Ediacarian fauna. Cambrian explosion. The emergence of a variety of animals
More recently, in the middle of our century, all the earliest of the known fossils belonged to geological formations from a stratigraphic interval called the Cambrian, which includes rocks that were formed from about 570 to 505 million years ago.
Among the Cambrian fossils there are a lot of animals, according to their plans, structures similar to a number of living forms. Thus, the fossil chronicle asked 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 lowermost Cambrian strata and their absence in Precambrian made the boundary between Precambrian and Cambrian the most important division on the scale of geological time.
Approximately 570 million years ago there was an unprecedented upsurge in the diversity of animals. New construction plans, new lifestyles, and with them - a new type of community, characterized by complex food chains.
But in recent years, animal fossils, more ancient than the Cambrian, have been discovered in different parts of the world. And yet the fact remains that in the Cambrian period there was a huge explosion of life. The beginning of the Cambrian is marked by the appearance of a large number of large groups of animals. Many of them still exist today, and many are so unusual that they cannot be attributed to any of the known types (the type is a group of animals that share the same general structure plan). Although most of the structure plans inherent in modern animals first appeared in the Cambrian, only a few of the animal species that inhabited the Cambrian seas became the ancestors of the now-living forms. Most of the other species now, in hindsight, can be considered short and unsuccessful experiments of nature. In the Cambrian period, there were more such “experimental” groups of animals than in any other interval of the Earth’s history.
The emergence of such a wide variety of new forms during the transition from Precambrian to the Cambrian radically changed the nature of the relationship between animals. During the Cambrian period, organisms appeared that were able to fill ecological niches that had been empty before. Organisms that feed on living matter have spread more widely, instead of eating dead organic matter or relying on a symbiotic relationship with photosynthetic algae. Predators appeared. The Cambrian animals were interconnected by a network of relationships, quite similar to those that exist in modern animals. Thus, the transition from Precambrian to the 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, belonging to this stage) and the so-called Ediacaran fauna — flat, soft-bodied animals, the first samples of which were found in the hills of Ediacara 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 disappearance of the Ediacaran fauna and the appearance of the first communities of shell faunas with low diversity (together there are about 5 species of animals). This stage lasted for about 5–15 million years.
The third stage, which lasted about 10-20 million years, is characterized by the shell fauna of moderate diversity (more than 5 but less than 15 species of shell animals are found together) and the emergence of a group of unusual creatures resembling a goblet and known as archeocytes. At the fourth stage, which lasted about 15–30 million years, the shell fauna with high diversity appeared and the first trilobites appeared. Trilobites are extinct arthropods, the body of which consisted of three sections (the head, the trunk and the tail section) and was covered with a thyroid shell, which was an exoskeleton. Like modern arthropods, trilobites shed from time to time in the process of growth, that is, they changed the shell; trilobite casings dropped - very common fossils.
The Ediacarian 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 tillits are evidence of long-lasting episodes of worldwide glaciation. Consequently, the Ediacarian fauna was to appear soon after the last major glaciation in the last Precambrian. Fossils of Ediacarian soft-bodied animals are found together with fossil traces (traces of movement preserved in the rock and shallow animal minks) formed on the surface of the sediments, which were then the seabed. In general, the fossil residues of animals in the layers of that time are much less than in later sediments, but this fact is partly due to the fact that soft-bodied organisms that quickly deteriorate are usually poorly preserved and rarely remain from fossils; and one should not conclude that the Ediacarian fauna was not widespread and abundant.
HELICOPLACUS is unlike any other living organism. This creature, about 5 cm long, resembling a lemon in shape, was covered with a spiral system of protective plates. His structure plan can be called experimental - now it does not occur. Helicoplacus appeared about 490 million years ago, and after 20 million years it became extinct. This time refers to the transition from Precambrian to Cambrian. Then in the animal world, many new building plans emerged, but most of them turned out to be unsuccessful, for example, Helicoplacus.
There are also a few shell fossils left from the first stage, including tubular fossils of animals with a calcium carbonate outer case: Cloudina from Namibia and Sinotubulites from southern China. In this time interval, tube-like fossils, called sabellitids and vendotenides, also appear. Sabellitids, as a rule, are several centimeters in length, and in diameter from one to several millimeters. They are fossilized tubular shells, which initially consisted of flexible organic matter and, apparently, belonged to worm-like animals, feeding on water filtration. Sabellitids along with the remnants of Cloudina and Sinotubulites are evidence of the existence of ancient filter-feeding animals surrounding themselves with a pipe and leading an attached lifestyle. Vendotenides are also fossils of tubes originally comprised 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 possibly the shells of colonies of Precambrian bacteria, formed from substances that were secreted by cells to the outside.
