Ediakar fauna. Cambrian explosion. The emergence of animal diversity
MORE recently, in the middle of our century, all the earliest known fossils belonged to geological formations from the stratigraphic interval called the Cambrian, which includes rocks formed from about 570 to 505 million years ago.
Among the Cambrian fossils there are many animals that, in their plans for the structure, are similar to a number of living forms. Thus, the fossil record posed a complex riddle: where are the ancestral forms that gave rise to these numerous, already quite highly developed and diverse animals of the ancient seas?
The sudden appearance of animal fossils in the lowest Cambrian strata and their absence in the Precambrian made the boundary between Precambrian and Cambrian the main division on the geological time scale.
About 570 million years ago there was an unprecedented surge in the diversity of animals. New construction plans, new lifestyles arose, and with them a new type of community, which is characterized by complex food chains.
MARK E.S. MAKMENAMIN
But in recent years, fossils of animals more ancient than Cambrian have been discovered in different parts of the world. And yet the fact remains - in the Cambrian period there was a huge explosion of life. The beginning of the Cambrian was marked by the appearance of a large number of large groups of animals. Many of them exist today, and many are so unusual that they cannot be attributed to any of the known types (type is a group of animals that share the same general plan of structure). Although most of the building plans inherent in modern animals first appeared in Cambrian, only a few of the animal species that inhabited the Cambrian seas became the ancestors of the living forms. Most of the remaining species now, in hindsight, can be considered short and unsuccessful experiments of nature. In the Cambrian period, there were more such “experimental” animal groups than in any other interval in the history of the Earth.
The emergence of such a wide variety of new forms during the transition from Precambrian to Cambrian radically changed the nature of relations between animals. During the Cambrian time, organisms appeared that were able to fill ecological niches that were empty before. Organisms that feed on living matter have spread more widely, instead of eating dead organic matter or relying on symbiotic relationships with photosynthetic algae. Predators appeared. Cambrian animals were interconnected by a network of relationships, quite similar to those that exist in modern animals. Thus, the transition from Precambrian to Cambrian was marked by the emergence of not only many modern types of animals, but also the modern type of animal community.
The transition from Precambrian to Cambrian can be divided into four main stages. The first of them is marked by the appearance of the very first shell fossils (only 1-2 such species are known from this stage are known) and the so-called Ediacar fauna - flat soft-bodied animals, the first samples of which were found in the Ediacar hills in southern Australia (hence the name). According to very rough estimates, this stage took place in the range of about 700-570 million years ago. The second stage, which began 570 ± 40 million years ago, is characterized by the extinction of Ediacar faunas and the appearance of the first communities of conch fauna with a low diversity (about 5 animal species are found together). This stage lasted approximately 5-15 million years.
The third stage, which lasted approximately 10–20 million years, is characterized by shell fauna of moderate diversity (together they find more than 5, but less than 15 species of shell animals) and the appearance of a group of unusual creatures that look like a goblet and are known as archaeocytes. At the fourth stage, which lasted about 15-30 million years, conch fauna with high diversity arose and the first trilobites appeared. Trilobites are extinct arthropods, the body of which consisted of three sections (head, trunk and tail section) and was covered with a thyroid carapace, which was an exoskeleton. Like modern arthropods, trilobites molted from time to time, that is, changed the shell; discarded trilobite shells are very common fossils.
The Ediakar fauna, corresponding to the first of these four stages, is found in the layers lying above the Precambrian sediments of marine sedimentary rocks of glacial origin, called tillites. These tillites are evidence of long-lasting episodes of global glaciation. Consequently, the Ediakar fauna was to appear shortly after the last major glaciation in the Late Precambrian. Fossils of Ediacar soft-bodied animals are found together with fossil tracks (traces of movement preserved in the breed and shallow burrows of animals) formed on the surface of sediments, which were then the seabed. In general, the fossil remains of animals in the layers of that time are found much less than in later sediments, however, this fact is partially explained by the fact that soft-bodied organisms that are rapidly destroyed are usually poorly preserved and fossils rarely remain from them; and it should not be concluded that the Ediac fauna was not widespread and abundant.
