is used as a synonym for the word Metazoa. Very often the monophylum remained simply nameless. Only occasionally were further suggestions for names submitted, which were called "Choanimal" and "Apoikozoa". Both did not prevail. In 2017, the US biologists Thibaud Brunet and Nicole King introduced the word “Choanozoa” as a new name for the clade made from flagellates and animals. Although it had appeared eight years earlier with the exact same word meaning in a picture, it had received no further attention at the time. In addition, "Choanozoa" had been proposed in 1981 and 1983 by the British evolutionary biologist Thomas Cavalier-Smith as a name for a strain that would have exclusively included the flagellates. As a result, the name had long since become a synonym for the word Choanoflagellata since the 1980s. In the Cavalier-Smith literal sense, "Choanozoa" was used until the 2000s. Then Thibaut Brunet and Nicole King introduced their new meaning of the term. They also argued that "Choanozoa" in Cavalier-Smith's use would describe a paraphylum . Because in its then most recent version from 2008, the name would have included two other unicellular groups, the Ichthyosporea and the Filasterea . The revised classification of eukaryotes from 2019 took up the nomenclature proposal by Thibaud Brunet and Nicole King. According to her, the monophyletic group of frilled flagellates (Choanoflagellata) and animals (Metazoa) now bears the name Choanozoa .

characteristic

Single cell of the species Salpingoeca rosetta .

The cellular feature of the Choanozoa is the collar scourge cell. It is often viewed as a common derived trait ( autapomorphy ). The cell carries a single flagellum on one cell pole. It is surrounded by the eponymous “collar” - a wreath of stereovilli , which stretch a fine curtain of slime between them. The interplay of the scourge and collar serves the filtering nutrition. The beating scourge creates a stream of water. The incoming water passes through the slime curtain. Food particles in the water are held up by the mucus, then captured by filopodia and then taken up by the collar whip cells by means of endocytosis .

Collared flagellates are made up of one collar flagellate cell, there are also small cell colonies made up of several collar flagellates. In contrast to the frilled flagellates, animals consist of many cells. Among them, the sponges have the choanocytes as one of several cell types. In the other animals, the collared flagellum cells probably underwent various modifications or were completely lost in the course of evolution .

It is often assumed that the collar scourge cell was developed from the last common ancestor of all today's Choanozoa. That would mean that the collar flagellate cells of the collar flagellates and the collar flagella cells of the sponges are homologous with each other . Despite all the great similarities, both cell types also have a number of subtle differences. Such differences can provide reasons to find the homology of the cell types less convincing. Perhaps the two collar scourge cells could also converge independently of one another .

Systematics

External classification of the Choanozoa

Holozoa Teretosporea Ichthyosporea Pluriformea Filozoa Filasterea Choanozoa

Internal systematics of the Choanozoa
Choanozoa   Choanoflagellata  Craspedida     Acanthoecida  Metazoa  Porifera     Epitheliozoa

According to phylogenomic studies , the relationship of the Choanozoa consists of protozoa of very diverse appearance. The sister taxon is the group of Filasterea . With them they form the Filozoa community of descent . The Filozoa are combined with the Teretosporea to form the Holozoa . The Teretosporea, in turn, includes the ichthyosporea , which are often parasitic, and the pluriformea . Of the latter group, only one marine unicellular organism of the genus Corallochytrium and one limnic unicellular organism of the genus Syssomonas have so far been discovered. This current system replaces a somewhat older and competing hypothesis, according to which the Pluriformea ​​are not regarded as part of the Teretosporea, but as a sister taxon of the Filozoa.

The Choanozoa themselves are divided into collar flagellates and animals. Each of the two groups can in turn be divided into two parts. Choanoflagellates of the first order are called Craspedida . They envelop their cells with organic compounds that usually form a theca . Few of them, however, also cover themselves with a fine film of mucus, which in their case is called glycocalyx . Collared flagellates of the second order are called Acanthoecida . They surround their cells with a basket of braces, which is mainly made up of silicon dioxide and is called Lorica .

While frilled flagellates only live as single cells or in tiny cell colonies, animals developed their own form of multicellularity . Their bodies consist of many or very many cells and always of several differentiated cell types. The two groups of animals probably include on the one hand the sponges (Porifera) and possibly on the other hand the epitheliozoa . The cell associations of the sponges have hardly any resemblance to the tissues of the other animals. On the other hand, all epitheliozoa differentiate as a common feature an covering tissue. The covering tissue cells are firmly connected to one another with the help of desmosomes in so-called Zonulae Adhaerentes . The multicellularity of animals represents a completely independent evolutionary path and arose convergent to the other multicellularities of fungi , slime mold , egg fungi , candelabrum algae , land plants , red algae and brown algae .