THE GAME OF THE POINT OF PEACE on the transition from Precambrian to the Cambrian has become much more complex and diverse. (All organisms shown here lived in the sea.) The so-called Ediacaran fauna, whose representatives are characterized by a soft flattened body, existed 700-570 million years ago, at the first stage of transition. Cloudina and Sinotubulites, tubular calcium carbonate shell fossils, belong to the same period. 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 prey; apparently, it is the predatory grasp of some early predator. Another shell fossil of the same age is Anabarites. Both species are distinguished by wide geographic distribution: they are found in the rocks of Asia, Australia, the Middle East and North America. In the rocks of the second stage of the transition time, the variety and number of traces of animal movement sharply increase. Very complex footprints are encountered: for example, Phycoides pedum, reflecting a series of movements associated with feeding and burying some kind of ground animal; the grooves forming the chevron pattern that the arthropod could have left. In the rocks that were formed at that time, there are, in addition, the first deep vertical mink. In the third stage, brachiopods, or brachiopods, and a group of strange organisms, called archeociates, appeared that had porous double-walled skeletons consisting of calcium carbonate. By the middle of the third stage are the first Lapworthella; these animals were covered with a protective sheath (the figure given here is a reconstruction based on the fossils of individual sclerites, i.e. parts of “armor”). The fourth stage of transition is marked by the appearance of trilobites; these arthropods had a thyroid exoskeleton, which periodically dropped as they grew.
In 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 long; they form a group of ancient phosphate conch fauna with low diversity. An example is a tusk shaped fossil called Protohertzina. In terms of microstructure, it is very similar to the sharp bristles with which prey, the miniature voracious predators of the type Chaetognatha (bristle maxillary) living in our time, seize prey. What we call Protohertzina was, apparently, the grasping "armament" of some predatory organism, like the bristle maxillary. This is the first fossil, about which we can say with confidence that we are part of a predatory animal. Along with the phosphate fossils from the second stage remained shell fossils made of calcium carbonate. It is 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 the details of the structure are only a few thousandths of a millimeter.
At about the same stratigraphic level, the number and diversity of fossilized traces of movement and activity in sedimentary rocks that made up the seabed increases dramatically. Deep vertical minks appear for the first time, there are many complex fossil tracks, 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 Ediacaran fauna (perhaps with rare exceptions), apparently, died out. However, some paleontologists believe that the disappearance of fossilated remains of animals of the Ediacaran fauna is explained by the fact that the number and activity of burrowing animals that were feeding on carrion significantly increased.
The third stage of the transition from Precambrian to the Cambrian is marked by shell fauna with moderate diversity. In most parts of the world, they appear in the strata lying directly above the strata containing fauna with low diversity. On the so-called Siberian Platform (occupies the center and west of Siberia), moderately diverse faunas are accompanied by the first archaeociates - cup-shaped fossils, in which the porous skeleton with double walls consists of calcium carbonate. Archaeocytes are somewhat similar to corals and sponges, but in reality they are not related to any of the now living groups and are now referred to as a separate type. Archaeocytes along with calcareous algae (multicellular algae, in which photosynthetic organs were strengthened with calcium carbonate needles), coalescing, formed hill-like aggregates of skeletal material, called bioherms, which were the first surf-resistant reefs.
GEOGRAPHY OF THE WORLD in 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 was split, which created extensive coastal areas suitable for settlement. Formed continents were located at the equator, so that the climate there was warm and even. The divergence of the continents in the 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 outlines of the continents of that time.)
At the fourth stage of the transition, which is associated with highly diverse shell faunas, the geographic distribution of the archaeocyat has greatly expanded, capturing many areas far from the Siberian platform. In addition, the first trilobite shell fossils appear in the sedimentary rocks formed at this stage.
In the XIX century. the boundary between the Precambrian and the Cambrian was relatively easy to determine, since in the regions studied at that time there were large interruptions or disagreements (which reflects the time periods when the sedimentation did not occur or they did not persist) between the Precambrian sedimentary layers and the Cambrian. The much larger amount of data available to the paleontologist in the twentieth century, in fact, complicated the task of accurately establishing the boundary, since many areas are now known in which Precambrian formations imperceptibly turn into the Cambrian. But knowledge of the boundaries of each specific geological formation is necessary in order to be able to correlate data collected in different places and determine the relative age of the formations under consideration.
