Ediacaran fauna. Cambrian explosion. The emergence of animal diversity
STILL recently, in the middle of this century, all the earliest known fossils were geological formations from a stratigraphic interval called the Cambrian, which includes rocks that originated from about 570 to 505 million years ago.
Among the Cambrian fossils are many animals, in terms of their construction plans similar to a number of living forms. Thus, the paleontological chronicle set a complex puzzle: where are the ancestral forms that gave rise to these numerous, already highly developed and diverse animals of the ancient seas?
The sudden appearance of animal fossils in the lowest Cambrian beds and their absence in the 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 of animal diversity. There were new plans for the structure, new ways of life, and with them a new type of community, for which complex food chains are characteristic.
But in recent years fossils of animals, more ancient than Cambrian, have been found 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 is 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 can not be attributed to any of the known types (the type is a group of animals that share the same general plan of the structure). Although most of the structure plans inherent in modern animals first appeared in the Cambrian, only a few of the species of animals that inhabited the Cambrian seas have become the ancestors of today's living forms. Most of the other species now, in hindsight, can be considered as short-lived 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 appearance of such a wide variety of new forms in the transition from the Precambrian to the Cambrian radically changed the nature of relations between animals. During the Cambrian time, organisms appeared that could fill the ecological niches that had been previously empty. More widely spread organisms that feed on living matter, instead of eating dead organic matter or rely on a symbiotic relationship with photosynthetic algae. There were predators. The Cambrian animals were interconnected by a network of relationships, quite similar to those that exist in modern animals. Thus, the transition from the Precambrian to the Cambrian is marked by the appearance not only of many modern types of animals, but also of the modern type of animal community.
The transition from the Precambrian to the Cambrian can be divided into four main stages. The first of them is marked by the appearance of the earliest shell fossils (only 1-2 such species are known to belong to this stage) and the so-called Ediacaran fauna - flat soft-bodied animals, the first samples of which have been found in the hills of Ediacar in southern Australia (hence the name). According to very approximate estimates, this stage was 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 Ediacaran faunas and the emergence of the first communities of shell faunas with low diversity (together there are about 5 species of animals). This stage lasted about 5-15 million years.
The third stage, which lasted about 10-20 million years, is characterized by shell fauna of moderate diversity (together finding more than 5 but less than 15 species of shell animals) and the appearance of a group of unusual creatures that resemble a goblet and are known as archeocyaths. At the fourth stage, which lasted about 15-30 million years, there were shell faunas with high diversity and the first trilobites appeared. Trilobites are extinct arthropods, the body of which consisted of three divisions (head, trunk and tail) and was covered with a shield-like shell, which was an exoskeleton. Like the modern arthropods, trilobites in the process of growth from time to time shed, that is, they changed their shells; discarded trilobite shells are very common fossils.
The Ediacaran fauna, corresponding to the first of these four stages, is found in layers lying above the Precambrian deposits of marine sedimentary rocks of glacial origin called the Tillites. These Tillites are evidence of long periods of global glaciation. Consequently, the Ediacaran fauna should appear soon after the last in the Late Precambrian by a large glaciation. The fossils of Ediacaran soft-bodied animals are found together with fossil tracks (preserved in the rock by traces of movement and shallow mink of animals) formed on the surface of sediments, which were then the seabed. In general, fossil remains of animals in the layers of that time are found much less than in later sediments, but this fact is partly due to the fact that soft-bodied organisms that are rapidly destroyed tend to be poorly preserved and rarely remain fossils; and one should not conclude that the Ediacaran fauna was not widespread and abundant.
HELICOPLACUS does not resemble any living organisms. This creature, about 5 cm long, shaped like a lemon, was covered with a spiral shielding system. His building plan can be called experimental - now it does not occur. Helicoplacus appeared about 490 million years ago, and after 20 million years it died out. This time refers to the transition from the Precambrian to the Cambrian. Then in the animal world a lot of new building plans arose, but most of them were unsuccessful, such as Helicoplacus.