Although the Epitheliozoa do not differentiate Choanocytes, they belong to the Choanozoa. This is because the Choanozoa taxon is primarily determined phylogenomically. First, because of similarities in their genomes, the frilled flagellates are grouped together with the animals as Choanozoa. Second, the epitheliozoa along with the sponges are identified as animals. As a result, all animals - i.e. all animals including the Epitheliozoa - also belong to the Choanozoa at the same time. Overall, the Choanozoa show greater genomic similarities with one another than with any other life form.

evolution

The evolution of the Choanozoa is part of the evolution of animals. Two events particularly stand out from the development. The first consists in the formation of the characteristic collar scourge cells, the second in the formation of the animal variant of multicellularity. Both events probably happened with fragile unicellular life forms or with soft and very few-celled organisms. They possibly took place more than 900 million years ago in the lower Proterozoic and apparently left no fossil traces.

Urchoanozoa

Possible pedigree of the Choanozoa.

If the collared scourge cell is regarded as a common original characteristic of all Choanozoa, the evolution of the Choanozoa would have started with the development of the collared scourge cell. This should have happened within a population of simply flagellated unicellular organisms from the Filozoa group. Presumably those organisms possessed some form of filopodia for which the name "Filodigiti" has been suggested. They are thread-like and always unbranched cell extensions that have the same thickness over their entire length and do not taper at the ends. Filopodia occur to this day in many unicellular organisms and in certain cell types of certain multicellular organisms. Recent choanoflagellates also form filopodia. Such cell extensions are given their shape by an internal structure made of microfilaments . The filaments consist of ten to thirty parallel strands of the structural protein actin . Some filopodia may have been shortened to stereovilli. The stereovilli were arranged in a ring around the flagellum and formed the collar of the collared scourge cell. The Urchoanoza could have evolved the typical collar scourge cells. They probably lived sessile and resembled today's frilled flagellates from the Craspedida group . According to a molecular clock , the Urchoanozoa may have separated from the rest of the Filozoa around 980 million years ago. The earth was in the neoproterozoic period of the Tonian and the supercontinent Rodinia was beginning to break up into smaller land masses. The oxygen content of the earth's atmosphere was less than a hundredth of today's value.

Cadherine

A path to multicellularity developed within the Choanozoa. According to the molecular clock, this event could have occurred a good 930 million years ago. This multicellularity is based crucially on certain adhesion proteins from the group of cadherins . The proteins are anchored in the cell membrane and protrude into the extracellular space . There they bind to the cadherins of neighboring cells. In this way, multicellular associations can arise. The genes for protein biosynthesis of the first cadherins originated in Holozoic protozoa before the Urchoanozoa had developed. Microfossils of the genus Bicellum presumably represent cell aggregates of such actually unicellular living organisms. About a billion years ago they were able to combine with the help of their cadherins to form temporary cell agglomerations . The evolution of the Cadherins continued. Their genes duplicated and diversified and exchanged gene sequences for protein domains with other genes. Horizontal gene transfer with prokaryotes probably also occurred . The gene sequence of a protein domain was transferred, which occurs in today's bacteria of the genus Clostridium in the cellulosomes . This enzyme complex brings about the enzymatic breakdown of cellulose . The Urchoanoza had already developed cadherins from three families of proteins . Today these proteins are synthesized by the single-celled choanoflagellates of the genus Monosiga . Thus, cadherins should not originally have served to hold cells together, but rather had other functions. That changed when the first classic cadherins evolved. The classic cadherins represented a further and new family of the cadherins. Only these new proteins allowed permanent cell cohesion and thus multicellularity. In fact, today the genes of the classic cadherins can only be found in the genomes of animals. The cell adhesion of the classic cadherins enabled the evolution of multicellular animals from choanoflagellates. Multicellularity probably represented an adaptation to predators. This could be proven in experiments. For example, unicellular green algae of the species Scenedesmus acutus were kept in an aquarium. Water fleas of the genus Daphnia , which fed on the green algae, were then placed in their basin . After several days it was observed that the green algae increasingly formed cell colonies . In another experiment, unicellular green algae of the species Chlamydomonas reinhardtii were brought together with predators of the species Paramecium tetraurelia . After fifty weeks, permanent and lumpy associations with many Chlamydomonas cells had formed in two of five experiments . The tendency to form such cell clusters is hereditary. They take up a larger volume and thus make it difficult for their predators to be held and digested.