The stratigraphic subdivisions of Precambrian and Cambrian from different regions are difficult to reconcile with each other 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 therefore are not suitable for subdividing stratigraphic sequences into biostratigraphic units.
Many early fossils have widespread not only stratigraphic, but also geographic distribution. One such organism is a tubular fossil with three distinct internal fins, called Anabarites. This species first appeared at the stage of shell fauna with low diversity, but remained a member and highly diverse fauna. Anabarites are 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 Ediacarian fauna and 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 assume that in the late Precambrian, many of the modern continents were part of a single supercontinent. J. Bond and his staff from the Geological Observatory of Lamont-Doherty found evidence that during the transition from Precambrian to Cambrian there was a split of the supercontinent along the lines of modern North America. The resulting continents in the early stages of their drift were close to each other and at about 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 traces speak of the complexity of the animals that left them. Above - shallow horizontal strokes made by animals in the sediment on the seabed (their width is about 1 mm); This sample refers to the Precambrian. Below - a trace (its width is about 1.5 cm), left by a crawling mollusc-like organism that lived in the Cambrian time.
When archaeocytes first spread and trilobites appeared, zoogeographical features began to take shape in the world, that is, differences in species composition between the faunas of different regions. The tendency to the emergence of such differences, of course, intensified by the expansion of water spaces between the continents, which continued to diverge in the early Cambrian. In addition, E. Palmer of the American Geological Society showed that at that time, so-called carbonate belts began to form along the margins of some continents. These are the sandbanks resulting from the accumulation of calcium carbonate shells. Animals that live in the gap between the coast and the carbonate belt, which restricts access to the open sea, can develop in isolation from the faunas of other continents. This effect is particularly pronounced in some groups of trilobites.
BECAUSE in Late Precambrian, the continents were mostly grouped near the equator, the climate after the last Precambrian glaciation was probably fairly even. As global climate warms, food stocks in the shallow waters have stabilized at a relatively low level. Reducing the range of temperature differences in the world was supposed to contribute to the stabilization of marine forage resources. In general, the lower the temperature gradient between the poles and the equator, the weaker the seasonal mixing of ocean waters, and this leads to a decrease in the supply of nutrients from the bottom layers of deep-water areas to the surface layers of the sea. A constant, non-fluctuating flow of food is very important for many marine animals, especially for the inhabitants of the tropics, who are more accustomed to constant living conditions than organisms living in those areas where seasonal changes in conditions are more pronounced.
Apparently, the unusual flat shape of the body of Ediacara organisms is associated with the limited availability of food in Precambrian. Due to flatness and small thickness (for example, the Dickinsonia pancake-like organism had a thickness of no 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 with such methods of nutrition as photo-autotrophy and chemo-autotrophy, which are especially encouraged by natural selection in those habitats where there are few nutrients.
In order to feed by photoautotrophy, the animal must enter into a symbiotic relationship with photosynthetic algae. Inside the tissues of the host organism, the 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 implement such a symbiosis, a significant part of the host body must be accessible to sunlight in order to allow for efficient 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 radiolarian cells, in the tissues of many hydras, some flatworms and freshwater sponges. - Approx. Transl. ).
Another nutritional method, chemoautotrophy, is to extract energy from nutrients entering the body from seawater through direct absorption. Chemoautotrophy is sometimes also carried out in a situation of internal symbiosis, when chemosynthetic bacteria inhabit the tissues of the animal. This method of feeding is common, for example, among animals living near deep-sea hydrothermal sources on the ocean floor. ("See: J. Childress, X. Felbeck, J. Somero. Symbiosis in the depths of the ocean," In the world of science ", 1987, No. 7 Note. Ed. ) Some animals, apparently, are able to absorb dissolved nutrients independently , without the help of bacteria (The ability to suck dissolved in water organic nutrients possess many small hydrobionts. - Approx. transl. ).
The flattened shape of the body of Ediacian animals was supposed to facilitate the absorption of nutrients from sea water or the absorption of light by photosynthetic algae. A recent study by P. Hellock-Muller from the University of South Florida in St. Petersburg showed that symbiotic relationships thrive in nutrient-poor waters because they are especially beneficial: thanks to the symbiotic, the host organism can immediately use its waste products instead of throwing them into the environment. Thus, the Ediacaran fauna was probably well adapted to the conditions that prevailed in the seas in the late Precambrian, when it was believed that the shallow waters were poor in nutrients.
