Where were trilobites found
“Age of fossils” actually says it all: with the beginning of the Cambrian era 541 million years ago, fossils (see box) suddenly appeared in the rocks - it was these fossils that made it possible for the >> geologists in the 19th century to have different layers of rock Assign regions to each other. With the knowledge that the more recent material is higher up in the sediment layers, they were able to set up an initial geological time scale based on what are known as key fossils (fossils that are typical for a period) - they did not yet know the exact year; but what was below must be older. On this basis, the history of the earth was divided into sections, whereby geographical names were often adopted: The first section of the ancient world is called Cambrian after the Latin name for Wales. (The fact that English regions were particularly often named after rock strata is a consequence of the importance of English geologists in the 19th century.) When the age of rocks could be determined in the 20th century (more on this >> here), the geological time scale with Provide years (for the >> current version).
Ammonite. Photo: GD Berlin, from wikipedia >> Fossil,
Retrieved January 13, 2009. License: >> cc 2.0
Fossils are evidence of past life from the history of the earth. The fossils that are important for the study of the history of life are mostly Fossils: When they are formed, water containing minerals penetrates into bones or other, mostly solid structures; the minerals deposit and replace the tissues over time. Rock can also fill cavities and thus create casts. Only under special circumstances can soft tissues become fossilized; especially when living beings are “stored” in the absence of oxygen - for example in deep lakes or on the seabed; a lack of oxygen, which ideally also kills predators who would otherwise attack the dead organisms. Because molluscs without skeletons are also preserved, paleontology (the science of researching past geological ages) owes a great deal to such deposits; one of the most famous is the Burgess Shale in the Canadian Rockies - more in the following chapter. Together with the molecular clocks, fossils represent the most important source of knowledge about the history of life on earth.
See also >> here for fossils.
The oldest fossils
All over the world, in limestone formations from the Lower Cambrian (as the oldest layers of the Cambrian, which began 541 million years ago are called) are found small tubes that do not dissolve in dilute acid: tiny shells and carapaces made of calcium phosphate. They are the living tubes of one named after the American paleontologist Preston Cloud Cloudina named animal. So for the first time you can Animals with a skeleton - as a skeleton, biologists refer to all biological structures in which minerals are stored (such as our bones, which consist of the protein collagen, in which the mineral apatite, a calcium phosphate, is stored). Calcium phosphate is not the only mineral; most shells of marine animals are made of calcium carbonate and the skeletons of sponges are made of silicon dioxide. And all three minerals can also be found in the small-shell fossils of the Lower Cambrian. It is not said that “small-shell fossils” also means small animals: in 1990 an animal was named in the Sirius Passet site in Greenland Halkieria found wearing a kind of mail shirt made of skeletal plates arranged in rows that were previously thought to be separate species.
Trilobite. Photo: Gryffindor, from wikipedia >> Trilobite, accessed January 13, 2009, license: >> FDL 1.2.
The most common of all fossils can be found right above these small-shell fossils: the Trilobites. These animals, reminiscent of woodlice, belong to the group of arthropods, all of which have articulated, articulated limbs and to which crustaceans and insects belong today. They owe their name (“Dreilapper”) to the back, which is divided lengthways into three sections (see photo on the right). The shallow seas of the Cambrian must have swarmed with trilobites, otherwise the fossils would not be so common. For a long time the trilobites were the only known arthropods from the Cambrian. But in 1909, the American paleontologist Charles Doolittle Walcott discovered a deposit in the Canadian Rocky Mountains with many extremely well-preserved fossils from the early Cambrian: Burgess Shale, which is over 500 million years old. The living beings had been buried under a landslide - their bad luck, but our luck, because this also preserved soft tissues. In this way sponges, seaweed and jellyfish were preserved; many worm-like animals; the fishy but headless one Pikaia, the oldest >> chordates at the time - but above all lots of arthropods. The trilobites were therefore members of a species-rich group at that time. Other living beings could not be assigned at first, and they were to make the site world famous: In 1989 Stephen Jay Gould published his book “Zufall Mensch”, in which he reported on numerous unknown animal strains in the deposit, including those that later became extinct again.
Was there a “Cambrian Explosion”?
