The planet Mercury has a temperature variation of 600℃ between night and day. On Venus there is an enormous atmospheric pressure of 90 times the pressure on Earth. The planet Mars has a very thin atmosphere of CO2 and an average temperature of -63℃ Celsius. The outer planets are uninhabitable gas planets.
Today, we have found just over 5.000 exoplanets - that is, planets that orbit stars other than the Sun - and we have not yet found one that even remotely resembles Earth.
Planet Earth is unique, the closest we get to paradise. It has all the prerequisites for life to unfold. It has a suitably stable temperature, abundant amounts of free water, an atmosphere that contains both oxygen, which can be breathed by living beings, and CO2, which nourishes plants.
Just over 500 million years ago, visible tangible life appeared on Earth in its many diverse forms. This unique eon of life is called Phanerozoic, which is Greek for "the age of visible animals". It is the first epoch is the Paleozoic, which name means "the old animals".
1. The epoch of ancient animals
The Phanerozoic refers to a part of Earth's history where there has been life. It has lasted 542 million years until now. The flourishing of life on Earth is called the Cambrian explosion, as the Cambrian was the first period of the Phanerozoic.
In this diagram, time progresses from right to left. In the upper line, the æons succeed each other. Hadal was the glowing inferno just after Earth's creation. In the Archean, the first rocks that we know of were formed, water vapor condensed and an atmosphere of nitrogen and methane arose. In the Proterozoic, cyanobacteria produced oxygen, which oxidized iron and methane. But at the end of the Proterozoic, life appeared on the sea floor. Phanerozoic refers to the period in Earth's history, when there was life. It has lasted 542 million years until now.
The Phanerozoic, the age of life, is divided into the Paleozoic, Mesozoic and Cenozoic eras, which we can call Earth's ancient, medieval and modern times. The Paleozoic is the epoch of early life and the subject of this article. The Mesozoic is the era of the dinosaurs, and the Cenozoic is the era of the mammals in which we still live.
The Paleozoic is divided into six geological periods. In the Cambrian, visible tangible life arose in the shallow waters along the coasts of the continents, and it continued in the Ordovician, which was ended by the long Andean-Saharan Ice Age, which caused the extinction of many species. In the Silurian, the climate became warm again, and the hitherto rather barren land was overgrown with green plants. In Devon, plants and four-legged animals finally appeared on land. In the Carboniferous, large amounts of vegetation sank to the bottom of the swampy landscape and were thereby transformed into coal. The period was ended by the Karoo Ice Age, which caused another mass extinction of species. In the Permian, all the Earth's continents were united in the gigantic supercontinent Pangea. Own work.
The early geologists, who defined and named the geologic periods knew nothing of the fossilized bacteria of the Archean or the exotic Ediacaran fauna of the late Proterozoic. They had found the fossilized trilobites from the Cambrian, and therefore they believed that it was during this time that life arose on Earth, and this should be marked by the start of a special geological era that was characterized by life, namely the Phanerozoic.
The Sun's brightness, radius and temperature, as a function of its age in billions of years - after Ignasi Ribas: "The Sun and stars as the primary energy input in planetary atmospheres" - Proceedings of the International Astronomical Union, IAU Symposium.
The Paleozoic is the first and longest of the Phanerozoic geological eras. It was the era of early life, when plants, corals, molluscs, insects, fish and many other living organisms arose. The Paleozoic is divided into six periods, which are the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian.
Despite the low brightness of the Sun, the Cambrian climate is assumed to have been a kind of temperate. The Ordovician and Silurian periods had a warm greenhouse climate, however interrupted by the Andean-Saharan Ice Age. The Devonian and Carboniferous were periods characterized by considerable stability - until the Karoo Ice Age at the end of the Carboniferous. In the Permian, a harsh and dry continental climate prevailed on the huge Pangea continent.
Variations in atmospheric oxygen content through the Phanerozoic by Robert Burner of Yale University - added to the geological periods. Time progresses from left to right. It can be seen that already at the end of the Proterozoic there seems to have been a considerable oxygen content in the atmosphere.
The Sun is a star in the main series of the Hertzsprung-Russell diagram. It will stay in the main sequence for about 11 billion years, during which time it will increase its luminosity threefold overall. The start of the Paleozoic happened just over 500 million years ago, and the Sun then had a brightness of just over 95% of its current brightness. At the end of the period, the Sun had reached almost 97% of today's brightness.
According to Robert Berner of Yale University, the oxygen content of the atmosphere has fluctuated between 15 and 35% during the Phanerozoic.
The content of CO2 in the atmosphere in the Phanerozoic according to Robert Berner of Yale University - however, the geological periods have been added. Time progresses from left to right. It can be seen that it had a maximum in the Cambrian, and then it has been decreasing except for a minimum in the Carboniferous period. There are several different reconstructions of the CO2 content in the past atmosphere, all of which are different. But the trend is the same: for the vast majority of time, the concentration has been much higher than in today's atmosphere.
The oxygen percentage reached a peak of over 35% in the late Carboniferous and early Permian, before falling dramatically to just over 15% at the transition from Permian to Triassic. It is believed that the high value in the Late Carboniferous was due to the fact that the oxygen produced was not used to oxidize organic material, which was instead buried in swampy areas. The subsequent decrease in oxygen content may be due to a cooling of the atmosphere and a drier climate on Pangea, which was not favorable for photosynthesis.
The evolution of the average temperature of the globe after Anton Uriarte - added to the geological periods. Time progresses from right to left. It can be seen that it has generally been slightly increasing until the middle of the Cretaceous period, after which it fell towards the Pleistocene ice ages. In addition, the curve has some corresponding minima at the other two ice ages in the Phanerozoic, namely the Andean-Saharan Ice Age at the transition between the Ordovician and Silurian, and the Karoo Ice Age in the late Carboniferous and early Permian.
Throughout almost the entire Phanerozoic, the content of CO2 in the atmosphere has been much higher than in the present. Only at the end of the Carboniferous and at the beginning of the Permian has the concentration been as low as in modern times. This is explained by the fact that dead trees and plants were not oxidized by oxygen, but instead sank to the bottom in shallow swampy areas with oxygen-poor water.