From the first stage there are also a few shell fossils, including tubular fossils of animals with an outer case made of calcium carbonate: Cloudina from Namibia and Sinotubulites from southern China. In this time interval, tubular fossils, called sabellitides and vendotenides, also appear. Sabellitides, as a rule, have a length of several centimeters, and in diameter - from one to several millimeters. They are fossilized tubular cases that originally consisted of flexible organic matter and, apparently, belonged to vermiform animals that fed by filtering water. The Sabellitides, together with the remnants of Cloudina and Sinotubulites, are evidence of the existence of ancient filter-animals that surrounded themselves with a tube and led an attached lifestyle. Vendotenides are also fossils of tubes originally composed of flexible organic matter, but they are much smaller than sabellitides (less than a centimeter in length and approximately 0.1 mm in diameter). Vendotenides are probably the shells of colonies of Precambrian bacteria, formed from substances that are excreted by cells outward.
The animal world on the transition from the 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 the transition. By the same time are 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, as some modern carnivorous organisms grab their victims; Apparently, this is a grasping device of some early predator. Another shell fossil of the same age is the Anabarites. Both species have a wide geographical distribution: they occur in the rocks of Asia, Australia, the Middle East and North America. In the rocks of the second stage of transitional time, the diversity and number of traces of movement of animals sharply increases. There are very complex traces: for example, Phycoides pedum, reflecting a series of movements associated with feeding and burrowing some bottom animal; grooves, forming a pattern of "chevron", which could leave an arthropod. In the rocks formed at that time, in addition, the first deep vertical mink are also found. In the third stage, there were brachiopods, or brachiopods, as well as a group of strange organisms called archeocytes, which had porous skeletons with double walls, consisting of calcium carbonate. By the middle of the third stage are the first Lapworthella; these animals were covered with a protective shell (the figure given here is a reconstruction based on the fossils of individual sclerites, i.e., parts of "armor"). The fourth stage of the transition is marked by the appearance of trilobites; these arthropods had a thyroid exoskeleton, which was periodically discarded as it 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 have a length of a few millimeters or less; they form a group of ancient phosphate shell fauna with a low diversity. An example is a fossil in the form of a tusk, called Protohertzina. By microstructure, it is very similar to the sharp setae that the miniature, gluttonous predators of the Chaetognatha type (bristle-chamomile), which currently live, catch the prey. What we call Protohertzina was, apparently, a grasping "weapon" of some predatory organism, like the bristle-jawed. This is the first fossil, of which it can be said with certainty that we are in front of us a part of a predatory animal. Along with the phosphate fossils from the second stage, there were shell fossils from 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 composition of the exoskeleton to such an extent that only a few thousandths of a millimeter of the structure remains.
At about the same stratigraphic level, the number and diversity of fossilized traces of movement and vital activity in the sedimentary rocks constituting the seabed sharply increase. For the first time deep vertical mink appears, there are many complex fossil tracks, for example Phycoides pedum - a trace from a series of movements associated with feeding and burrowing some bottom animal. There are also traces in the form of oblique parallel grooves formed when the appendages of a crawling or burrowing arthropod scrape the sediment.
To this level, the Ediacaran fauna (possibly with rare exception), apparently, has died out. However, some paleontologists believe that the disappearance of fossilized animal remains of the Ediacaran fauna is due to the fact that the number and activity of burrowing animals feeding on carrion increased significantly.
The third stage of the transition from the Precambrian to the Cambrian is marked by shell faunas with moderate diversity. In most parts of the world, they appear in strata lying directly above layers containing fauna with low diversity. On the so-called Siberian platform (occupying the center and west of Siberia), moderately diverse fauna is accompanied by the first archeo-zoates - cup-shaped fossils, in which a porous skeleton with double walls consists of calcium carbonate. Archeocyats partly resemble corals and sponges, but in fact they are not related to any of the now living groups and now they are classed as a separate type. Archeocyanates, together with calcareous algae (multicellular algae, in which the photosynthetic organs were hardened with needles of calcium carbonate), fused, formed hill-like accumulations of skeletal material, called bioherms, which were the first resistant to surf reefs.
GEOGRAPHY OF THE WORLD in the transition from the Precambrian to the Cambrian time favored the emergence of 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 split, which created extensive coastal areas suitable for colonization. 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 zoogeographical features - the differences between territorially separated faunas. (On the contours of the continents of that time, a modern grid of parallels and meridians is superimposed.)