Urmetazoa

According to a modern hypothesis, the transition from the frilled flagellates to the animals took place through certain cell colonies from the parent group of the choanoflagellates, which were able to synthesize classic cadherins. The individual cells would have been firmly connected to one another through these proteins. They would have formed a single cell layer. The cell layer would have wrapped itself around a fluid-filled cavity, the first mesohyl , like the skin of a bladder . Presumably the cell colony would have sat on solid ground. The connection between the cell colony and the subsurface could have accelerated the evolution of a new cell type from specialized holding cells, the first pinacocytes . Taken together, such organisms would have had two cell types, choanocytes and pinacocytes, and each had a central mesohyl.

According to an older hypothesis, the path to multicellularity began not with a sessile but with a spherical planktonic cell colony made up of stem group choanoflagellates. The cells would have been lying on top of each other without any gaps. In this way they would have encompassed a central, liquid-filled cavity. This stage of the hollow sphere is called "Choanoblastaea". Then cells would have migrated from the envelope into the central cavity and differentiated into other cell types. The "advanced Choanoblastaea" would have formed, a cell colony filled with different cell types. The cell colony would then have sat on the ground and would have become sessile. According to both hypotheses, the sessile organisms would have resembled extremely simply built sections of the walls of today's sponges. In fact, it is now widely assumed that the first animals, the Urmetazoa , belonged to the sponges.

The two hypotheses presented call for a duo of decisive steps for the evolution of cuffed flagellates to early sponges. Firstly, a permanent multicellular to multicellular structure developed, which, secondly, comprised differently differentiated cell types. Correspondences have been discovered for both steps in today's frilled flagellates. Individuals of the species Salpingoeca rosetta from the order of the Craspedida can differentiate into five clearly distinguishable collar- whip cell types (morphotypes). A collar scourge cell type is also formed, which forms multicellular to multicellular and rosette-shaped cell colonies. The stimulus for the differentiation to the collar scourge cell type of the rosette colonies consists of the substance rosette inducing factor ( RIF-1 ). RIF-1 is a sulfonolipid . It is produced by bacteria of the genus Algoriphagus as well as closely related organisms from the bacterial strain of Bacteroidetes and may be released into the water. The bacteria feed Salpingoeca rosetta . The rosette colonies are likely to be able to consume a bacterial deposit more effectively than a single collagen scourge cell would be able to. The rosette colony is similar to the embryonic morula stage of the animals. In addition, individual collagen flagellum cells of the species Salpingoeca rosetta can fundamentally change their outer cell shape ( transdifferentiation ). The change in shape takes place in a reversible manner, so it can also be reversed. Usually within a few minutes the wreaths of stereovilli and the flagella are withdrawn into the cell body and broken down. The cells lose their round shape. They transform into amoeboid cells . The transformations take place when the collar flagellates are spatially very cramped. Their second shape enables them to perform amoeboid movements with which they can crawl out of the constrictions. Transdifferentiations to amoeboid cells were also observed in five other species of collared flagellate.

On the one hand, recent choanoflagellates differentiate into a few different types of collagen scourge cell. The collar flagellate cells transdifferentiate into amoeboid cells. The amoeboid cells transdifferentiate again into collar flagellum cells. On the other hand, today's sponges have few different types of sponge cells . The amoeboid archaeocytes belong to the sponge cell types. The archaeocytes differentiate from the other sponge cell types. The other types of sponge cells also include collar scourge cells. These collar flagellate cells transdifferentiate back into archaeocytes. The pattern of the expressed genes in the archaeocytes of the sponges is very similar to the pattern that is present in the collar flagellate cells of the collar flagellates. On the other hand, the gene expression patterns of the collared flagellate cells and the sponge-collar flagellate cells differ from each other to a much greater extent. The observations led to another hypothesis regarding the Urmetazoa. They would not have started out as colonic flagella cell colonies. Instead, their bodies would have been made up of associations of one cell type that could transdifferentiate into other cell types - as can be seen in a similar way today using archaeocytes. As one of those other cell types, collagen flagellum cells would have established themselves again. From this third hypothesis, however, it would also follow that the collared flagella cells of the choanoflagellata and the choanocytes of the sponges are not directly homologous with one another.

Early fossils

A lack of fossils makes it difficult to reconstruct the Choanozoa tribal history for hundreds of millions of years. A number of possible animal fossils have been described, but none of these supposed sponges was generally convincing. The first less controversial evidence of the Choanozoa is fossilization of animals from the last millions of years shortly before the onset of the Phanerozoic .