HELICOPLACUS is not like any of the living organisms. This creature, about 5 cm long, resembling a lemon in shape, was covered with a spiral system of protective plates. His structural plan can be called experimental - now he does not meet. Helicoplacus appeared about 490 million years ago, and after 20 million years it died out. This time refers to the transition from Precambrian to Cambrian. Then in the animal world many new building plans arose, but most of them were unsuccessful, for example Helicoplacus.
There are also few shell fossils from the first stage, including tubular animal fossils with an outer case of calcium carbonate: Cloudina from Namibia and Sinotubulites from southern China. In this time interval, in addition, tube-like fossils appear, called sabellitides and wendotenides. Sabellitides, as a rule, are several centimeters in length, and in diameter from one to several millimeters. They are fossilized tubular cases, which initially consisted of flexible organic matter and, apparently, belonged to worm-like animals, fed by filtering water. Sabellitides, along with the remains of Cloudina and Sinotubulites, are evidence of the existence of ancient animal-filtering animals that surrounded themselves with a pipe and led an attached lifestyle. Vendotenides are also fossils of tubes originally consisting of flexible organic matter, but they are much smaller than sabellitides (less than a centimeter in length and about 0.1 mm in diameter). Vendotenides are probably shells of colonies of Precambrian bacteria, formed from substances that are secreted by cells to the outside.
W SALON WORLD on the transition from Precambrian to Cambrian has become much more complex and diverse. (All the organisms shown here lived in the sea.) The so-called Ediac fauna, whose representatives are characterized by a soft flattened body, existed 700-570 million years ago, at the first stage of transition. At the same time, Cloudina and Sinotubulites - tubular shell fossils from calcium carbonate. The characteristic fossil of the second stage of the transition is Profoherfzina. It consists of calcium phosphate and resembles a spike, which some modern predatory organisms grab their victims; Apparently, this is a grasping adaptation of some early predator. Another conch fossil of the same age is Anabarites. Both species are distinguished by wide geographical distribution: they are found in the rocks of Asia, Australia, the Middle East and North America. In the rocks of the second stage of transition time, the variety and number of traces of animal movement sharply increase. There are very complex traces: for example, Phycoides pedum, which reflects a series of movements associated with feeding and digging in some kind of bottom animal; the grooves forming the chevron pattern that the arthropod could leave. In the rocks formed at that time, in addition, the first deep vertical minks are found. In the third stage, brachiopods, or brachiopods, appeared, as well as a group of strange organisms called archaeocytes, which had porous skeletons with double walls, consisting of calcium carbonate. The first Lapworthella belong to the middle of the third stage; these animals were covered with a protective shell (the figure shown here is a reconstruction based on the fossils of individual sclerites, that is, parts of the “armor”). The fourth stage of the transition is marked by the appearance of trilobites; these arthropods had a thyroid exoskeleton, which was periodically discharged as they grew.
At the beginning of the second stage of the transition period, fossils of animals and their fragments appear, the basis of the solid parts of which was calcium phosphate. The earliest of these fossils are several millimeters or less in length; they form a group of ancient low-diversity phosphate conch fauna. An example is a tusk-shaped fossil called Protohertzina. In microstructure, it is very similar to the sharp bristles that grab prey living in our time, miniature voracious predators such as Chaetognatha (bristle-jaw). What we call Protohertzina was, apparently, a grasping “weapon” of some predatory organism, like the bristle-maxillary. This is the first fossil, which we can confidently say that we have before us a part of a predatory animal. Along with phosphate fossils from the second stage, shell fossils from calcium carbonate remained. It can be difficult to draw conclusions about the initial mineralogical composition of these early shell fossils, since calcium phosphate can replace calcium carbonate in the exoskeleton so accurately that structural details of only a few thousandths of a millimeter are preserved.
At about the same stratigraphic level, the number and variety of fossilized traces of movement and activity in sedimentary rocks that make up the seabed increases sharply. For the first time, deep vertical minks appear, there are many complex fossil traces, for example Phycoides pedum - a trace from a series of movements associated with feeding and burying some kind of bottom animal. Traces are also known in the form of oblique parallel grooves, formed when the appendages of a crawling or digging arthropod scratched the sediment.
To this level, the Ediakar fauna (possibly with rare exceptions) seems to have died out. However, some paleontologists believe that the disappearance of fossilized animal remains of the Ediac fauna is explained by the fact that the number and activity of burrowing animals, which have been feeding on carrion, have significantly increased.