Scientists have calculated the temperature of the past from analysis of sediments on the bottom of the Arctic Ocean, as the amount of certain oxygen isotopes depends on the temperature when the sediment was formed. By far the largest part of the Phanerozoic, the temperature of the globe has been significantly higher than in modern times. Only during the ice ages has the temperature been lower. Apart from these Ice Age minima, the temperature throughout the period has been slightly increasing until the Cretaceous maximum, which can perhaps be attributed to the Sun's ever-increasing brightness.
The length of the day through the Phanerozoic. Time progresses from right to left. A research team from the "Geophysical Observatory" at the "Institute of Physics of the Earth", Karelian RAS led by V.V. Shecherbakova has prepared a database for the "magnetic moment values" of the Earth (VDM). On this basis, they have calculated the Earth's orbital speed through the Phanerozoic - See link below. Points and triangles show direct measurements of rotation and VDM values, respectively, the dotted black and red lines are regression curves matched to points and triangles, and the straight black line is a statistical linear regression, which represents the Earth's rotational speed, i.e. the length of the day, through the Phanerozoic.
A research team from the Russian "Institute of Physics of the Earth" in Karelia has drawn up a curve for the speed of the Earth's rotation through the Phanerozoic, calculated on the basis of magnetic measurements. It can be seen that at the start of the Phanerozoic, a day lasted a good 21 hours and of course, at the end of the Phanerozoic - i.e. the present day - the day lasts 24 hours.
2. Cambrian
Petrified beach from the Cambrian. We see the characteristic imprints from waves and tides, there are some washed-up goplets as well as the so-called "Climactichnites trackways", which are believed to be traces of some kind of snails. If so, it must have been the first animal to go on dry land. The gople in the foreground is 10 cm. in diameter - from Blackberry Hill in Wisconsin, USA.
The Cambrian is the oldest part of the Paleozoic. It lasted 53.7 million years, namely from 542.0 to 488.3 million years before the present. Cambria is the Latin name for Wales, where Britain's Cambrian rocks are best exposed.
The Cambrian is special because of the unusually large number of fossils that have been preserved to this day. This means that our understanding of the Cambrian fauna surpasses that of many later periods. We all know the Cambrian from the fossilized trilobites, of which there are so many.
At the beginning of the Paleozoic, about 542 million years ago, almost all of Earth's landmasses were located in the southern hemisphere. North of 30 degrees north latitude everything was covered with water. Many of Earth's continents joined together to form the supercontinent Gondwana, which stretched from the equator to the South Pole and consisted of the cratons of South America, Africa, Arabia, Antarctica, Australia and India. Baltica, which included the part of the Earth's crust that would later become Denmark, formed an island of its own.
The continents of the world in the Late Cambrian. Gondwana has been formed close to the South Pole. Photo scotese.com
The continent of Laurentia, which is named after the Laurentian Mountains north of St. Lawrence River in Canada, also formed a separate island. The name Gondwana is derived from an indigenous Indian tribe called "Gond" and means something like "Gond land". But it is only a name; as everyone knows, neither animals, humans nor actual plants existed on dry land at this time.
Experts disagree widely about when the oxygen content of the atmosphere began to rise and reach today's level of 21%. Some believe that it happened as early as the transition between the Archean and Proterozoic. Others believe that it first happened at the end of the Cambrian.
Variations in global sea level throughout the Phanerozoic. Time progresses from right to left.
It can be seen that the water level largely follows the temperature, when it is hot, the water level is high, and when it is cold, the water is bound as ice at the poles, and therefore the water level is low. The Karoo ice age at the end of the Carboniferous appears clearly with a very low water level, likewise today's Pleistocene ice age, whereas the Andean-Saharan ice age at the transition between Ordovician and Silurian doesn't seem to have caused a very low water level - at least in this chart.
X: Time scale in millions of years.
2: (Red curves) Hallam et al.
3: (Blue curve) Sea level.
4: (Black bar) Extent of fluctuations in sea level in the Pleistocene. Photo Robert A. Rohde modified by user Zimbres Wikimedia Commons.
The climate was warmer than it is today, according to Anton Uriarte. But how much warmer is hard to say, maybe 5-1℃0. No evidence of ice has been found at the poles in the Cambrian. Earth's oceans also covered a larger portion of Earth's surface than they do today, which also indicates a warm climate.
Typical fossilized trilobites from the Cambrian - Note the incredible details. The trilobites get their name from the fact that they have three rows of shells or shields down their backs. Photo Spoonkymonkey Wikipedia.
Despite the oxygen in the atmosphere, no generally accepted evidence of life on land has been found in the Cambrian. The solid land must in the main have lain like sand and rocks, barren and desolate. Life developed and unfolded exclusively in shallow water on the coasts of the continents. It is possible that some areas of the land have been covered by a green film consisting of bacteria, algæ, or lichen. Such a green cover may have developed even before the Cambrian, although there is no direct evidence for it. Films of cyanobacteria have been found even in modern deserts, so it is easy to imagine that something similar could have existed in the geological past, before there were any plants.
Fossilized mandibles of cambrian crustaceans found at Lake Riley in the Deadwood Rock Formation of western Canada. It can be calculated that the body length of the crustaceans was 10-15 mm. Photo "Exceptionally preserved crustaceans from western Canada reveal a cryptic Cambrian radiation" Thomas H. P. Harveya, Maria I. V lez and Nicholas J. Butterfield
In the Cambrian, the content of CO2 in the atmosphere rose to an absolute maximum for the entire Phanerozoic, namely 25 times today's concentration of 380 ppm, apparently without a corresponding increase in temperature.
It is thus that actual plants need soil to grow in, but soil is formed from organic decay, i.e. also from plants. It may have been such a contradiction that, over millions of years, prevented plants from gaining a foothold on land. But an organic layer formed by millions of years of decay of blue-green bacteria and similar organisms may have created the breeding ground for actual plants, which were to come later.
The soft bodies of the Ediacaran creatures disintegrated quickly after their death and disappeared without a trace on a very large scale. But in the Cambrian, animals with hard external skeletons developed, which were suitable for forming fossils after death.