At the fourth stage of the transition, which is associated with highly diverse shell fauna, the geographical distribution of archeocyathas has greatly expanded, and has captured many areas far from the Siberian platform. In addition, in the sedimentary rocks formed at this stage, the first fossils of the trilobite shells appear.
In the XIX century. the boundary between the Precambrian and the Cambrian was relatively easy to determine, since there were major breaks or disagreements in the areas studied at the time (which reflects the intervals at which rock deposits did not exist or were not preserved) between 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 precisely establishing the boundary, since now many areas are known in which Precambrian formations imperceptibly pass into the Cambrian formations. But knowledge of the boundaries of each specific geological formation is necessary in order to be able to relate the data collected in different places and determine the relative age of the formations in question.
Stratigraphic subdivisions of the 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 (that is, they occur in layers formed over a long period of time) and therefore are of little use for subdividing stratigraphic sequences into biostratigraphic units.
Many early fossils have wide not only stratigraphic, but also geographical distribution. One of these organisms is a tubular fossil with three distinct internal ribs, called Anabarites. This species first appeared on the stage of shell faunas with low diversity, but remained a member of highly diverse faunas. 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 Ediacaran 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 from the Lamont-Doherty Geological Observatory found evidence that during the transition from the Precambrian to the Cambrian, the supercontinent split according to the outlines of modern North America. The resulting continents in the early stages of their drift were close to 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.
FOSILIZED TRACKS speak of the complexity of the animals that left them. Above - shallow horizontal moves made by animals in the sediment on the seabed (their width is about 1 mm); this sample belongs to the Precambrian. Below - a trace (its width is about 1.5 cm), left by a crawling mollusciform organism that lived in the Cambrian time.
When archeocyaths first spread and trilobites appeared, zoogeographical features began to take shape in the world, that is, differences in the species composition between faunas of different regions. The trend towards such differences, of course, was enhanced 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 marine shoals arising from the accumulation of shells from calcium carbonate. Animals living in 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 especially evident in some groups of trilobites.
Since in the Late Precambrian the continents were basically grouped near the equator, the climate after the last Precambrian glaciation was probably fairly even. As the global climate warms, food reserves in the shallow water have stabilized at a relatively low level. Reducing the magnitude of the temperature differences in the world should contribute to the stabilization of marine fodder resources. In general, the smaller 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 the deepwater regions to the surface layers of the sea. A constant, non-fluctuating inflow of food is very important for many marine animals, especially for the inhabitants of the tropics, who are more accustomed to unchanging conditions of existence than organisms living in areas where seasonal changes in conditions are more pronounced.
Apparently, with the limited supply of food in the Precambrian, the unusual flat form of the body of the Ediacaran organisms is connected. Due to the flatness and small thickness (for example, the pancake-like organism of Dickinsonia had a thickness of no more than 6 mm and its diameter could exceed 1 m), the maximum possible ratio of body area to volume was achieved. And the higher this ratio, the better with such methods of nutrition as photoautotrophy and chemoautotrophy, 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, algae are protected from animals that could eat them. In turn, they supply the host with nutrients and eliminate the waste of its vital activity. To implement this symbiosis, a significant part of the host's body should be accessible to sunlight to allow efficient photosynthesis in algal 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 . )
Another way of feeding, 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 inhabit the tissues of the animal. This way of feeding is common, for example, among animals living near the deep-sea hydrothermal springs on the ocean floor. (See: J. Childress, H. Felbeck, J. Somero, Symbiosis in the Depths of the Ocean, "In the World of Science", 1987, No. 7 Ed. ) Some animals seem to be able to absorb dissolved nutrients independently , without the help of bacteria (Many small hydrobionts possess the ability to absorb dissolved organic nutrients in water . )
The solid form of the body of the Ediacaran animals was supposed to promote the absorption of nutrients from sea water or the absorption of light by photosynthetic algae. Studies conducted recently by P. Hallok-Muller of the University of South Florida in St. Petersburg showed that symbiotic relationships thrive in waters that are poor in nutrients, since they are particularly beneficial: thanks to the symbiont, the host organism can immediately use the waste of its vital activity , 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 is believed that the shallow waters were poor in nutrients.