The third stage of the transition from Precambrian to Cambrian is marked by shell fauna with moderate diversity. In most parts of the world, they appear in strata lying directly above strata containing fauna with low diversity. On the so-called Siberian platform (occupying the center and west of Siberia), moderately diverse faunas are accompanied by the first archaeocytes - cup-shaped fossils, in which the porous skeleton with double walls consists of calcium carbonate. Archaeocytes are somewhat reminiscent of corals and sponges, but in fact they are not related to any of the living groups and now they are assigned to a separate type. Archaeocytes together with calcareous algae (multicellular algae, in which photosynthetic organs were strengthened with needles of calcium carbonate), growing together, formed hill-like accumulations of skeletal material, called bioherms, which were the first surf-resistant reefs.
GEOGRAPHY OF THE WORLD during the transition from Precambrian to Cambrian time favored the emergence of a new fauna and flora. At the end of the Precambrian, most of the land was a single supercontinent. By the beginning of the Cambrian, this massif had split, which created vast coastal areas suitable for settlement. The formed continents were located at the equator, so the climate there was warm and even. The divergence of continents in early Cambrian contributed to the emergence of zoogeographic features - differences between geographically separated faunas. (A modern grid of parallels and meridians is superimposed on the contours of the continents of that time.)
In the fourth stage of the transition, which is associated with highly diverse shell faunas, the geographic distribution of the archaeociate has greatly expanded, capturing many areas far from the Siberian platform. In addition, the first fossils of trilobite shells appear in sedimentary rocks formed at this stage.
In the XIX century. the boundary between Precambrian and Cambrian was determined relatively easily, since in the regions studied then there were major breaks or disagreements (which reflects the time intervals when rock deposition did not go or they did not remain) between the sedimentary layers of Precambrian and Cambrian. The much larger amount of data available to the paleontologist in the 20th century actually complicated the task of accurately establishing the boundary, since now many regions are known in which Precambrian formations imperceptibly turn into Cambrian. But knowledge of the boundaries of each specific geological formation is necessary in order to be able to correlate the data collected in different places and determine the relative age of the formations in question.
The stratigraphic units of Precambrian and Cambrian from different regions are difficult to reconcile for two reasons. First, the number of species of ancient fossils is small. Secondly, many of these species have a wide stratigraphic distribution (i.e., they are found in layers that have formed over a long period of time) and are therefore unsuitable for dividing stratigraphic sequences into biostratigraphic units.
Many early fossils have widespread not only stratigraphic, but also geographical distribution. One of these organisms is a tubular fossil with three clearly visible internal ribs, called Anabarites. This species first appeared at the stage of conch fauna with low diversity, but remained a member of highly diverse fauna. Anabarites is found in Australia, India, China, Iran, Kazakhstan, Mongolia, in the west and east of North America and in Siberia. The wide distribution of some elements of the Ediacar fauna and the early shell fauna is due to the location of the continents at the end of the Precambrian - the beginning of the Cambrian. Most of the land was then at the equator, and there is reason to believe that in the Late Precambrian, many of the modern continents were part of a single supercontinent. J. Bond and his staff at the Lamont-Dougherty Geological Observatory found evidence that the supercontinent split along the outlines of modern North America during the transition from Precambrian to Cambrian. The resulting continents in the early stages of their drift were not far from each other and at approximately the same latitude. This allowed the animals to spread: there were no huge oceans or sharp temperature differences that would prevent the migration of organisms from one continental shelf to another.
FOSSILIZED TRACKS indicate the complexity of the animals that left them. Above are shallow horizontal passages made by animals in sediment on the seabed (their width is about 1 mm); this sample belongs to Precambrian. Below is a trace (its width is about 1.5 cm), left by a crawling mollusk-like organism that lived in Cambrian time.
When archaeocytes first spread and trilobites appeared, zoogeographic features began to take shape in the world, i.e. differences in species composition between faunas of different regions. The trend towards the emergence of such differences, of course, was reinforced by the expansion of water between continents, which continued to diverge in early Cambrian. In addition, E. Palmer from the American Geological Society showed that at that time so-called carbonate belts began to form along the outskirts of some continents. These are the shallows resulting from the accumulation of calcium carbonate shells. Animals that live between the coast and the carbonate belt, which limits access to the open sea, can develop in isolation from the fauna of other continents. This effect is especially pronounced in some trilobite groups.