Trilobites on the sea floor, painted by Heinrich Harder in 1916. There were over 20,000 different species, it is said, widespread and found all over the Earth. The largest trilobite ever found is 70 cm long and is called Isotelus Rex, it is dated to the Ordovician. On the other hand, the smallest trilobite found so far was only 1.5 mm long, it is called Acanthopleurella Stipulæ. The trilobites were a group of animals that existed for a very long time. They originated in the Cambrian and managed to survive two of the world's worst mass extinctions, namely the Ordovician-Silurian and Devonian-Carboniferous mass extinctions, before the great extinction at the end of the Permian finally got them.
Trilobites lived in the sea along the coasts of the Cambrian continents. They were one of the most successful of all prehistoric animals. Several thousand different species of trilobites have been recorded, spanning the entire Paleozoic period. Some were decidedly predators, they probably hunted other trilobites, others lived on a form of plankton or similar early organisms.
The oldest find of trilobite fossils can be dated to the early Cambrian, 526 million years ago. The last trilobite species became extinct in connection with the mass extinction at the transition between Permian and Triassic about 250 million years ago.
The Burgess Shale rock formation in the Canadian Rocky Mountains is a very important site for Cambrian fossils. The fossils have a unique wealth of detail that also show soft parts of their bodies.
Fossil of Anomalocaris from the Burgess Shale. The predator Anomalocaris is characterized by a circular toothed mouth and a single pair of jointed frontal appendages, perhaps for grasping prey. The most complete Anomalocaris specimen is 25 cm long, although isolated fragments suggest that individuals may grow up to 100 cm. long. Photo Burgess Shale in Yoho National Park, which is a World Heritage Site located in the Canadian Rockies of British Columbia.
The first complete fossil of Anomalocaris was also found here, which was a kind of giant crustacean that could be a meter long, a true giant for this time. It is believed that it lived on trilobites. It must have been a feared predator in the Cambrian waters.
Near the city of Kunming in the province of Yunnan in China, a petrification of an animal that looks like a small fish, only 28 mm, has been found. long. They have given it the name Myllokunmingia. It has been dated to 524 million years before the present, and is believed to be the world's first vertebrate.
Graphical reconstruction of Anomalocaris, which ravaged the Cambrian waters. Photo Junnn11 - Own work Wikipedia
The first crustaceans, sponges and coral reefs appeared during this period. The Cambrian seas were also home to molluscs, such as snails and slugs, which according to some already appeared in the Ediacara period at the end of the Proterozoic.
Some believe that the Cambrian was ended by an ice age or some other form of mass extinction, because so many of the species of the period cannot be found in the Ordovician. But no evidence of such an ice age has been found in analyzes of bottom deposits in the seas.
3. Ordovician
Ordovician was really a natural evolution of Cambrian, and the reason that the period got its own name is said to be a discussion between two British geologists from the nineteenth century, Sedgwick and Murchison. One claimed that the period belonged to Cambrian, and the other said that the period belonged to the Silurian. Eventually, they settled to define it as a brand new period. Ordovician is named after a native Welsh tribe that the Romans called the Ordovicians.
Earth's continents in Ordovician. The super-continent Gondwana was located at the South Pole. Laurentia, Baltica and Sibir were scattered in the Southern Ocean. Above 30 degrees north latitude was only water. The craton Avalonia had torn itself away from Gondwana and joined Baltica.
Ordovician lasted 44 million years that is from 488 to 444 million years before present.
The land was still deserted, in all probability void of plants and life. However there have been found a few fossils of plant spores, but it is not clear what kind of plants they represent. One can imagine that parts of the land could have been covered with bacteria cultures, perhaps cyanobacteria, primitive mosses and lichens all being growths, which do not leave fossils.
Part of the shale oil, that in a not distant future will be transformed into natural gas and exploited, has been created in the Ordovician. Shale oil is found in Estonia and Sweden among other places, which were more or less dry land in the Ordovician; then perhaps one should imagine that at least some places seethed with life also on the land in Ordovician, perhaps it was in lakes.
Ordovician Landscape - The artist describes that life in the Ordovician unfolded underwater, and that the land was generally barren and bare. - However, in order to create mud for the many mud-eating animals on the bottom, there must have been some organic growth on land which could decay and be carried out to sea with the rivers. Photo JoseBonner Wikipedia.
The supercontinent Gondwana has moved towards the South Pole and Laurentia, Siberia and Baltica were still isolated continents in the ocean.
The area of the earth's crust which was to become Denmark was part of the continent Baltica; however, apart from Lolland-Falster and Southern Jutland, which belonged to the craton Avalonia, which had broken away from Gondwana and united with Baltica. You can thus say that the foundation for Denmark was created in the Ordovician.
The Moon was still closer to Earth than it is now, how much closer is disputed. Consequently, due to the physical law of the conservation of rotational energy, its orbital speed must have been correspondingly faster, that is, the time between two full moons was shorter.
In addition, the Moon has a tidal effect on the Earth, so that the speed of the Earth's rotation, i.e. the length of the day, becomes increasingly slower. Therefore, the length of the day in the Ordovician was somewhat shorter than it is now, namely around 21.5 hours.
Fossils of graptolites. Photo Leptograptus fossil graptolites (Viola Formation, Ordovician; Murray County, Oklahoma, USA) James St. John Wikipedia.
The content of CO2 in the atmosphere fell from the Cambrian maximum to a more low 17-18 times the present level, apparently without a corresponding temperature change. The oxygen content of the atmosphere seems to have been just around 20%.
The sun's brightness was about 95% of today's, but nevertheless the climate at the beginning of the Ordovician was mild, perhaps 3-10℃ above today's temperature. The water level in the world ocean was the highest both before and since during the Phanerozoic, and therefore there were many warm shallow areas along the coasts of the continents, where many different creatures lived.
Fossil of hyolite from the Ordovician - The Stone Museum. Found in Nymølle Grusgrav, Pårup on East Fyn by Peter Mortensen..
Strata from the Ordovician are characterized by rich deposits of fossils of trilobites, graptolites, corals, bryozoans, echinoderms and brachiopods.
The trilobites continued to thrive in the warm waters. There were many different species of trilobites. Some were mobile mud eaters and others were predators. Some species developed the ability to curl up like bench-biters, others developed the ability to swim, where before they could only crawl over the bottom.
