The beginning of life

The following era, the Mesozoic Era, is characterized by the extraordinary evolution of reptiles, the undisputed masters of this Era. Reptiles evolved from amphibians, with the “invention” of the egg, that made them independent from water also for reproduction. Some reptiles returned to the water and became good swimmers. Most reptiles were herbivorous, however many carnivorous species evolved. Some, like the feathered dinosaur of the Falcarius genus, which has been recently the subject of much attention, returned to a herbivorous style of life.
During the Mesozoic Era also some particular reptiles developed, perhaps they were hot-blooded, their size was small, they were ungainly and they swayed as they walked. From these, towards the end of the Jurassic Period, mammals evolved. Among the reptiles, those which stimulate our fantasy most are the dinosaurs, whose name means “terrible reptile”, even though only some of them were really terrible. In fact, they occupied all the ecological niches with species of all sizes and were mostly herbivorous. More precisely, up to the end of the Jurassic Period they were mostly small, and their gigantic size that fascinates us so, developed in the Cretaceous Period, not long before the mass extinction cancelled most of the species. The first flying reptiles appeared at the end of the Triassic Period, approximately 70 million years before real birds, and, however they were not their ancestors. Birds appeared approximately 140-150 million years ago, at the end of the Jurassic Period; they evolved from a class of reptiles known as Ornithischian dinosaurs (“ bird-hipped”, to which the Stegosaurus and Triceratops belong). The extremely famous Archaeopteryx, found in a cave in southern Germany, is one of the first representatives to be classified as a bird due to the extraordinary preservation, in a very fine grain limestone, of the traces of the extremely thin structures of the feathers. Researchers are still discussing the appearance of the attitude to fly and the role of feathers : recent discoveries , in fact, in China, in Utah and in Alaska, show the existence of many dinosaurs covered with feathers which were quite unsuited to fly, like the Beipiaosaurus, the Falcarius or the Troodon.
The feathers seem to have appeared much before the possibility of flying in the sky. Insects, that were less attractive than the reptiles, were killed by the dozen in the Paleozoic extinction. These diversified remarkably during the Mesozoic Era. The role of insects became very important at the end of the Mesozoic Era (in the Cretaceous Period, 100 million years ago), with the appearance of flowering plants (angiosperms) that further enriched the scenario of life, making the terrestrial environment increasingly similar to what is known to us. In the Cretaceous-Tertiary boundary, 65 million years ago, another great mass extinction took place, which drastically reduced the number of living species, as had already occurred at the end of the Paleozoic Era. These are the two best known extinctions, and the most drastic, but other episodes of this kind were repeated several times during the course of the history of the Earth. With regard to the possible causes of this extinction, many theories have been proposed, some are rather fanciful (like the one that states that the great quantity of excretion produced by the large herbivorous dinosaurs caused an increase in the concentration of methane in the atmosphere that poisoned most of the living creatures); among the more reliable theories, there is the hypothesis of a meteorite that fell in the Gulf of Mexico (the Chicxulub meteorite in Yucatan) whose dusts produced by the impact, caused the opacification of the atmosphere, with a consequent drop in the temperatures and decrease of photosynthesis, thus drastically decreasing the food available for the herbivorous animals and the death of most of the living creatures. The crater of the impact is no longer visible because it has been buried by tertiary sediments, but proof of the asteroid’s fall lies in the presence of anomalous quantities of iridium in the geological levels of this age worldwide. The following Era, the Cenozoic Era, was the era of mammals. After the disappearance of the most powerful antagonists the reptiles, mammals experienced an enormous diversification of species during the Cenozoic Era. In fact, during the Mesozoic Era, due to the competition with the stronger reptiles, mammals remained small and not very striking, but during the 10 million year period after the mass extinction, approximately 130 kinds of mammals appeared, more than the existing amount up to then!

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