Tonium

Construction plans of the recent sponges.
Yellow : pinacocytes.
Red : choanocytes.
Gray : mesohyl.
Blue arrow : outflow of water through the osculum.

In contrast to those hypothetical Urmetazoa, fully developed sponges have more complex blueprints . You have a few more cell types. Their bodies reach macroscopic proportions and are often (but not always) supported by a framework of hard skeletal needles, the sponge spicules . In its thin outer walls there are small pores, the ostia. Water flows in through them. It then gets into the chalice-like, central suboscular space and is then expelled again through the osculum located in the middle above. It is possible that remains of an early form of this blueprint have been preserved in fossil form. The oldest come from the earth period of the Tonium.

Vermiform microstructures

In the Stone Knife Formation (northwest Canada), fossil stromatolites were found that had been built up by cyanobacteria . The age of these shallow water structures has been dated to approximately 890 million years. So-called vermiform microstructures were discovered immediately on and next to the stromatolites and on their sides . These are chalky traces of irregular networks of tiny tubes, each a few millimeters to a few centimeters in length. The vermiform microstructures may have been left behind by keratose ("horny") horned silica sponges ( Demospongiae ). They were sponges that did not build a supporting structure from hard sponge spicules, but only from the structural protein spongin . Keratosis horn silica sponges are still found in today's seas, even if they are considerably larger in size. The common bath sponge ( Spongia officinalis ) is one of them. Presumably, the area of ​​those very early sponges was limited to the shallow water in close proximity to the stromatolites, because there the cyanobacteria provided oxygen gas (O ₂ ) that is essential for survival through their oxygenic photosynthesis . While all other areas of the ocean were as good as oxygen-free, the sponges were able to survive with the local, albeit still very low, oxygen concentrations.

Otavia

Other early forms of the sponge blueprint could have been preserved in fossil form and are almost 760 million years old. They were found in marine shallow water sedimentary rocks of Namibia and were given the generic name Otavia . The tiny fossils were no more than five millimeters long and are distantly ovoid or spherical in shape. Its thin walls are perforated by many tiny openings that have been interpreted as ostia. In addition, there are several larger recesses that could be viewed as oscula. Overall, however, it does not seem obvious to interpret Otavia as an early sponge. Perhaps they are just grains of calcium phosphate that have been roughened, perforated, and hollowed out by sand.

Cryogenium

During the period of existence of Otavia , the earth changed into the period of the cryogenium . The Sturtic Ice Age began 717 million years ago , and in a few thousand to tens of thousands of years it almost completely covered the planet with ice. It lasted for 47 million years. It was followed again 20 million years later by the Marino Ice Age , which similarly made almost the entire earth disappear under ice for about 5 million more years. The sponges had to adapt to the harsh environmental conditions. Shortly before the start of the cryogenium in the oceans, a thin layer of slightly oxygenated water was established near the surface. But in the dark under the mighty closed ice sheets, the organisms quickly used up the scant oxygen, so that there was soon a very low level of oxygen. Perhaps the sponges survived at the lower end of crevasses that reached down to the sea water, or they limited their areas to those small-scale zones near the equator that might have remained ice-free. At the time of the onset of the Sturtic Ice Age, the atmospheric oxygen content had already risen to around 1% of today's value and wherever the air came into contact with the sea water, oxygen gas could have dissolved in the water.

But the rocks of those volcanoes and mountains that pierced the ice continued to weather. Dust was created that was captured by the winds and deposited on the tops of the glaciers. Such mineral dusts were rich in nutrient salts. The salts were dissolved in liquid water when shallow meltwater lakes formed on the glaciers in summer. Cyanobacteria in particular probably thrived in these shallow and nutrient-rich waters. The bacteria operated oxygenic photosynthesis when enough sunlight penetrated them through the dusty air. Oxygenic photosynthesis further enriched the near-earth air with oxygen gas, so that towards the end of the Sturtic Ice Age the oxygen concentration in the air was possibly a little more than a hundredth of its current value. Some of the oxygen also got under the ice into the sea. The places where the ice shelf lifts from the sea floor and begins to float on the water are called touchdown lines. Even during the Ice Ages of the Cryogenium, subglacial melt water probably emerged at the touchdown lines. There was some oxygen in them. The oxygen came from air bubbles that had previously been trapped in the glacial ice and were now dissolved in the meltwater. The subglacial meltwater mixed with the seawater at the touchdown lines. Thus it was enriched with oxygen. With the help of this meltwater oxygen pump, slightly oxygenated bodies of water were created under the floating ice in front of the touchdown lines. Sponges and other benthic aerobic organisms could have settled in strips a few hundred meters wide . Beyond that, oxygenated plumes of water reached out, tapering progressively towards the bottom of the ice sheet until they ended about two thousand meters from the touchdown lines. Pelagic aerobic life forms could have survived in these plumes of water .

