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The Mesozoic of the earth lasted approximately 185 million years. It is divided into three periods: Triassic, Jurassic, and Cretaceous. It is the era ofreptiles or dinosaurs.
The Triassic, the first period of the earth's Mesozoic, which started approximately 248 million years ago and ended 213 million years ago, received its name from a threesome of sediments found in central Europe: coloured sandstone, limestone, and upper trias.
The distribution of the continents was at that time similar to the situation that existed during the Permian. In view of the great mass of the supercontinent Pangea, which still existed, the climate on land was of continental, mostly arid nature.Ferns and horsetails, widely scattered during the Permian, were now concentrated mainly in the humid and coastal regions, while conifers began to spread. Different kinds of araucaria were among the plants that were able to adapt to drier conditions of the climate, since their long needles facilitated the condensation of the air humidity.
At the end of the Permian period, many animal species inhabiting the seas became extinct. One possible cause is that the sea salt was gradually binding with the enormous arid continent, exhausting the oceans. The animals that could not adapt to new conditions disappeared. Only some of the ammonites, nautiloids, and different kinds of fish and amphibians survived. On the other hand, clams, various kinds of crustaceans, and reptiles began to flourish.
First clams began to appear, while reptiles of the order pelycosauria, similar to mammals, vanished. Ophiacondon was one of these reptiles.
A variety of other reptiles, however, such as marine reptiles andtherapsids, flourished. The real dinosaurs appeared in the late Triassic period. One of them was the plateosaurus, a large herbivore. A great number of fossils of this dinosaur were found between 1919 to 1932 in Trossingen, Germany. It is also probable that small, rodent-size mammals already existed. Fossils attesting to their existence were found in China, South Africa, and Europe.
The Jurassic, the following period of the Mesozoic, takes its name from the Swiss mountains of Jura, where many fossil sediments were found.Cephalopods, ammonites, and belemnites are some of the most common findings.
During the Jurassic period, ammonites were again abundant and varied. Warm climate and the formation of large, shallow seas bathing the coastal areas supported their development. Many new forms evolved in crinoids. Part of these animals separated from their stalks and moved as beautiful, colourfulcrinoids in the water. Another part anchored their stalks in floating wooden debris, because the seafloor of the shallow seas, where there was only a slight current, was too muddy and there was not enough oxygen to support life.
Reptiles and dinosaurs, however, inhabited Jurassic seas as well. They were also able to remain part of the time on the coast. Crocodiles hunted in the shallow waters, but they were able to swim kilometres into the sea, far away from the coast. In contrast to today's crocodiles, their environment was mainly salt water.
During the Jurassic period, the gigantic continental mass fractured, forming smaller continents. The expansion of shallow seas brought rain to formerlyarid regions. This led to diversification of reptiles on land. It concerns especially sauropods, including dinosaurs, pterodactyls, therapsids, snakes, crocodiles, and birds.
Another branch of reptiles, the mammal-like reptiles (therapods), flourished already during the Permian and the Triassic periods. During the Jurassic period, only their more developed descendants –mammals – survived. They were small carnivores, reaching approximately the size of today's rat.
At the end of the Jurassic period, 145 million years ago, the northern and the southern continents started to separate. North America separated from Africa, but continued to be connected to northern Europe. India shifted towards the north. Shallow seas formed between Africa and the still contiguous masses of North America and Europe, as well as between North America and Asia, covering low-lying regions. Hydrophilous plants (Poleiskappen), which would bind quantities of water, were not yet in existence. Warm climate continued into the Cretaceous period. Due to heavy rains,ferns, conifers, and ginkgoe trees reached gigantic proportions. Palm-like ferns, cycads, appear for the first time. Some of them had flowers and fruits, and were somewhat similar to the angiosperms of today. These types, however, became extinct. Today's angiosperms originated during a later, parallel evolution.
These swampy forests provided excellent environment for large herbivores, such as brachiosaurus and brontosaurus. In addition to the herbivores, numerous large and small carnivorous reptiles were spreading on all continents. The largest ones were dilophosaurus and allosaurus. The predecessors of pterodactyls were probably also carnivores. They walked on their hind extremities and so the membranes between the talons of their front extremities were not occupied and could be used for flying.
The first known prehistoric bird, archeopteryx, dates approximately to the same period. Its fossils were found in a limestone plate near Solnhofen (southern Germany). It may well represent the link between reptiles and birds.
The last period of the earth's Mesozoic was named for the white, chalky limestone, characteristic of this period. The conception of "Cretaceous period" was coined in 1815 by the German geologist Karl von Raumer. Chalky cliffs dating to that period are still visible today on the English coast (Dover). The Cretaceous started 144 million years ago and ended 65 million years ago.
This was the time when dinosaurs reached their climax before disappearing. Their complete extinction is still a mystery. Probably it was not the result of a single event, such as afall of a meteorite or volcanic eruption, but more likely the result of the shifting of the continents and environmental changes that followed.
The extinction of ammonites (cephalopods), jaw-boned birds, and pterodactyls should be also considered in this context. Snails and sea urchins fared much better under the changed conditions, in fact, they became more diverse. As far as fish are concerned, in addition to cartilaginoidsharks and manta rays, only the more developed bony fish survived.
During the Cretaceous period, the already divided Pangea fractured into additional parts. Northern and southern regions drifted further away from each other and the Atlantic ocean widened. A major part of the continents was covered by shallow seas. North America was one of the most affected continents. First, the flooding waters proceeded from the south. The water formed a large gulf, which covered parts of today's Texas and Mexico. Water proceeded also from the north, so that only a narrow isthmus connected the two large parts that were not under water.
Later on, even this link disappeared and both seas became one. North America was divided into two parts. At that time, a great variety of marine reptiles inhabited the enormous ocean. This is documented by findings of numerous fossils. When the water finally receded, it left behind lakes and swamps that provided an appropriate environment for the last dinosaurs.
At the end of the Cretaceous period, or the beginning of the Tertiary era, the reptiles were represented only by crocodiles, snakes, therapsids, sphenodons, and turtles. In contrast, mammals were rapidly expanding. There were also more and more kinds of birds. Marsupials emerged in Asia.
Great changes took place on land when the waters receded. Folding occurred in both greatmountain ranges of America, the Rocky Mountains and the andes. We have to imagine that In Europe, which was divided into northern and southern parts, existed mountains such as the Alps of today .
Together with the tectonic changes and the changes in the distribution of the oceans, the climate also changed. While in the Jurassic period the climate was rather benign, a global cooling occurred in the Cretaceous period.
Now there were differentclimate zones. This resulted in the change of flora in the impacted regions. As the sea levels gradually dropped, the climate was getting drier. The previously abundant forests changed into savannahs supporting horsetails and ferns. Conifers and ginkgoes grew in the higher altitudes.
First plants, which had seeds enclosed in a carpel and to which belong the angiosperms of today, started to grow on the bare rocks. Smaller leafy trees made a gradual appearance, together with mixed coniferous forests. These greatly expanded in the following Tertiary era. Plant fossils, which were found in certain sedimentary layers, help scientists determine what type of climate prevailed in the various regions during the individual eras of the earth's history.
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