Fossils of graptolites are characteristic of Ordovician and Silurian rocks. They were free-floating or stationary colonial animals with chitinous armor that filtered the ocean water for plankton. A graptolite colony was made up of many short tubes that were less than 1 mm. long, each housing a graptolite individual, a "zooid".
The brachiopod Vinlandostrophia Ponderosa from the Ordovician. Platystrophia ponderosa, Maysvillian (Upper Ordovician) near Madison, Indiana. Now Vinlandostrophia ponderosa. Photo Wilson44691 Wikipedia.
Brachiopods or brachiopods are similar to clams, but unlike these, which have right and left shells, brachiopods have upper and lower shells. The shell on the ventral side is mostly larger than the dorsal side.
South of the island of Bornholm, layers from the Ordovician come to the surface, and fossilized graptolites can often be found here. The Swedish island of Øland is formed almost entirely of Ordovician rock.
The world's first coral reefs were formed in the Ordovician.
Bryozoans were small colony-forming coral-like animals. They could sit on firm surface such as seaweed, stones or mussels. Bryozoans existed in the Earth's oceans for billions of years, from the Ordovician to the Cretaceous period, where their skeletons are one of the main components of the Cretaceous.
Modern sea teeth, Antalis entalis, are mud eaters. They live buried in the seabed. Photo Wikidata.
Echinoderms are such animals as starfish, sea urchins and sea urchins.
The newly evolved cephalopods were the great dangerous predators of the Ordovician. They could grow up to 4.5 meters long and had an unknown number of arms.
Many of these species had already appeared in the Cambrian and flourished in the Ordovician. They came to form the marine fauna throughout the Paleozoic until the great mass extinction at the transition from Permian to Triassic. Some of the species, however, continued their existence throughout the rest of the Phanerozoic, indeed, as is known, many of them are still found in the sea today.
In the Ordovician there were also many mud-eaters, i.e. animals that got their nutrition by passing large amounts of mud or dung, something similar to earthworms and sandworms. Antalis entalis are mud-eating molluscs that are fairly unchanged since the Ordovician. Hyolites were a type of mud-eating animal. Many trilobite species also lived by eating mud.
Ordovician fossil algæ found in the United States, winnipegia cuneata, manitobia patula and kinwowia articulata. Photo Berkeley Edu.
But mud in shallow coastal waters is formed, when organic decay on land is washed into rivers and from there it is carried out to sea. Therefore, the existence of mud-eating animals in the Ordovician indicates that there must have been some form of organic vegetation on the land which produced the organic parts in the mud.
In 2010, five different types of fossil liverworts were found in Argentina, which were dated to the Middle Ordovician. These are thus the oldest known fossils of stem plants. Liver mosses are very primitive plants, more primitive than ordinary mosses, they exist as both aquatic and terrestrial plants. In addition, fossilized algæ from the Ordovician have been found in the USA.
Stromatolites are a symbiosis between blue-green bacteria and other bacteria; they existed billions of years before the Ordovician, and they still exist. Therefore, they must also have existed in the Ordovician. There may well have been various forms of such symbioses.
It is reasonable to assume that the land was then overgrown with various primitive mosses, algæ, lichens and bacteria, which were generally so soft that they did not leave fossils.
The front of the Kufra Basin in southern Libya is made up of sediments from the Andean-Saharan Ice Age in the Late Ordovician and Early Silurian. The ice spread towards the northwest. Photo unknown origin.
At the end of the Ordovician, a violent event occurred on Earth, which wiped out 85% of all species. It is generally assumed that it was one or more ice ages, which are called the Andean-Saharan Ice Age. It covered what is now the Sahara and the Amazon, which was then part of the supercontinent Gondwana, located at the South Pole.
Artistic Reconstruction of the Andean-Saharan Ice Age in the Late Ordovician and Early Silurian. Photo u-teti-soni.blogspot.com
Geological findings indicate that the desert was then covered by a thick layer of ice. The ice sheet extended over more than 8 million km2. from the Massif du Hoggar in southern Algiers to the Atlantic coast of Mauritania. Sea levels dropped drastically, and the extensive shallow areas along the coasts of the continents dried up. This led to the great mass extinction at the transition from the Ordovician to the Silurian.
The Ice Age lasted for about 2 million years. The remains of large furrows and depressions created by the Andean-Saharan Ice Age ice sheets can still be seen today.
During the Andean-Saharan ice age, the CO2 concentration in the atmosphere remained much higher than today's level. Some studies even show a CO2 concentration which was up to 16 times higher than today. It shows that there is probably no cause-and-effect relationship between temperature and CO2 concentration in the atmosphere.
The extent of the Andean-Saharan glacial ice sheet according to "Late Ordovicium sedimentary environments, glacial cycles." by J.F. Ghienne. There are different estimates of how large an area was covered by the Andean-Saharan Ice Age ice sheet. The maximum estimate states that all of Africa and most of South America were covered in ice. The minimum estimate states that West Africa and the areas around the Red Sea, the Andes, and South Africa were covered in ice.
Therefore, it is likely that geographical or astronomical factors, rather than the chemical composition of the atmosphere, were the decisive factor in the creation of the Hirnantian Ice Age, which is also called the Andean-Saharan Ice Age.
4. Silurian
Silurian is named after another ancient Celtic tribe in Wales, called the Silurians by the Romans. The period lasted 28 million years, namely from 444 to 416 million years before present.
A small piece of petrified seabed from a shallow Silurian coastal water. The trilobite Dalmanites, the coral Favosites, bryozoans including Favositella, brachiopods including Atrypa can be seen. From Wenlock Limestone, Dudley, West Midlands, England. Photo Adrian Pingstone Wikipedia
After the Andean-Saharan Ice Age, temperatures rose again and the climate remained warm and humid throughout the rest of the Silurian, Devonian and most of the Carboniferous. The land grew with lush vegetation. The new vegetation consisted of vascular plants, which had a stiff stem that allowed them to grow upwards. They were such as ferns and toadstools.
We know that gardeners can increase the CO2 content of the air in their greenhouses and this will make the plants grow much faster. Something similar happened in Devonian times; the warm, humid climate and the high concentration of CO2 in the atmosphere caused vascular plants to colonize all continents with a host of new fast-growing plant types. There were probably extensive "forests" of moss and lichen, ferns and toadstools.