The environment of the cryogenium acted on the sponges with strongly pronounced abiotic selection factors. A certain part of the then evolved survivability could have been passed on to this day. Recently, the permanent stages ( gemmulae ) of the freshwater sponge Ephydatia muelleri form new sponge bodies even after they have been stored under anoxic conditions for 112 days . The crawling sea sponge Tethya wilhelma performs its regular full-body contractions with the usual repetition rates at 4% of today's atmospheric oxygen concentration. Even at 0.25%, he still does not change the reading rates of his genes. Furthermore, more than 80% of the gemmulae of the freshwater sponge Eunapius fragilis survive a one-hour cooling to −70 ° C. In addition, a hole was drilled through 872 thick ice on the Antarctic Filchner-Ronne Ice Shelf in 2016 . At the drilling site, the top of the ice shelf protruded 111 meters above sea level. With the help of a diving camera, the underlying 472 meters of free water column was overcome and the sea floor was reached. There grew a stalked sponge and fifteen sessile sponges, each a few centimeters in size, on the sides of a boulder. The sponges lived in complete darkness at a total sea depth of 1,233 meters (1,344 meters below the shelf ice surface) and at a distance of 260 kilometers from the edge of the shelf ice and at a water temperature of −2.2 degrees Celsius.

Steroid biomarkers

The oldest chemical fossils of sponges may have come from a more recent section of the cryogenium. They could in two biomarkers from the group of steroids be seen, the 24-Isopropylcholestan and 26-methyl stigma Stan hot. The molecules were found in rocks and petroleum from Oman . Today they are produced exclusively from sponges. The biomarkers could suggest that sponges actually existed at least 635 million years ago. However, the substances could also come from other organisms. Protozoa from the group of rhizaria produce predecessors of both molecules, the same applies to green algae. The predecessor molecules could then have been changed by geochemical processes, so that they are available today as 24-isopropylcholestane and 26-methylstigmastane.

Ediacarium

Volcanoes probably protruded through the ice sheets throughout the entire cryogenium. They gradually enriched the earth's atmosphere with carbon dioxide over several million years. The gas reached hundreds of times its current atmospheric concentration and increased the natural greenhouse effect considerably. In this way, the icing was finally ended. The earth went into a pronounced warm phase. The ice melted, the earth-wide sea level rose by up to 500 meters. The Ediacarian geological period began .

Weng'an Biota

The very fine-grained phosphorites of the Doushantuo Formation (southwest China) contain the Weng'an Biota . The microfossils are likely a little over 609 million years old and have been interpreted as eggs or early embryos from animals. However, they could just as well represent the remains of algae cysts or come from very large sulfur bacteria that resembled the recent genus Thiomargarita . Nevertheless, at least the weng'an biota fossils of the genera Tianzhushania , Spiralicellula and Megasphaera seem to have more similarities with early animal embryos. The criteria with which the three genera had been declared animal embryos were nonetheless rejected as not valid. On the other hand, megasphaera even seem to be able to distinguish several stages, which are reminiscent of processes during the early embryonic development of animals.

Eocyathispongia

Further traces of early sponges may have been recovered from the rocks of the Doushantuo Formation. A single and tiny specimen of the genus Eocyathispongia was found there in 600 million year old marine shallow water sedimentary rocks . The fossil has a volume of about three cubic millimeters and shows a nodular, spherical shape. Its interior consists of three chambers, each of which was connected to the surrounding water through an opening. The openings were interpreted as an oscula. Eocyathispongia consisted of hundreds of thousands of cells. The walls of the three chambers were made up of several layers of cells. Inside the fossil, fields made up of hundreds of tiny, pot-shaped honeycombs were discovered close together. Choanocytes could possibly have been lined up there. Otherwise there is no trace of these cells. The fossil's sponge affiliation can be questioned in several ways. The fossil has neither ostia nor sponge spicula. In terms of its external shape, it does not resemble any other known sponge. In addition, the inner surfaces of Eocyathispongia may have been too small to feed the entire organism. The choanocytes supposedly sitting there might not have been able to filter off enough food particles to adequately supply all of the body's cells.