BECAUSE in the Late Precambrian, the continents were mainly grouped near the equator, the climate later than the last Precambrian glaciation was probably quite even. As the global climate has warmed, food supplies in the shallow waters have stabilized at a relatively low level. The decrease in the range of temperature differences in the world was supposed to contribute to the stabilization of marine feed resources. In general, the smaller the temperature gradient between the poles and the equator, the weaker the seasonal mixing of the ocean waters, and this leads to a decrease in the supply of nutrients from the bottom layers of deep sea areas to the surface layers of the sea. A constant, non-fluctuating food supply is very important for many marine animals, especially for tropics, who are more used to unchanging living conditions than organisms living in areas where seasonal changes are more pronounced.
Apparently, the unusual flat body shape of Ediacar organisms is associated with limited food supplies in Precambrian. Due to flatness and small thickness (for example, the pancake-like organism of Dickinsonia had a thickness of not more than 6 mm, and its diameter could exceed 1 m), the maximum possible ratio of body area to its volume was achieved. And the higher this ratio, the better for such nutritional methods as photoautotrophy and chemoautotrophy, which are especially encouraged by natural selection in those habitats where there are few nutrients.
In order to eat by photoautotrophy, the animal must enter into a symbiotic relationship with photosynthetic algae. Inside the tissues of the host body, algae are protected from animals that could eat them. In turn, they supply the host with nutrients and eliminate the waste of his life. To achieve this symbiosis, a significant part of the host body must be accessible to sunlight in order to ensure the possibility of effective photosynthesis in algae cells. Photosynthetic algae are found in the tissues of many modern reef-forming corals, as well as some tropical mollusks (Such symbionts also live in radiolar cells, in the tissues of many hydras, some flatworms and freshwater sponges. - Approx. Translation ).
Another way of eating, chemoautotrophy, is to extract energy from nutrients that enter the body from sea water through direct absorption. Chemoautotrophy is sometimes also carried out in a situation of internal symbiosis, when chemosynthetic bacteria live in the tissues of the animal. This method of feeding is common, for example, among animals living near deep-sea hydrothermal springs on the ocean floor. ("See: J. Childress, X. Felbeck, J. Somero. Symbiosis in the Deep Ocean," In the World of Science ", 1987, No. 7, Ed. ) Some animals appear to be able to absorb dissolved nutrients on their own. , without the help of bacteria (Many small hydrobionts possess the ability to absorb organic nutrients dissolved in water. - Note transl. ).
The flattened body shape of ediac animals was supposed to facilitate the absorption of nutrients from sea water or the absorption of light by photosynthetic algae. Recent studies by P. Hallock-Muller of the University of South Florida at St. Petersburg have shown that symbiotic relationships thrive in nutrient-poor waters because they are especially beneficial: thanks to the symbiont, the host can use its waste immediately. instead of throwing them into the environment. Thus, the Ediakar fauna was probably well adapted to the conditions prevailing in the seas in the Late Precambrian, when, as it is believed, shallow waters were poor in nutrients.
WOUNDED ANIMALS - a clear evidence of the existence of predators in transition. Above is a damaged and healed Hyolithellus shell (approximately x40 magnification). Below - crippled and overgrown shell of trilobite Olenellusrobsonensis (3/4 of life size). The fact that the wounds healed indicates that they were inflicted on the victim in life, and not later, when she became a corpse or empty shell. - Visual evidence of the existence of predators in transition. Above is a damaged and healed Hyolithellus shell (approximately x40 magnification). Below - crippled and overgrown shell of trilobite Olenellusrobsonensis (3/4 of life size). The fact that the wounds healed indicates that they were inflicted on the victim in life, and not later, when she became a corpse or empty shell.
