Precambian, Paleozoic, Mesozoic, and Cenozoic Eras Help (page 2)

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By — McGraw-Hill Professional
Updated on Aug 30, 2011

Paleozoic Era

Roughly 300 million years ago, the oxygen in the atmosphere arrived at its present level. With its buddy the ozone layer to protect life from harmful ultraviolet radiation, the atmosphere of the planet allowed the development of life on land. This era was most favorable to the development and growth of invertebrates (spineless creatures like shrimp and jellyfish), fish, and reptiles. This generally tropical climate was divided by wide temperature swings of different ice ages. By the end of the era, the continents were pushed up into the giant continent of Pangea.

As the landmass got drier, the humid swamps receded along with their unique plants and animals. This change caused the largest sweeping extinction of any of the eras. More life forms were lost than at any other time.

Mesozoic Era

This era can best be remembered as the era of the dinosaurs. Lasting only about half the time as the Paleozoic era, the Mesozoic era was a happening time. A time when plants, fish, shellfish, and especially reptiles were “super-sized,” it was like everything on the Earth was on mega vitamins. Dinosaurs stomped around gigantic ferns and huge trees, while Pterosaurs (flying reptiles) cruised the skies. The climate was warm everywhere.

Although geologists can only guess at the forces that caused the break up of the supercontinent, Pangea, into Laurasia and Gondwana during this time, Antarctic samples hint at global “hot spots” of magma that built up causing cracks. Local dinosaurs and plants were separated for millions of years and became more specific depending on their own areas and the food and temperatures locally. Even small mammals began to pop up underfoot as chance appetizers for the meat-eating dinosaurs like Tyrannosaurus Rex .

During the Mesozoic era, more recent forms of insects, coral, ocean life, and flowering plants developed. All was really going great until suddenly, the dinosaurs and many other animals died off. Many scientists think this was caused by the impact of a large asteroid and the following years of global smoke, volcano eruptions, and generally nasty weather. The Sun couldn’t shine through the ash and smoke, the water was contaminated, and the Earth was definitely not a great vacation spot.

Cenozoic Era

With the threat of the big dinosaurs gone, mammals thrived during the Cenozoic era. Early mammals lived pretty peacefully with birds, regular reptiles, and invertebrates. The climate became cooler and drier as the continents drifted apart and took on roughly their current positions. Some scientists suggest the Himalayas were pushed up during this time.

Perennial grasses grew and allowed the herds of grazing animals to flourish along with now-extinct side lines of the evolutionary tree. Temperatures continued to dive with the Antarctic landmass being formed. The appearance of the homo sapiens line of mammals appeared during the last few minutes of this era (geologically speaking) along with the use of primitive tools, discovery of fire and the wheel, along with extinction of more ancient species.

Look again at Fig. 2-4 for the portion of time in which our modern world has existed. If you were to draw the history of the Earth along a horizontal line, the few thousand years of known human existence would be little more than a sliver. Our modern, technological society of today would be less than a pin point.

Geological Time Precambrian


Fig. 2-4. Compared to ancient eras, our modern time period has been very small.

Practice problems of this concept can be found at: Geological Time Practice Test

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