WOUNDED ANIMALS - clear evidence of the existence of predators during the transition period. Above - the damaged and healed Hyolithellus shell (increase approximately x40). Downstairs - a crippled and overgrown trilobite shell of Olenellusrobsonensis (3/4 full size). The fact that the wounds healed indicates that they were inflicted on the victim during his lifetime, and not later, when she became a corpse or an empty shell. - clear evidence of the existence of predators in the transition period. Above - the damaged and healed Hyolithellus shell (increase approximately x40). Downstairs - a crippled and overgrown trilobite shell of Olenellusrobsonensis (3/4 full size). The fact that the wounds healed indicates that they were inflicted on the victim during his lifetime, and not later, when she became a corpse or an empty shell.
In the Late Precambrian and during the transition to the Cambrian, food resources have changed, apparently due to chemical changes in the ocean and other ways of feeding have become more relevant. By the end of the Precambrian, the value of heterotrophy began to increase - feeding on other organisms (animals or plants). 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 upwards towards the sun. Algae layers alternate with layers of sediment particles that adhere to the algal mat. Although the thickness of each layer is less than a millimeter, massive structures were formed as a result of their gradual accumulation: some Precambrian stromatolites have a height of more than 10 m.
About 800 million years ago, a variety of stromatolites plummeted. S. Oramik of the University of California at Santa Barbara connects this with the appearance of animals that feed on algae: algal mats are very sensitive to excessive exploitation by consumers, and their violation can halt the formation of stromatolite.
Indications on the spread of heterotrophy are also found in siliceous shales containing micro-stones. These are sedimentary rocks consisting of microcrystalline quartz. In thin thin sections of such shales under a microscope with appropriate illumination one can see the fossil microbes contained in the rock. The shells of microorganisms 700–800 million years ago became more powerful — perhaps to provide protection from ancient herbivorous animals. At about the same stratigraphic level, fossils of traces left by animals that fed on carrion and sediment appear. The animals to which these tracks belonged were almost certainly the ancestors of the Cambrian shell organisms.
NUMEROUS PORES , or the tubules in the brachiopod shells of the mycitece group, may have been a device for protection against predators. These holes could serve to expel scare agents.
PLEASE , the most striking feature of the Cambrian faunas is that so many radically different types of animals 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 Sechta-Barbara and D. Erwin from the University of the piece. Michigan, so far the leading genetic rearrangements were then possible because the genome (the complete set of the body's genes) in multicellular animals was much less complicated during that transition period than it is today. The links between different parts of the genetic developmental program were much simpler, and fewer mutations were lethal. Valentine and Erwin consider this genetic plasticity to be one of the two reasons why many high-ranking taxa — of types and classes — suddenly appeared on the border between Precambrian and Cambrian (class is the largest subdivision of the type). The second reason is probably that then there were still many unoccupied ecological niches. Therefore, in the Cambrian, types and classes could appear with a speed unprecedented after that: fundamentally new animals, without meeting competition, became the founders of new large taxa.
The explosion of diversity in the Cambrian, quickly giving new types and classes, marked the beginning of the first complex animal communities linked 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 development of the animal community is predation, which makes up the food pyramid. Previously it was thought that predators did not play a large role in the Cambrian communities, but recently evidence was found that predation was significant: the fossils of predators themselves, damaged by predator (and sometimes partially regenerated) predators and adaptations to fight predators in some animals, were found.
ANOMALOCARIS is a predator living in the early and middle Cambrian. This organism was much larger than most of its modern animals - it was about 45 cm long. He had prehensile appendages that sent food into his mouth. He ate, apparently, mostly trilobites. The animal moved, making wavy movements a kind of lateral fins, retreating from the lower side of the body.
The above-mentioned Protohertzina, a fossil resembling the grasping bristles of the modern bristle bristles, is, in all likelihood, a remnant of some early Cambrian predator. Another predator, Anomámacaris, 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 like any of the organisms of our time. His body, in the form of a flattened drop, was equipped with lateral fins. On the broad head there was a pair of articulate appendages, with which the predator sucked the victim into its terrible ring-shaped maw, resembling a pineapple chunk, seated with teeth. This organism, apparently, is a representative of the rapidly disappearing "experimental" type. Briggs and Whittington suggest that other important evidence of the existence of predators in Precambrian — the fossils of injured trilobites — are traces of mainly Anomalocaris activity.