This phase of supposedly special creativity of nature with suddenly appearing and later disappearing again was called the "Cambrian explosion". In the meantime, however, paleontologists, including in particular Simon Conway Morris - one of the “heroes” in Stephen Jay Gould's book - have been able to attribute many of these life forms to known animal phyla (as Walcott had done from the beginning). The >> Ediacara organisms found in the meantime also showed that the development of new animal phyla was probably not that sudden. The evaluation of the molecular clocks today indicates that today's animal phyla came into being at least 650 to 600 million years ago, that is, deep in the Precambrian (more on this >> here).
But there was a good reason for the deception: The ancestors from the Precambrian were suddenly visible 541 million years ago as they grew larger and developed hard shells and armor - that is, easier to survive and find than fossils. Growth in height and hard shells and armor are apparently connected, apparently the growth in height made supporting skeletons necessary (>> more). So the question today is: What triggered this development spurt at the beginning of the Cambrian? Since all three skeletal building materials that are widespread today appeared at about the same time, it makes sense to look for the cause not within individual living beings, but in the environment. Was it because of the increasing oxygen content in the air that made the growth possible? Was it because of an improved supply of the nutrient phosphate due to changed ocean currents? The cause could have been a geologically troubled environment: the supercontinent Gondwana had collapsed; North America, Siberia and Baltica have been separated, the Iapetus Ocean (a forerunner of the Atlantic) formed, and sea levels rose. This also made the nutrient-rich continental shelf accessible to the animal world - which could also have been conducive to their growth in size.
Distribution of land masses in the early Cambrian about 540 million years ago. Own illustration based on a map by Dr. Ron Blakey, >> Global Paleographic Views of Earth History.
Perhaps, however, the development of life itself was the trigger: In the Cambrian, two ecological stages were given a new quality: those organisms that fed on bacteria and algae (the primary consumers) and those that in turn lived off them - the "predators" or Secondary consumer. An example are the trilobites. They have the oldest known eyes - compound eyes like today's insects. The science journalist Volker Arzt once called the creation of eyes “The Invention of Light”: Solar radiation existed before, but it only became visible, light, with the invention of the eye. (Although there were light-sensitive tissues in earlier life forms, the different types of eyes developed from this. The receptor molecule opsin in the light collecting cells of the retina can even be traced back to the bacteria.) The advantages of sight for predators are obvious: You help them as a “fantastic exploration system” (again Volker doctor) to find their prey. And they practically forced their prey to develop eyes as well - as an early warning system. The perception of the environment that is possible with sight apparently had so many advantages that over time better and better sensory organs have developed from a basic model - natural developments have no goal, but must prove themselves in daily life and increase the chances of their wearer, to reproduce (>> The theory of evolution); With enough time, however, this development can - as the eyes show - lead to quite extraordinary results.
But with the seeing robbers, life on earth changed. The peaceful life of cyanobacteria, algae and bacteria was over; suddenly there were living things out to devour them. Ecosystems emerged that we can still find today in the sea; with hunters and the hunted. Could the tanks and shells be an adaptation to the appearance of predators? According to this hypothesis, predators and prey developed more and more sophisticated strategies of attack and defense in the sense of an “arms race”, which rocked each other upwards and ultimately led to the diversity of organisms. One can speculate about the reasons, the stones say one thing: In the sea, ecosystems that resembled today's marine ecosystems developed; there were algae gardens with rich fauna in exposed coastal zones, plankton in the upper part of the open sea, which consists of photosynthetically active algae and animals living on them; an animal world in dark deep sea areas that lives on sinking dead plankton and finally in warm, shallow seas reefs that consisted of sponges and sponge-like animals in the Cambrian.
In the meantime, two somewhat older sites than the Burgess slate have been found, Sirius Passet in Greenland and Chengjiang in China (see box). Here, too, there is an astonishing variety of organisms; here, too, most of the species could be assigned to today's animal phyla. To come back to the question from the headline: The term “Cambrian Explosion” seems to be wrong - the diversity did not arise all at once, but has only become recognizable due to the growth in size of living beings.
The Chengjiang site
The Chengjiang site is in the 525 to 520 million year old Maotianshan slate of the Chinese province of Yunnan. It became famous in 1984 for a very well-preserved trilobite - soft tissues have also been preserved here, the living beings were probably buried by sediment (“turbidity currents”) that had slipped off the undersea slopes. So far 185 species have been described, about half of them arthropods. Many of them still have relatively soft exoskeletons made of chitin, like today's insects - hard shells were probably only created as a second step. In Chengjiang, as in Burgess slate, it was up to two meters long Anomalocaris, a trilobite-like predator was found. One eighth of the species cannot be clearly assigned to today's phyla. Perhaps the most amazing find is Myllokunmingia, the oldest (jawless) fish - and thus the oldest vertebrate - on earth. Over 500 specimens of this three centimeter long fish have been found so far. So far, they are unique to Chengjiang Vetulicoliathat resemble a constricted sausage with a thick and a thin half; the systematic classification of this animal strain is still controversial.