REDED ANIMALS - a clear evidence of the existence of predators in the transition period. Above is the damaged and healed hyolithhellus shell (an increase of about x40). Below is the crippled and overgrown trilobite armor of Olenellusrobsonensis (3/4 of a natural size). The fact that the wounds healed indicates that they were inflicted on the victim during life, and not later, when she became a corpse or an empty shell. - a clear evidence of the existence of predators in the transition period. Above is the damaged and healed hyolithhellus shell (an increase of about x40). Below is the crippled and overgrown trilobite armor of Olenellusrobsonensis (3/4 of a natural size). The fact that the wounds healed indicates that they were inflicted on the victim during life, 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 methods of nutrition have become more relevant. By the end of the Precambrian, the importance of heterotrophy-feeding by other organisms (animals or plants) began to increase. The evidence for an increase in the role of heterotrophy is found, for example, in the paleontological chronicle of stromatolites. Stromatolites are formations on the seabed in the form of domes, columns or cones, formed from sequentially formed layers of algal mats. They grew layer by layer up to the sun. The layers of algae in them alternate with layers of sediment particles, which adhered to the seaweed mat. Although the thickness of each layer is less than a millimeter, as a result of their gradual accumulation, massive structures have formed: some Precambrian stromatolites have a height of more than 10 m.
About 800 million years ago, a variety of stromatolites fell sharply. S. Oramik from the University of California at Santa Barbara associates this with the appearance of animals feeding on algae: algal mats are very sensitive to excessive exploitation by consumers, and their violation can stop the formation of stromatolite.
Indications of the distribution of heterotrophy are also found in siliceous shales containing microfossils. These are sedimentary rocks consisting of microcrystalline quartz. In thin sections of such shales under a microscope, under appropriate illumination, it is possible to see the fossils of microbes contained in the rock. Shells of microorganisms 700-800 million years ago have become more powerful - perhaps to provide protection from ancient herbivorous animals. Approximately at the same stratigraphic level, fossils of traces left by animals that feed on carrion and sediment appear. The animals that belonged to these tracks were almost certainly the ancestors of the Cambrian conch shells.
Multiple pores , or tubules in the shells of the brachiopod group of myquiticide, may have been a device for protecting against predators. These holes could serve to isolate the deterrents outside.
PLEASE , the most striking feature of Cambrian fauna is that so many radically different types of animals have appeared in a relatively 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.Ervin from the University of Pa. Michigan, so far leading genetic rearrangements were then possible because the genome (a full set of organism genes) in multicellular animals was much less complicated in that transition period than it is today. The connections between different parts of the genetic development program were much simpler, and fewer mutations turned out to be lethal. Valentine and Erwin consider this genetic plasticity to be one of the two reasons why many taxa of high rank-types and classes-suddenly appeared on the border between the Precambrian and the Cambrian (the class is the largest subdivision within the type). The second reason is probably that at that time there were still many unoccupied ecological niches. Therefore, in the Cambrian, types and classes could appear with an unprecedented speed: 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, laid the foundation for the first complex communities of animals connected by food chains. The emergence of new types of communities in turn has created ecological niches for new types of animals. The key element in the formation of the animal community is predation, because of which the food pyramid forms. 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 predators (and sometimes partially regenerated), victims and adaptations for predator control in some animals were found.
ANOMALOCARIS - predator, living in the early and middle Cambrian. This organism was much larger than most modern animals - it reached a length of about 45 cm. He had grasping appendages, with which food was sent to the mouth. He ate, apparently, mostly trilobites. The animal moved, making wave-like movements a kind of lateral fins that moved away from the lower side of the body.
The above mentioned Protohertzin, a fossil reminiscent of the grasping bristles of the modern combed stubble, is in all probability the remnant of some early Cambrian predator. Another predator, Anomalocaris, is known, its shape was recently reconstructed by the well-preserved remnants of D. Briggs from the University of Bristol and G. Whittington of Cambridge University. Anomalocaris is an animal that is giant for the Cambrian (about 45 cm in length) and does not resemble any of the organisms of our time. His body, in shape resembling a flattened drop, was provided with lateral fins. On the broad head there was a pair of articulated appendages, with which the predator was dragging the victim into its horrible ring-shaped mouth, like a pineapple slice, seated with teeth. This organism, apparently, is a representative of the rapidly disappeared "experimental" type. Briggs and Whittington suggest that other important evidence of the existence of predators in the Precambrian - the fossils of wounded trilobites - are traces of activity in the main Anomalocaris.