LATER in Precambrian and during the transition to Cambrian, food resources changed, apparently, due to chemical changes in the ocean and other ways of feeding became more relevant. Towards the end of the Precambrian, the importance of heterotrophy began to increase - nutrition by other organisms (animal or plant). Evidence of the increasing role of heterotrophy is found, for example, in the fossil record of stromatolites. Stromatolites are formations on the seabed in the form of domes, columns or cones, formed from successively formed layers of algal mats. They grew layer by layer up to the sun. Layers of algae in them alternate with layers of sediment particles that adhere to the algal mat. Although the thickness of each layer is less than a millimeter, as a result of their gradual accumulation, massive structures formed: some Precambrian stromatolites are more than 10 m high.
About 800 million years ago, the variety of stromatolites fell sharply. S. Oramik from the University of California, Santa Barbara, associates this with the appearance of animals that feed on algae: algal mats are very sensitive to excessive exploitation by consumers, and violation of them can stop the formation of stromatolite.
Indications of the spread of heterotrophy are also found in siliceous schists containing micro-fossils. These are sedimentary rocks consisting of microcrystalline quartz. In thin sections of such shales under a microscope, with appropriate lighting, you can see the fossils of microbes enclosed in the rock. The shells of microorganisms 700-800 million years ago became more powerful - perhaps in order to provide protection from ancient herbivorous animals. At about the same stratigraphic level, fossils of tracks left by animals that feed on carrion and sediment appear. The animals that owned these tracks were almost certainly the ancestors of the Cambrian shell organisms.
NUMEROUS PORES , or tubules in the shells of the brachiopods of the mikvititsid group, may have been a defense against predators. These openings could serve to release outward repelling substances.
PLEASE , the most striking feature of the Cambrian faunas is that so many types of animals that are radically different from each other appeared in a relatively very short time. What are the reasons for this sudden explosion of diversity? According to J. Valentine from the University of California at Sect-Barbara and D. Erwin from the University of pc. Michigan, such far-reaching genetic rearrangements were then possible because the genome (the complete set of body genes) in multicellular animals was much less complicated during this transitional period than today. The connections between different parts of the genetic development program were much simpler, and fewer mutations were fatal. Valentine and Erwin consider this genetic plasticity to be one of two reasons why suddenly many taxa of high rank — types and classes — suddenly appeared on the border between Precambrian and Cambrian (class is the largest division within a type). The second reason is probably because there were still many unoccupied ecological niches. Therefore, in Cambria, types and classes could appear at an unprecedented rate thereafter: fundamentally new animals, without encountering competition, became the founders of new large taxa.
The explosion of diversity in Cambria, quickly giving rise to new types and classes, laid the foundation for the first complex communities of animals connected by food chains. The emergence of new types of communities in turn created ecological niches for new types of animals. A key element in the formation of the animal community is predation, which forms the food pyramid. Previously, it was believed that predators did not play a large role in the Cambrian communities, but recently evidence has been found that predation was significant: fossils of predators themselves, damaged by a predator (and sometimes partially regenerated) of victims, and adaptations to combat predators in some animals were found.
ANOMALOCARIS is a predator living in early and middle Cambrian. This organism was significantly larger than most of the animals modern to it - it reached a length of about 45 cm. It had grasping appendages with which food was directed into the mouth. He ate, apparently, mainly trilobites. The animal moved, making wave-like movements with a kind of lateral fins extending from the lower side of the body.
The Protohertzina mentioned above - a fossil resembling the grasping bristles of the modern maxillary bristles - is, in all likelihood, a remnant of some early Cambrian predator. Another predator, Anomalocaris, is known, its appearance was recently reconstructed from the well-preserved remains of D. Briggs from the University of Bristol and G. Whittington from the University of Cambridge. Anomalocaris is an animal, giant for the Cambrian (about 45 cm in length) and not similar to any of the organisms of our time. His body, resembling a flattened drop in shape, was equipped with lateral fins. There was a pair of jointed appendages on the broad head, with which the predator pulled the victim into its terrible ring-shaped mouth, similar to a chunk of pineapple planted with teeth. This organism, apparently, is a representative of the rapidly disappeared "experimental" type. Briggs and Whittington suggest that another important evidence of the existence of predators in Precambrian - the fossils of wounded trilobites - these are traces of the activity of mainly Anomalocaris.
Among the finds of fossilized trilobites in numerous specimens, pieces were snatched from the shell. In most cases, these wounds were partially healed, from which it is clear that the shell was damaged when it was worn by a living trilobite, and was not broken after molting by some animals that fed on carrion. There are other traces of predator activity, such as holes in small shell fossils. These holes look like holes left by some modern predators drilling holes in search of soft meat inside them.
