Earth’s History and the Mesozoic Era Essay

Earth’s history began about 4 billion years ago. The atmosphere contained almost no oxygen, and would have been toxic to humans, and most other modern life. Since then Earth has been a part of very important events, such as the formation of the solar system, which formed from a large, rotating cloud of dust, and gas called a solar nebula. Earth has also had other significant events like the formation of the moon, the creation of oceans, the origin of life, and the first continents. It has also been through many eons, starting with the Hadean eon about 4. billion years ago, followed by the Achaean eon about 3. 8 billion years ago, and both are a part of the super eon Precambrian. All eons are organized on the Geologic Time Scale. After the Achaean eon ended the Proterozoic eon began about 2. 5 billion years ago. This eon included the oxygen revolution, which made a development in life. Life developed into eukaryotes, which were organisms whose cells contain complex structures enclosed within membranes. These cells have a nucleus with which genetic material is carried. During this eon there were severe ice ages. In the early Proterozoic eon the Earth cooled dramatically, and some scientists say that the Earth experienced ice ages so severe, that it was once completely frozen. This hypothesis is called Snowball Earth. When the Proterozoic eon ended, the Phanerozoic eon began around 542 million years ago. This eon included three eras. The first era was the Paleozoic era. During this time many modern groups of life came into existence, and life began to colonize the land, first plants, and then animals. This era saw the Cambrian explosion, which was a sudden emergence of new species, and forms during this time. This burst of animals forms led to most of the major animal groups we know today. The Cambrian explosion gave rise to the first animals with shells, and exoskeletons, like the Trilobites. The Phanerozoic eon continued with the second era, which was the Mesozoic era. This era is subdivided into three periods, and lasted from 251 million years ago to 65. 5 million years ago, beginning with recovery from a previous extinction during the Paleozoic era. This era has been called the â€Å"Age of Reptiles,† probably because of their dominance during this time. There were new diversifying plant groups including conifers, reproducing by means of seeds borne in cones. This era also saw many new predators in the seas, on air, and in the skies. The first period in the Mesozoic era was the Triassic period. This period began recovering from a mass extinction in the Paleozoic era, which had wiped out 95% of Earth’s species at the time, so life gradually developed in the beginning of this period. Still this period saw 450 new families of plants, and animals come into existence. The Triassic marked the beginning of many important advances in plant life. The conifers seemed to join the already flourishing ferns, cycads, and now extinct species like seed ferns. The plants of the Triassic had thick, waxy coverings, and trees did not usually grow as tall as modern trees. The Triassic saw that amphibians continued to be well represented and diverse. One common group of amphibians were the labyrinthodonts. These flat-headed creatures grew several feet in length, had sharp, conical teeth, with deeply folded enamel. They had small limbs, and very weak backbones. They spent their time in the backwaters of the Triassic rivers, and later became extinct while other amphibians like frogs were established. Towards the end of the Triassic is when reptiles began to rise. They were completely adapted to life on land, unlike the amphibians, so they occupied a variety of habitats. The dinosaurs appeared, descending from the thecondonts. The earliest thecondonts lived during the early Triassic, and were small, and medium sized predators. One group of thecondonts even took to the water, and became large fish eaters, like the phytosaurs, and crocodilians. At the end of the Triassic period, a meteorite slammed into the Earth. It is thought to have been responsible for the Manicouagan impact structure. The Mesozoic era continued setting the stage for warmer climates to follow. After the Triassic period came the second period in the Mesozoic era, which was the Jurassic period. This period began around 206 million years ago, and is known for the increase in number, and diversity of dinosaurs. This period is also when birds appeared in, and ruled the skies, descending from thecondonts. The vegetation on land mainly stayed the same with cycads, seed ferns, conifers, and other plants. The climate was warm, and humid. Reptiles had adapted to climates during this period, and were more diverse. Dinosaurs attained their longest life spans, and larger sizes during this period. Although at the beginning of this period dinosaurs were small, and lightly boned, feeding on insects and other small dinosaurs; but the dinosaurs weren’t the only reptiles. There were also reptiles that ruled the skies. These birds were warm-blooded to obtain the maximum of metabolic efficiency needed for controlled flight, but kept the reptilian mode of reproduction, by laying eggs. Some birds also adapted to life at sea, by flying underwater to catch fish. Predators also occupied the seas like the fast swimming ichthyosaurs, shaped like dolphins, with toothed, beaklike jaws. By the close of this period dinosaurs were massive predators, like the Allosaurs and Ceratosaurs. These dinosaurs had heavy bodies, powerful hind legs, front limbs used for grasping, and holding prey, and long sharp teeth for spearing, and stabbing. The Seismosaurus was one of the longest known dinosaurs, possibly reaching a length of more than 140 ft. The Jurassic period ended with a minor-mass extinction event, and many shallow water species died out. The cause of this is unknown. When the Jurassic period ended, the final period of the Mesozoic era began, which was the Cretaceous period. During this period coral reefs were the most widespread thing, ranging a thousand miles farther away from the equator, whereas today they are restricted to the tropics. Animal life continued to develop, as new species of dinosaur became important. The Tyrannosaurus Rex appeared in this period, and ruled during the end of the Cretaceous period. This dinosaur was a reptile-hipped dinosaur. It walked on two legs, which made it a therapod, and all therapods are carnivorous. Flowering plants developed also, and grew in this period. The ancestors of many modern tree species appeared in this period, like the magnolia, and maple. Flowers brought many insects. The flowering plants provided food for the insects, while the insects made sure to spread pollen from flower to flower. This kept the flowers producing seeds to keep reproducing. At the end of the Cretaceous period, there seems to have been a major event that caused mass extinction. A very popular theory is that one or more large asteroids or comets struck the Earth. Dinosaurs and over 70% of other species went extinct. In fact all land animals over 50 pounds seemed to have gone extinct. This event ended the Cretaceous period, and completely ended the Mesozoic era. The Mesozoic era was definitely a time of dominance for reptiles, as they could adapt to many habitats during this time. Flowering plants developed, and many other plants came into existence. Overall this era was a time of rebirth, as it began after a mass extinction, and saw many new plants, and animals in a variety of habitats. The Mesozoic era will always remain an important part in Earth’s history.