Among the finds of fossilized trilobites in numerous specimens from the shell are snatched pieces. In most cases, these wounds are partially delayed, from which it is clear that the shell was damaged when it was worn by a living trilobite, and not broken after moulting by some carrion-eating animal. There are other traces of predator activity, for example, holes in small shell fossils. These holes are like holes, left by some modern predators, drilling shells in search of soft meat inside them.
The third proof of the prevalence of predation in the early Cambrian are some signs of animals of that time that are important for protection from predators. In a number of new types, a shell and an exoskeleton first appeared; probably, these structures served to protect against attack. Deep vertical mink, which appeared in large numbers during periods of shell fauna with low and moderate diversity, could be protected from predators not able to move in the bottom sediment. Some species of trilobites developed long spines, apparently obstructing the attack of such a predator as Anomalocaris.
In addition, S. Bengston from the Institute of Paleontology in Uppsala, E. Landing of the Geological Service pcs. New York and S. Conway Morris of the University of Cambridge showed that many small shell fossils actually are sclerites, that is, parts of the disintegrated barbed armor, which apparently was protected by the upper side of the body in slowly crawling animals. Such animals may have been similar to small sea urchins. Another device has so-called mikvitcidy from the class of brachiopods, or brachiopods (these are animals with a bivalve shell, outwardly reminiscent of bivalve mollusks). In the walls of the shells, they had numerous pores. Perhaps, through these pores, the animal blew out chemicals that repel predators and parasites.
Thus, in the Cambrian, predators 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 building plans. For example, without a bivalve shell, the brachiopod would not be able to have an internal water current, which ensures the effectiveness of food by filtration.
Although now it becomes clear how the Cambrian communities could arise, one central question remains unanswered: why did the "Cambrian revolution" take place at that time, rather than tens or even hundreds of millions of years earlier? This is very strange, because carnivorous animals and herbivorous animals appeared, judging by the traces of their life, much earlier - even 200 million years before the Cambrian.
The answer may lie in the chemical changes in the ocean. At the transition from the Precambrian to the Cambrian, the concentration of phosphates, as well as sulfur and strontium isotopes in seawater for unknown reasons, underwent strong changes. As suggested by P. Cook and J. Schergold from the Department of Mineral Resources, Geology and Geophysics in Canberra, the extensive phosphate deposits found in sedimentary rocks between the Precambrian and Cambrian, in many parts of the world, reflect the period of global phosphogenesis, when increased availability phosphates and other nutritional compounds made it easier for animals to form phosphate skeletons.
But this fact does not fit into the fact that at the Precambrian border with the Cambrian the calcareous shells are as abundant (if not more) as the phosphate ones. Perhaps, it is more correct 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. The environment, rich in nutrients, no longer forced animals to symbiotic relationships, so the number of organisms feeding on carrion and sediment could increase, which in turn led to an increase in the number of predators. When such heterotrophic organisms reached, so to speak, critical biomass, an "ecological chain reaction" began, according to M. Brazir of the University of Hull: evolutionarily new animals created niches that were filled with other, even newer species, and gradually emerged complex communities , in which there were many animals with shells.
EDiakar fauna and other Precambrian animals arose in a world characterized by a single massif of land, a decreasing glaciation and relatively small reserves of nutrients in the sea. The Cambrian animals, remarkable for their variety and abundance, appeared in a completely different world - during the split of the supercontinent (thanks 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 the global changes in 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, and maybe some other independent factors that mattered.
Whatever the reasons for their occurrence, the "innovations" of the early Cambrian (for example, shell organisms, predators, deep digging into the ground) quickly spread throughout the world. The combination of environmental conditions (such as the abundance of nutrients in marine waters) and biotic changes (in particular, the emergence of predators) led to a significant change in the nature of animal communities. Modern animals, including humans, are direct offspring of organisms that first appeared during the Cambrian explosion, and the style of ecological interactions that has developed between these ancient animals is typical of almost all animal communities of the last 570 million years.