Sponge spicula

In addition, fossil sponge spicules could be found in the Doushantuo Formation. The age of their place of discovery is estimated at 580 million years. Here, too, it is controversial whether it is really the remains of sponges. The fossil particles could also represent fossil fragments of radiation animals ( Radiolaria ). It is possible that it was only forty million years later and independently of one another that the various lines of the sponges began to evolve hardening biomineralizations of their spicules.

Tabulata

Dickinsonia is one of the oldest fossils of presumed Choanozoa.

All in all, no generally convincing sponge fossil from Tonium, Cryogenium or Ediacarium seems to have been discovered. According to the molecular clock, on the other hand, the other animals could have emerged from the parent group of sponges around 920 million years ago. Perhaps the last common ancestor of all other animals had a disk-shaped appearance ("placula") or it resembled a hollow sphere ("gastraea"). In both cases, this tiny organism is said to have consisted of two layers of cells. In addition, the cell type of the collar flagellum cells disappeared in this new line of development. The Weng'an Biota also includes a series of fine fossil tubes that have been interpreted as traces of a group of the other animals. They should come from cnidarians from the class of the flower animals ( Anthozoa ), more precisely from the extinct group of the Tabulata . However, the tubes could also have been left behind by, for example, algae threads.

Dickinsonia

South Australian rocks with an age between 571 and 539 million years contain imprints of a tiny and squat worm-shaped life form that possibly belonged to the animals, more precisely to the bilateria . She received the generic name Ikaria . The presumed bilateral animals Spriggina and Kimberella with about 555 million years, as well as the possible trunk group comb jelly ( Ctenophora ) Eoandromeda with 551 million years come from the same time horizon . The probably tube- dwelling worm Cloudina and the segmented worm Yilingia have similar ages . There is also a possible Lophotrochozoon named Namacalathus from 547 million year old Namibian rocks. Other representatives of this so-called Ediacara fauna have also been suggested as animals, such as the genus Dickinsonia . Certain steroids could have proven that the latter was an animal. The molecules were found in the fossil and are usually considered biomarkers for animals. May put Dickinsonia a genre particularly large plate Animals ( Placozoa is).

Cambrian

The Ediacarium was drawing to a close. In the southern hemisphere, land masses connected to the great continent of Gondwana . Many volcanoes erupted in the process. They enriched the earth's atmosphere with the greenhouse gas carbon dioxide. The global average temperature rose. The oceans went through a period of great oxygen deficiency. But under the warmer conditions, the weathering of the rocks accelerated . Their minerals were loosened from waters and ended up in the sea. There they acted as fertilizer for algae growth. The increased oxygenic photosynthesis caused the oxygen content to rise. Then the earth changed into the earth period of the Cambrian 541 million years ago. Presumably, the increased oxygen content turned out to be one of the main reasons for the Cambrian explosion that was now beginning . From then on, the animals formed hard substances in their supporting tissues and for shells and housings. Thereby they favored their fossilization . Therefore, fossils of the Choanozoa have existed in great abundance and diversity since the Cambrian. The first undisputed sponge spicules were also deposited during the transition to the Cambrian.

Individual evidence

1. ^ ^{A b c} Thibaut Brunet, Nicole King: The Origin of Animal Multicellularity and Cell Differentiation . In: Developmental Cell . Volume 43, 2017, doi : 10.1016 / j.devcel.2017.09.016 , p. 125.
2. ↑ Jean Vacelet, NicoleBoury-Esnault: Carnivorous sponges . In: Nature . Volume 373, 1995, doi : 10.1016 / j.ympev.2015.08.022 , p. 333.
3. ↑ Henry M. Reiswig: Bacteria as food for temperate-water marine sponges . In: Canadian Journal of Zoology . Volume 53, 1975, doi : 10.1139 / z75-072 , p. 582.
4. ^ Gonzalo Giribet, Gregory D. Edgecombe: The Invertebrate Tree of Life . Princeton University Press, Princeton / Oxford 2020, ISBN 978-0691170251 , p. 38.
5. ↑ Manuel Maldonado: Choanoflagellates, choanocytes, and animal multicellularity. In: Invertebrate Biology. Volume 123, 2004, doi : 10.1111 / j.1744-7410.2004.tb00138.x , p. 1.
6. ↑ Barry SC Leadbeater: The Choanoflagellates . Cambridge University Press, Cambridge 2015, ISBN 978-0521884440 , pp. 2, 4.
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