Map of the World in the Silurian. The continents Baltica and the craton Avelonia. has united with the North American Laurentia, Baltica and Greenland as well as some small cratons. Gondwana still exists It can be seen that there are still many small continents and probably long coastlines with large shallow areas. Photo Scotese.com.
The glaciers of the Andean-Saharan Ice Age melted, and therefore the water level of the world's oceans became quite high, creating extensive warm shallow sea areas where life thrived. Here reefs were built by corals and other reef builders such as bryozoans and some prehistoric organisms called stromatoporoids.
The Sun was shining at about 95-96% of its current brightness. The day was just under 22 hours long. The temperature in the Silurian after the ice age was perhaps about 3-12℃ higher than today. The oxygen content of the atmosphere was around 16-17%. The CO2 content in the atmosphere was about 20 times today's level.
A fossil of a Eurypterid or sea scorpion. The elongated body consisted of a large forebody and a hindbody with 12 movable joints, as in shrimps, and a tail joint. The front two of the six pairs of limbs functioned as grasping tools, the next four as walking legs, while the rear pair was transformed into oar-like swimming organs. Eurypterids are predominantly found in deposits from brackish and fresh water, but they were also found in salt water. Individual species could probably move onto land for shorter periods. The most common size was around 100 cm, at the river Ems in Germany specimens have been found, which are believed to have been 45 cm long. Photo Fossils-UK.
The supercontinent Gondwana was still in the southern hemisphere surrounded by some smaller continents. Including Baltica and Avelonia, which had merged with Greenland and Laurentia as well as some smaller cratons. These continents pushed against each other, and thereby the Caledonian mountain range was built up, which included the mountains along the east coast of Greenland, the English and Scottish mountains, the mountains along the northern part of the American east coast and the North German mountains in Avelonia.
The northern hemisphere was for the most part a single large sea.
In the same way as in the waters south of Cape Horn today, the waves could roll around the globe and get stronger with each round. We must believe that the northern sea has been characterized by raging storms and huge waves.
Finally, life arose on land. The earliest signs of life on land are scorpions, arachnids and centipedes, as are spiders, mites, mealybugs and centipedes. The first leeches also seemed to have appeared at this time.
Much like today, the interior of the continents seem to have been drier and more desert-like than the coastal areas.
The decisive event for marine life in the Silurian was the development of actual fish with internal skeletons, backbones and jaws. In South China, a fossil of a Silurian fish with jaws, backbone and internal skeleton has been found. It has been named Psarolepis and is about 10 cm. long.
The fish Psarolepis had a backbone, internal skeleton and jaws. It was found in South China. The length is approx. 10 cm. - Drawing by NTamura.
In the shallow muddy waters near the shores and in fresh water also lived a formidable predator called the Eurypterid or sea scorpion. They probably lived on tribolites and fish.
5. Devonian
The Devonian lasted 56.2 million years, namely from 416.0 to 359.2 million years before the present. The period is named after the county of Devon in southern England.
Map of the World in Devon. The supercontinent Gondwana still exists, but it has moved slightly away from the South Pole. Lauretia, Baltica and a number of other small continents have united into a new large continent, which some have named Euramerica. The part of the Earth's crust that will become Denmark is part of both Baltica and Avalonia. Photo Scotese.com.
Although the Sun was only shining at about 96% of its current brightness, the warm climate in Devon continued and lasted throughout the period. The temperature was perhaps about 3-10℃ higher than today. The water level in the World Ocean was high, but for unknown reasons it fell slightly compared to the Silurian.
The day was about 22 hours long.
The oxygen content of the atmosphere was quite low, around 16-17%.
The CO2 content in the atmosphere had decreased compared to the Silurian, so that it was now only about 15 times today's level. Towards the end of the period, the CO2 content fell significantly towards the Carboniferous and Permian minimum. It is assumed that carbon, bound in organic growths, was not recycled to the atmosphere, but instead sank to the bottom in swamps and marshlands, forming fossil hydrocarbons and coal. Part of the fossil reserves that we extract today were formed at the end of the Devonian.
The Devonian is often called the Age of Fish because so many different species of fish were found from this period. Both lobe-finned, ray-finned fish and primitive sharks originated in Devon.
An artist's reproduction of a landscape from the Devonian period. Painting by Eduard Riou from The World Before the Deluge 1872 United States Wikipedia
The most fearsome fish were the armored placoderms. There have been identified more than 200 types of placoderms, the first appeared in the early Silurian, but most can be attributed to Devonian. A few of them could be up to 10 m long.
Trilobites, brachiopods and various builders of large coral-reefs were still widespread.
In the Devonian, the early vegetation on land developed into actual plants with primitive roots. The earliest were the original vascular plants, i.e. ferns and toadstools. They were not very tall, maybe about a meter, and their growing places probably remained close to the water. But by the Late Devonian, land plants had evolved into true trees with trunks, and the first ever forests appeared. Yes, fossils of tree-like growths almost 30 m. high have been found.
At the end of the Devonian, the first seedlings appeared. The soil surface received a steady supply of decomposed organic substances, which mixed with the original sand and gravel became soil, which in turn provided soil for new plants.
Fossil evidence has been found that wingless insects, mites, scolopanders, centipedes, scorpions and spiders lived in Devon's forests of mosses, large ferns and toadstools.
Towards the end of the Devonian, amphibians evolved from fish.
Left: Fossilized footprints of a Devonian four-legged animal found in the Zachelmie quarry in Poland. - This one appears to have had six toes or claws.
Right: The animal's gait has been reconstructed by Polish and Swedish palæontologists from the fossilized footprints. Photo and drawing Per Ahlberg et al.
An amphibian is a four-legged animal with lungs to breathe, it can live on land, but it still lays its eggs in the water. Frogs, toads and salamanders descend from the early amphibians.
The fish from which the amphibians evolved probably lived in tidal ponds or other small lakes. They had both primitive lungs and gills for breathing. Eventually they evolved into true air-breathing amphibians with fish-like heads and tails, but with four legs. Many types of amphibians evolved quickly because they had little competition from other land animals. The amphibians could eat the plants and insects that were already abundant on land.
