Earth Science
Earth Time - The Mesozoic and Cenozoic Eras

Earth Time - The Mesozoic and Cenozoic Eras
Reading Level
     edHelper's suggested reading level:   grades 9 to 10
     Flesch-Kincaid grade level:   9.36

     challenging words:    anklylosaurs, iridium, life-forms, now-extinct, ornithischians, rodent-like, saurichian, saurischians, bird-hipped, ceratopsians, life-form, following, sapiens, paleontologists, skin-covered, subcontinent
     content words:    Paleozoic Era, Permian Period, Mesozoic Era, South America, Sierra Nevada, North America, Triassic Period, Jurassic Period, Cretaceous Period, Cenozoic Era

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Earth Time - The Mesozoic and Cenozoic Eras
By Trista L. Pollard

1     If you traveled back in time to the end of the Paleozoic Era, you might be a little disappointed. You would have found very few life-forms on the planet. Mass extinction occurred at the end of the Permian Period. As a result, about 90 percent of the marine animals and about 78 percent of the land animals became extinct. For the few species that survived, such as amphibians and reptiles, there were a lot of resources and space. Time for new organisms to appear!
2     As the Mesozoic Era began, the life-forms that survived needed to evolve and adapt to the new environment. This era, which began 251 million years ago and ended 65 million years ago, started with Pangaea breaking into smaller continents. The movement of the Earth's tectonic plates caused the continents to drift and collide. Mountain ranges like the Andes in South America and the Sierra Nevada in North America were uplifted. Our climate was mostly warm and humid. There were also shallow seas and marshes. Perfect conditions for the survival of reptiles!
3     The Mesozoic Era saw an increase in the populations of lizards, turtles, crocodiles, snakes, and dinosaurs. That's right; our planet had already existed for about 4.3 billion years before the first dinosaurs made their debut. This is why the Mesozoic Era is known as the "Age of Reptiles." Due to the abundance of fossils from this era, scientists have divided the era into three periods. The Triassic Period is when the dinosaurs first appeared. Imagine seeing dinosaurs as small as squirrels running around with others that weighed almost 15 tons and measured about 30 meters long. The average dinosaurs during the period, however, were about 4 meters to 5 meters long. These dinosaurs were also very quick in speed. The forests where they roamed were filled with cone-bearing trees and cycads, whose modern relatives are palm trees. Some of the first mammals, small rodent-like creatures, also roamed the forest.
4     One type of reptile called an ichthyosaur lived in the ocean. There were also ammonites which resembled shellfish and existed in large numbers. Scientists use them as index fossils for the Mesozoic Era. As the Jurassic Period began, dinosaur populations started to increase. They became the dominant life-form during this period. According to the fossil record, two major groups of dinosaurs existed. The saurischians or "lizard-hipped" dinosaurs contained both herbivores and carnivores or meat eaters. One type of saurichian, an herbivore called the Brontosaurus (now called the genus Apatosaurus), weighed almost 50 tons and measured up to 25 meters long.

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