The third evidence of the prevalence of predation in early Cambrian is some of the signs of animals of the time that are important for protection against predators. In a number of new types, the conch and exoskeleton first appeared; probably these structures served as a defense against attack. Deep vertical minks, which appeared in large numbers during periods of shell fauna with low and moderate diversity, could protect against predators who were unable to move in the bottom sediment. Some trilobite species developed long spikes, apparently making it difficult to attack a predator such as Anomalocaris.
In addition, S. Bengston of the Institute of Paleontology in Uppsala, E. Landing of the Geological Survey New York and S. Conway Morris from the University of Cambridge showed that many small shell fossils are actually sclerites, that is, part of the decayed spiny armor, which, apparently, was protected by the upper side of the body in slowly crawling animals. Such animals may have looked like small sea urchins. Another device is possessed by the so-called mikvititsidy from the class of brachiopods, or brachiopods (these are animals with a bivalve shell, resembling bivalve mollusks). In the walls of the shells they had numerous pores. It is possible that through these pores the animal released chemicals that scare away predators and parasites.
Thus, in Cambrian, predators were an important element of the marine habitat. Exoskeletons, originally appearing probably as a protective device, have also become a key factor in the development of some amazing new building plans. For example, without a bivalve shell in a brachiopod, an internal flow of water would be impossible, providing them with nutrition efficiency by filtration.
Although it is now being clarified how the Cambrian communities could have arisen, one central question remains unanswered: why did the “Cambrian revolution” take place at that time, and not tens or even hundreds of millions of years earlier? This is very strange, because carrion-eating animals and herbivorous animals appeared, judging by the traces of their life activity, much earlier - even 200 million years before the beginning of the Cambrian.
The answer may lie in the chemical changes of the ocean. During the transition from Precambrian to Cambrian, the concentration of phosphates, as well as sulfur and strontium isotopes in sea water underwent strong changes for unknown reasons. As P. Cook and J. Shergold of the Canberra Office of Mineral Resources, Geology, and Geophysics suggest, the vast phosphate deposits found in sedimentary rocks from the Precambrian – Cambrian boundary in many parts of the world reflect a period of global phosphogenesis when increased availability phosphate and other nutrient compounds facilitated the formation of phosphate skeletons in animals.
But this hypothesis does not fit the fact that on the Precambrian – Cambrian border, calcareous shells are as (if not more) plentiful as phosphate ones. It may be more appropriate to consider the phenomenon of phosphate deposition as part of a larger-scale process - a sudden increase in the amount of available nutrients in the oceans. An environment rich in nutrients no longer forced animals to symbiotic relationships, so the number of organisms that feed on carrion and sediment could increase, which in turn led to an increase in the number of predators. When such heterotrophic organisms reached critical biomass, so to speak, according to M. Brazier of the University of Hull, an “ecological chain reaction” began: evolutionarily new animals created niches that were filled with other, even newer species, and complex communities gradually emerged in which there were many animals with shells.
The EDIAKAR fauna and other Precambrian animals arose in a world characterized by a single land mass, decreasing glaciation and relatively small reserves of nutrients in the sea. Remarkable in their diversity and abundance, Cambrian animals appeared in a completely different world - during the split of the supercontinent (due to which long coastlines with a tropical climate arose) and the abundance of food in sea waters. What brought Cambrian fauna to life? It is still difficult to give a definite answer to this question. Whether global changes in environmental conditions played a decisive role, or a number of successful changes in the animal genetic programs, or the reason for some combination of these phenomena, or maybe some other factors independent of them were significant.
Whatever the reasons for their occurrence, the “innovations” of the early Cambrian (for example, shell organisms, predators, deep burial in the ground) quickly spread around the world. The combination of environmental conditions (such as the abundance of nutrients in seawater) and biotic changes (in particular, the appearance of predators) led to a significant change in the nature of animal communities. Modern animals, including humans, are direct descendants of organisms that first appeared during the Cambrian explosion, and the style of ecological interactions that developed between these ancient animals is characteristic of almost all animal communities of the last 570 million years.