Galathea 2 steams out in 1950. The expedition lasted for two years. The frigate had a crew of 100 sailors and scientists. Foto Wikipedia.
In the Zachelmie quarry in the "Holy Cross Mountains" in southeastern Poland, footprints of a four-legged animal have been found in layers from the Middle Devonian period. There are traces of several individuals. It is estimated that the length of the animals varied between half a meter and two meters from snout to tip of tail. They have resembled lizards or crocodiles and had a similar gait. They have had six toes or claws on each foot.
The Polish and Swedish palæontologists who examined the footprints believe that a herd of such animals lived on a muddy coast behind a coral reef in the Devonian period. Every day at low tide they ran across the muddy bank to eat the clams and other animals left by the receding sea.
The blue fish, coelacanth, Latimeria, in the Zoological Museum of Copenhagen. Alive the fish is blue, but the color fades away very quickly after it has been taken up from the sea. Foto FunkMonk (Michæl B. H.) Wikimedia Commons.
This is the first evidence of four-legged animals on land. It is believed that they have evolved from brush-finned fish, which were a group of large predatory fish known, among other things, from fossils. They were distinctive in that their tasseled fins were supported by muscles and bones built almost like the limb skeleton of terrestrial vertebrates. Common modern fish, such as cod and herring, are called ray-finned.
In 1950, the Danish Galathea expedition caught a specimen of "The Blue Fish", Latimeria, in the sea off South Africa. Until then, the world had only known it from a half-rotten specimen landed by fishermen in South Africa in 1938.
The modern tasseled fins Panderichthys rhombolepis. Photo User: ArthurWeasley User: Bruce A.S.Henderson User: Mitch Ames User: DiBgd Wikipedia.
The blue fish belongs to the lobe-finned fish that before 1938 only was known from fossils. Until then it was thought that they had become extinct in the Cretaceous period 65 million years ago. Until this day only about 200 of these living fossils had been caught.
Their characteristic lobe-fins resemble terrestrial vertebrate's limbs. The fins are supported by an internal skeleton, which also bears some resemblance to terrestrial vertebrates. They have a joint behind the head, which allows them to move their head up and down independently of the body's position.
They live in deep waters in the world ocean, where they can be more than 100 years old. They can grow up to 2 m long and weigh up to 95 kilos, which is basically the same as humans. However, they are usually smaller, especially the males that often have the length of only about 1.65 m. Their eggs are fertilized inside the female like it is the case for mammals. The eggs are about 10 cm in diameter, and a newborn young fish is 35 cm long. The females get between 5 and 25 at a time.
But it becomes even more interesting. Lungfish, which are equipped with lungs in addition to gills, also belongs to the group of lobe-finned fish. Lungfish have been found as fossils in layers from the beginning of the Devonian period. The oldest forms lived in salty water, but they quickly adapted to life also in freshwater. In Devonian lungfish were widespread, and the number of species was the highest ever. Lungfish still live in Africa and Australia.
A tetrapod goes ashore. Painting of Ichthyostega.
Now, one can imagine that the first tetrapods (tetra means four, and pods means feet - that is four-legged animals) went ashore in the Devonian period as follows: Some lobe-finned fish with small bulges in the esophagus, which served as lungs, landed on some muddy shores. They used their powerful tassel-fins as legs. Maybe they in this way they could better escape giant octopus' and sea scorpions on the prowl. Now and then they went out into the water and found some food, where after they immediately crawled back to the safety on land. Over time they also developed the ability to obtain their feed on land in form of plants, snails, insects and other animals, which were already there. And from these first four-legged creatures with internal skeleton descended reptiles, dinosaurs, mammals and, ultimately, humans.
However, it is still puzzling that such a fish like the blue fish with the limb-like lobe-fins and corresponding skeleton could develop in the sea. It is easy to imagine that it could have been a kind of whale, so to say, that it could have been an animal that had developed on land and later on returned to the sea - like the whale.
6. Carboniferous
The Gondwana continent had left the South Pole and begun to float to the north, where it came ever closer to Euramerica, which was composed of Laurentia, Baltica, Avelonia and some other small continents. Ice had begun to form in the southern part of Gondwana.
The World in Early Carboniferous - Gondwana had moved to the north and approached Euramerica. There was beginning ice in the southern part of Gondwana. The entire northern hemisphere was mainly ocean. There were no continents extending from North to South obstructing the east-west currents in the ocean. One must believe that the seas have been characterized by enormous waves in the same way as today in the waters south of Cape Horn, where currents and waves can move freely around the globe and amplify for each encirclement. Map by Christopher Scotese - Scotese.com
In Carboniferous, the duration of day and night was just over 22 hours. The sun shone with 97-98% of its current brightness, but the climate was never the less hot and humid, maybe the temperature was 6-8℃ higher than present-day temperature. Some think that Carboniferous' average temperature was 20℃ that can be compared with world average temperature today that is about 14℃ and with Denmark's average temperature today that is 8℃.
Gladstone Mangrove Marsh in Australia - The Swamp forests of Carboniferous may have looked like that. Photo Tripadvisor.
Carboniferous got its name after the huge quantities of coal from that time, which we can retrieve from the underground. In Carboniferous, the old and fallen trees sank to the bottom of stagnant water in extensive swamps and marshes without rotting or burning. As time passed by, they were covered by other trees and new sedimentary layers, and in the course of time, they were transformed into coal.
One may wonder why there were so many swamps at this time, shouldn't we think that the amount of water in the world surely must remain relatively constant?
Earth's first continents were fragments of solidified crust that floated around on liquid magma. Basically, it is still the case, but the fragments have only become thicker and do not flow as fast anymore. The initial small continents were flat as pancakes, or should we say flat as ice floes, which after all roughly are created in a similar way.
A total presentation of atmospheric CO2 and average global temperature during Phanerozoic. - It is easily seen that the atmospheric CO2 and global temperature have only little correlation. Temperature after C.R. Scotese University of Chicago and CO2 after R.A. Bremer.
Only when the original small continents, which are also called cratons, assembled into larger continents did they begin to press against each other, and thereby they could fold and push back. It was in this way that the mountains were created.