Among the finds of fossilized trilobites in numerous specimens from the shell snatched pieces. In most cases, these wounds were partially healed, from which it is clear that the shell was damaged when it was worn by a live trilobite, and not broken after some molt by some animal that fed on carrion. There are also other traces of predator activity, for example, holes in small shell fossils. These holes are like holes left by some modern predators, drilling the shells in search of soft meat inside them.
The third evidence of the prevalence of predation in the 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 shell and the exoskeleton first appeared; these structures probably served as defense against attacks. Deep vertical minks, which appeared in large numbers during periods of low and moderate diversity shell faunas, could protect against predators who could not move in the bottom sediment. In some species of trilobites, long spines have developed, which apparently made it difficult to attack such a predator as Anomalocaris.
In addition, S. Bengston from the Institute of Paleontology in Uppsala, E. Landing from the Geological Survey of pcs. New York and S. Conway Morris of the University of Cambridge showed that many small shell fossils are in fact sclerites, that is, parts of broken barbed armor, which apparently protected the upper side of the body in slowly crawling animals. Such animals may have looked like small sea urchins. Another device is provided by the so-called mikvitsiydy from the class of brachiopods, or brachiopods (these are animals with a bivalve shell, which look like bivalve mollusks). In the walls of the shells they had numerous pores. It is possible that through these pores the animal released chemicals that frighten predators and parasites.
Thus, predators in the Cambrian were an important element of the marine habitat. Exoskeletons, originally appeared, probably as a protective device, became a key factor in the development of some amazing new plans for the structure. For example, without a bivalve shell, an internal water flow would be impossible for brachiopods, which would provide them with efficient feeding by filtration.
Although it is now clearing up how the Cambrian communities could arise, one central question remains unanswered: why did the "Cambrian revolution" occur at that time, and not tens or even hundreds of millions of years earlier? This is very strange, because carrion-fed animals and herbivorous animals appeared, judging by the traces of their vital activity, much earlier - 200 million years before the beginning of the Cambrian.
The answer, perhaps, lies in the chemical changes of the ocean. At the transition from Precambrian to Cambrian, the concentration of phosphates, as well as sulfur and strontium isotopes in seawater, for unknown reasons, underwent strong changes. As P. Cook and J. Shergold of the Mineral Resources, Geology and Geophysics Department in Canberra suggest, extensive phosphate deposits found in sedimentary rocks belonging to the Precambrian and Cambrian, in many areas of the world, reflect the period of global phosphogenesis, when increased availability phosphates and other nutrient compounds facilitated the formation of phosphate skeletons in animals.
But this hypothesis does not fit the fact that, at the border of the Precambrian with the Cambrian, the calcareous shells are as (if not more) abundant as the phosphate ones. It may be more appropriate to consider the phosphate deposition phenomenon as part of a larger process — a sudden increase in the amount of available nutrients in the oceans. The nutrient-rich environment no longer forced animals to symbiotic relationships, so the number of carrion-feeding organisms could increase, which in turn led to an increase in the number of predators. When such heterotrophic organisms reached a critical biomass, so to speak, an ecological chain reaction began, according to M. Brazier of the University of Halla: evolutionary new animals created niches that were filled with other, more new species, and gradually so complex communities emerged in which there were many animals with shells.
EDIAKARA fauna and other Precambrian animals have arisen in the world, the characteristic features of which were a single land mass, diminishing glaciation and relatively small reserves of nutrients in the sea. The Cambrian animals, remarkable for their diversity and multiplicity, appeared in a completely different world - at the time of the split of the supercontinent (due to which long coastlines with a tropical climate arose) and the abundance of food in sea waters. What caused the Cambrian fauna? This question is still difficult to give a definite answer. Whether global changes of environmental conditions played a decisive role, or a number of successful changes in the genetic programs of animals, or the reason for some combination of these phenomena, or perhaps some other factors independent of them.
Whatever the reasons for their occurrence, the “innovations” of the early Cambrian (for example, shell organisms, predators, deep burying in the ground) quickly spread throughout the world. The combination of environmental conditions (such as an abundance of nutrients in marine waters) 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 has developed between these ancient animals is characteristic of almost all animal communities of the last 570 million years.