In the Ordovician (after the Ordovicern, a Celtic tribe living in Wales), which followed the Cambrian and began 485 million years ago, the corals found their way into the reefs; they should experience a flowering period in the carbon. Coral reefs with their diversity of species represent something like the marine counterpart to the rainforests on land.
In addition to the corals, the “graptolites” appeared in the Ordovician: They look like drawn lines (their name means “written on stone”). They turned out to be colonies of collar animals (hemichordata) that lived as primary consumers of the plankton of the oceans; in the family tree of life they are somewhere before the chordates (which will be explained in a moment) - they are therefore our (very ...) distant ancestors. In the Ordovician, large predators such as the nautiloids (relatives of today's Nautilus), giant scorpions, sea urchins and starfish and other jawless fish also appeared. These jawless fish represent an early group of vertebrates, which probably arose from chordates in the early Cambrian. ChordatesLike the “lancet fish” still alive today, they have a support rod called a “notochord” that runs through their backs. The chordates “invented” the inner skeleton; In the vertebrates, the support rod in the adult animal is replaced by the spine, the notochord forms the intervertebral discs. In addition to the lancet fish, the chordates also include the sea squirts, which are still living today and which are attached to a rock and filter their food out of the water. Only their tadpole-like larvae are reminiscent of vertebrates. Charles Darwin already suspected that the sea squirts had ancestors that were more similar to the larvae, and from which a sedentary group developed on the one hand, from which today's sea squirts emerged, and on the other hand today's vertebrates. Darwin's conjecture is now supported by molecular biological findings. The animal kingdom is traditionally divided into vertebrates and invertebrates, but for systematics the vertebrates are “only” a sub-tribe of the chordates. (In addition to the tribe of the chordates, there are about 30 tribes of the invertebrates, including the molluscs (with the octopus), the flatworms, roundworms and annelids (each representing a separate tribe), the arthropods (with the insects, spiders, Crustaceans, ...) and the echinoderms (with the starfish) belong).
The end of the Ordovician was heralded by another Ice Age: The cooling resulted in over half of all living species becoming extinct. This is a recurring pattern: Catastrophes have also shaped the earth (see >> here). The variety of life forms was from time to time reduced by great events, and after that the earth was forever different. Life had to go through such "filters" several times. The cause of the Ice Age at the end of the Ordovician is still a mystery; A more recent theory assumes that the gamma-ray flash produced by a cosmic supernova damaged life and triggered a layer of smog via chemical reactions, which in turn was responsible for the cooling.
Trilobites and graptolites could not recover from the mass extinction; there are other species that are now being found. Formative for the Silurian (like the Ordovician, the name goes back to a Celtic tribe in Wales), which began 443 million years ago, but is the step of life on land. So far, the history of life has only played out in water - if the earth were a hundred-year-old man, life would have come into being at the age of 22; but did not go ashore until I was over 88! Plants took the first step ashore in the Ordovician about 470 million years ago: Many paleontologists believe that it was green algae that made the leap onto land (they still come from all the algae in the intertidal zone in the driest areas in front); possibly also in a symbiosis with fungi: The lichens formed in this way still occur today in extreme habitats in which neither partner could survive on their own. Only a fraction of the very first “real” land plants are known. Only the spores reveal that they must belong to the land plants, as they were protected from drying out. The spore sacs resemble those of today's liverworts; therefore, many paleontologists believe that these were the first land plants. Why the country was settled at all can only be guessed at: it may have happened as an adaptation to habitats in the coastal area that were drying up due to global warming; an adaptation that then opened up a huge habitat, the largely uninhabited land (bacteria and archaea at hot springs and volcanoes may have been other precursors alongside lichen and cyanophyte crusts on the edge of the sea).
Going ashore required many adaptations: Plants living in the water absorbed nutrients over their entire surface; the life processes of all living things were adapted to the water - therefore the humid interior had to be protected from the dry environment on land. Living beings can float in water, but on land they need a body that can defy gravity (more in the box on the right). When the colonization was successful, it changed the face of the earth: the green plants from space shape the face of the earth more than any other living being; soils formed on the earth's surface, which only consists of rocks and its weathering products (>> more); in short: the earth became what we know today. The settlement of the country also had another effect: Most of the biological diversity known today (>> more) is found on the mainland, which is only partially related to the much poorer knowledge of life in the sea. Settling the mainland gave life a further boost in development.