The Cambrian uplift, which took place in the Cambrian and Silurian, created some of the world's first and oldest mountains, which can be seen in Wales and East Greenland, among other places. But most and largest of the Earth's mountains are much younger than them. The Himalayas and the entire Tibetan Plateau, for example, were only created in the Tertiary period, after the Indian continent hit the Eurasian continent. The Alps were also created a few million years ago, when Africa began to press up against Europe.
An artist's reconstruction of a Carboniferous landscape showing trees sinking to the bottom in stagnant oxygen-poor water. Photo Studio 252 MYA Darwin's Door.
But in Carboniferous, the world was in general still flat as a pancake. The climate was hot and humid, and the water from the never-ending rain had difficulty to run away in the flat landscape. It created huge swamp landscapes with stagnant water poor of oxygen.
When the new fast-growing trees of the Carboniferous fell for storm and tear, they sank to the bottom of the stagnant oxygen-depleted water, without being oxidized by burning or rotting. In this way still more carbon was taken out of the world's carbon cycle and therefore the atmospheric CO2 sank to a historic minimum, as it shrank to less than 400 ppm (parts per million), which also roughly is the CO2 content of today's atmosphere.
Whitish Carboniferous fossil plants on black shale found near the town of St. Clair, Pennsylvania. It is 10.3 cm in cross-section where it is widest. Foto James St. John Wikipedia.
On the other hand, the oxygen content rose to extreme heights, namely from 25 to 35%, which must be compared with today's 21%. It was probably due to the fact that the oxygen, created by the photosynthesis of the many trees, could no longer find anything to oxidize over time. All iron, methane, carbon and other oxidizable compounds were already used up, and fallen trees were immediately covered by oxygen-poor water.
Charred trees found in coal mines show no signs of annual rings, so it can be concluded that differences in seasons were completely insignificant. The mild and humid climate across the globe was probably due to the continents being positioned in a way that allowed water to flow freely and thereby distribute the heat without being blocked by land masses stretching from north to south - as in the present day.
Fossil of Meganeura, which was a kind of dragonfly from the Carboniferous period, which could achieve a wingspan of 75 cm. Photo Ghedoghedo Wikipedia.
The Euramerica continent was located pretty near equator and was covered by tropical swamp forests, which by their decay formed most of the Western European and American coal deposits. The forests consisted partly of the fern-like, but seed-bearing, Pteridos Permian, the up to 35 m. high Lepidodendrale Lycopod with green stems and Sphenopsid Calamites, which could be 20 meters high.
A piece of calamites from a coal mine. Photo unknown origin.
Gondwana, which was located further down towards the southern Arctic regions, had its own more sparse fauna.
Many species in Carboniferous developed the ability to reproduce by
amniote eggs, which are eggs that are designed to develop on the ground and not in water. In the amniote egg, the embryo is surrounded by various membranes and a shell that protects it simultaneously making it possible to absorb oxygen.
Polished rock from the Karoo Ice Age near Douglas in South Africa. Foto Swiss Educ - Glaciers online.
Many different species of tetrapods thrived on land. Some looked like crocodiles or salamanders, others resembled lizards or snakes.
It is believed that already in Carboniferous developed two main types of tetrapods namely Saoropides and Pelycosaurs. From Sauropides descend lizards, snakes, crocodiles, dinosaurs and birds and from Pelycosaurs descends mammals.
Insects and flowering plants belong together, and they also appeared simultaneously in Carboniferous. Insects could be very large. Meganeura was a kind of dragon fly, which could have a wingspan of 75 cm. Many believe that Carboniferous' very high oxygen content in the atmosphere was very favorable for insects.
Karoo Ice Age is named after the Karoo Basin in South Africa, where typical glacial sediments from this period have been found. The Karoo ice age is divided into two periods, namely from 359 to 318 million before present, and from 318 to 299 million years before present.
This ice age lasted exceptionally 60 million years. Roughly it occupies the second half of the Carboniferous and the first half of Permian.
Schematic representation of the continents that formed Gondwana with the glaciation of the Karoo Ice Age plotted. Foto Wikimedia Commons.
The glaciations extended primarily on the southern hemisphere flowing from the base of Southern Gondwana. On all continents, that once were part of Gondwana, have been found evidence of the Karoo Ice Age. One can find moraine layers in the Karoo basin of South Africa, Talchir Boulder Beds in India, Wynyard rock formation on Tasmania and the Parana Basin in Brazil.
Layers from the Karoo Ice Age exhibit very often cyclical repetitions, which indicates that a sequence of actual ice ages was interrupted by inter-glacials at regular intervals. As we know, such cyclical repetitions are also a characteristic of our present Pleistocene ice age.
The northern hemisphere was covered by sea, probably with huge waves, and there could probably not have been formed real glaciations.
7. Permian
In the Permian, all the earth's continents gathered in one single large C-shaped giant continent over the equator called Pangæa. The sea within the C is called the Tethys Sea, and the remaining of the worlds ocean is called the Panthalassic Sea.
A simplified representation of Pangæa. User:Kieff Wikimedia Commons.
Permian was the last period of the Paleozoic. It lasted 48 million years, namely from 299 to 251 million years before present. The period has been named after the Russian city of the same name, as it was here that layers from this period first was found.
The sun shone with about 97% of its current brightness, and day and night lasted about 22.5 hours. Atmospheric CO2 concentration was very low and in line with the level of today; however, in the last quarter of the period, it again started to rise towards the maximum of the Cretaceous. Atmospheric oxygen content decreased throughout the period from an all-time high of 35% in Carboniferous to the more modest level of 22% that is in line with today's oxygen level.
The Karoo Ice Age lasted well into the Permian period, and the climate has likely been pretty cold. But at the end of the period, the temperature did reach a few degrees higher than today's temperature.
Rock that has been polished smooth by the Karoo glaciers in the Permian - Selwyn Rock, South Australia south of Adelaide near the town of Victor Harbor - In Ihis area also many other traces of the Ice Age can be found, among other various moraine deposits.. Foto Bahudhara Wikipedia.
Pangæa was a huge continent and contained extreme climate and environmental conditions. The southern part was cold and dry, with large parts of the region frozen in glaciers. The coastal areas were hot and humid, while the continent's interior suffered from dryness and desertification. Some lakes simply dried out during Permian. In Northwest Europe, especially in Germany thick layer of salt deposits from Permian have been found and on Spitsbergen has been found a thick layer of salt and gypsum, which is typical for dried out lakes.
The equatorial regions, especially near the Tethys Sea, were covered by
extensive swamps. This part of the Earth's crust is now the southern region of China, where large deposits from the Permian have been found.
It is believed that most of Pangæa had a very pronounced continental climate.
Monsoon conditions, characterized by highly seasonal rainfall, as we know them from India, probably prevailed along the coasts.
The monsoon is a steady wind that blows from sea to land or from land to sea due to temperature differences between land and sea. The bigger the oceans and the bigger the land masses, the more the monsoon wind will blow.
The most well known monsoon today is the southeast Asian monsoon that brings the rain to India and other countries. The hot and humid air is forced over high mountains and releases thereby its humidity as rain. In Permian the Pangæa was an even bigger continent, creating an even more powerful monsoon; however the mountains of that time were few and not very high, so it is assumed that the Pangæan monsoon was powerful but did not give very much rain to the interior of the contingent. Drawing unknown origin.
In summer, the sun heats both land and sea, but the temperature on land rises faster than the temperature above the sea. Rocks and soil have a poor thermal conductivity and a small heat capacity, and therefore the temperature on land rises quickly. Water, on the other hand, can absorb far more heat from the same solar radiation, as water has a good thermal conductivity and a high thermal density, and furthermore the heat is quickly distributed by waves and currents in the sea.
When land masses are heated in the summer, the warm air will rise into the air, and this will create a low pressure over the land. The warm moist air over the sea will then flow in to try to fill this low pressure, and this wind is the monsoon. In winter, the situation is reversed, as the sea is warmer than the land, and the monsoon then blows from the land and over the sea.
The larger the land mass that borders the larger the ocean, the stronger the monsoon winds will be. In principle, there can be monsoon winds all over the Earth, but the Southeast Asian monsoon is the best known, because there a very large continent borders a very large and warm sea.
Artistic production of the predator Dimetrodon from the Permian. - It
descended from Carboniferous' Pelycosaurer. It looks undeniably some lizard-like, but it has in all probability features characteristics of the later mammals. Many believe that it was warm-blooded and that its characteristic back-fin served the same purpose as the elephants' large ears, namely to form a big surface, where it could get rid of the heat. Foto Dimetrodon - Permian period - Desktop Nexus Wallpapers.
Pangæa was the largest continent ever, and it was surrounded by an equally huge ocean, namely the Panthalassic Sea covering the rest of the Earth's surface. It is believed that the shores of Pangæas must have been exposed to very strong monsoon winds.
We have all heard of the continental climate of the interior of Eurasia, shall we say in Kazakstan or Mongolia. The summers are terrible hot with temperatures reaching 40℃, and winters are icy cold with similar minus temperatures. It is, of course, because Eurasia is such a vast continent with a long distance from its interior to the sea and its big heat content.
Pangæa was a much bigger continent, probably with an even more pronounced continental climate. There can be no doubt that winters in central Pangæa must have been a very harsh experience, and summers must have been unbearably hot.
Several mountain ranges were created, when the many continents pushed toward each other creating the supercontinent. Among others, the Ural Mountains was created in Permian. We must also believe that their pressure against each other gradually created a higher level in the interior of the continent so that the rivers could drain the land more efficiently, than they did in the Carboniferous, making the water stream from the mountains and down into the sea, as it does today.
The mass extinction at the transition from Permian to Triassic. At intervals - as indicated by the red arrows - mass extinctions of Earth's species have occurred. But the Permian-Triassic extinction event was the most severe in Earth's history. It led to the extinction of up to 96% of all marine species and 70% of those that lived on land. It is also the only known mass extinction of insects. The incident has been called the "mother of all mass extinctions". Foto Hans Arne Nakrem Universitetet i Oslo.
But still no Alps and no Himalaya had emerged, and the monsoon wind blowing from the sea into the land was not forced to release its humidity in the same ultimate way, as it is the case today in southern Asia, when the wind is forced over high mountains. All that we know is that the interior of Pangæa was very dry and desert-like.
The lush swamp forests of the Carboniferous were gradually replaced by conifers, seed-bearing ferns and other drought-resistant plants. The oldest ginkgo trees have been found in layers from the end of the Permian.
In this barren and arid climate lived the descendants of the Carboniferous period's Sauropids and Pelycosaurs. The Sauropids became the ancestors of numerous bird and dinosaur species, and the Pelycosaurs became extinct in the following Triassic period, but however not before they had developed a separate side branch, namely mammals.
At the end of the Permian, the world experienced a mass-extinction in an unprecedented scale. Over 90% of all species disappeared during a short time. In the sea approximately 95% of all species became extinct. Trilobites, sea scorpions, many species of coral, moss animals, lizards, insects and plants disappeared forever.
Large Australian meteor craters. Distribution of uncertain, possible or probable impact craters. Green dots represent confirmed impact craters, red dots represent confirmed impact structures greater than 100 km in diameter, and red dots inside white circles represent craters greater than 50 km in diameter. Yellow dots represent likely impact structures. Foto Andrew Glikson og Franco Pirajno News.com.au.
It cannot be said with certainty what the cause was, but some point to extensive volcanic activity. At the end of the Permian, the continent of Siberia was on fire. Large areas in Siberia consist of basaltic rocks from this particular time. Huge volcanoes sent a stream of CO2, toxic gases and particles into the atmosphere. Acid sulphurous rain may have poisoned the marine environments. It has been proven that the global temperature increased by 5-10℃.
Another proposed explanation for what happened is that the Earth was hit by one or more giant meteors. It is believed to have proved that the disaster occurred on a single fatal day.
The underwater Bedout High crater is located 300 km northwest of the Australian coast. It has been dated to the end of the Permian period about 250 million years before the present. It is estimated that its original diameter was about 200 km, which is of the same size as the dinosaur killer that created the Chicxulub crater in Mexico.
In 2004, a large crater-like structure was recorded almost two kilometers under the East Antarctic Ice Sheet. The Wilkes Land crater may be more than five times the size of the Chicxulub crater. It has been tentatively dated to around the transition from Permian to Triassic.