A Reconstruction of Cooksonia. A shows the plant, B a cross-section through the stem with recognizable vessels and C shows the sporangia in which the spores were formed. © University of California Museum of Paleontology, used with permission.
The oldest land plant visible without a microscope is the 425 million year old, only a few centimeters high, forked branch Cooksonia; a primeval fern. The fact that it was able to stand upright - in contrast to algae - was due to the storage of the macromolecule lignin in the cell wall - lignin is the substance wood is made of. Owned inside the shoots Cooksonia already vessels in which water was transported from the soil to the upper parts of the plant (and which at the same time further increased the strength of the shoots); the spores with which it spread were spread by the wind. With stable cell walls and vessels, all the prerequisites were met for the plants to grow upwards - an advantage in denser stands, because they avoid being shaded by their neighbors.
With the plants, or shortly afterwards, the animals and mushrooms came ashore (the mobile animals had already undertaken excursions ashore, as animal tracks show 500 million years old, but probably found nothing of interest there). The oldest known fossil Land animals are the arthropods, especially species that resemble today's millipedes and trigonotarbids that resemble today's spiders. Their exoskeleton stabilized these groups of animals and somewhat protected them from dehydration; strong legs, with which they previously walked on the ocean floor, allowed them to move on land. Mushrooms had probably already accompanied the green plants on land: soil fungi were able to extract nutrients from the rock and in return were supplied with organic nutrients by the green plants; Even today the vast majority of plants live in a symbiosis with fungi (the mycorrhiza). But with the colonization of the land by animals and plants, the fungi were also able to survive on their own: They break down organic material from dead animals and plants. This is how communities developed on land: producing plants, herbivorous and animal-eating animals and decomposing fungi that utilize organic matter and add it to the soil again. In the Silurian, the fish must have made the transition from the sea to brackish and fresh water, and one of these fish species was to become the forerunner of the terrestrial vertebrates. The transition of vertebrates to land was not easy either: breathing and movement work very differently in water than on land. The lungfish, among other things, show that this transition is possible; they have - the name suggests - a lungs and also fleshy fins with which they can move on land (>> more).
The jump ashore was to be so successful that "already" 65 million years later the forests of the Carboniferous Period covered large parts of the continents (>> more). These forests resembled today's tropical rainforests; in fact, the Europe that was just forming was then on the equator. In the Silurian, the continent of Baltica (today's Northern and Eastern Europe) collided with Laurentia, the North American forerunner; "Avalonia" was created as a counterpart to the existing Gondwana. In this collision, the Iapetus disappeared and the Caledonian Mountains were unfolded - the Appalachians, the mountains in north-western Great Britain and the high mountains of western Scandinavia.
>> The Age of Fossils II: From the Age of Pisces to the Great Catastrophe
© Jürgen Paeger 2006 - 2013
The skeleton evidently emerged several times, today two basic types can be distinguished: the “exoskeleton”, which surrounds mussels and insects like a shell, and the internal skeleton of the chordates (>> here).
over Charles Doolittle Walcott Richard Fortey writes that his middle name could not have been more inappropriate: “There is next to nothing that Walcott has not done. He described Cambrian fossils from all over the world, including China (if lined up, his books take up almost a meter of shelf). He was a valued administrator, and he discovered the Burgess slate. " (from: >> "Life. A Biography")
Compound eyes occur frequently in the arthropods, they are composed of numerous small individual eyes. The lenses of the trilobites consisted of calcite sticks, the eye provided a mosaic-like image.
Jawless fish still exist today: The lampreys and Inger. The Lancet fish on the other hand, they are not fish at all, because they do not belong to the vertebrate group.
To earlier tracks for life on land see >> here.
Step ashore required a number of adjustments on the plants: to protect them from dehydration, they developed a layer of wax on the leaves; in order to still be able to absorb and release gases, stomata that are opened and closed hydraulically. Roots and bundles of pipes serve to supply water and nutrients, which at the same time stabilize the stem and thus help the plants to defy gravity.
Nevertheless, the country could only be settled after the oxygen produced by algae and plants had formed an ozone layer in the atmosphere that kept the sun's UV radiation away from the earth (>> more).
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