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The relative time scale comprises four major intervals, commonly known as eras. The oldest, or Precambrian, includes ancient rocks whose only life forms are micro-organisms and the layered mounds of stromatolites built by blue-green algae. The second oldest era, the Paleozoic, which was dominated by marine invertebrate life, although arthropods, mollusks, vertebrates, and plants that invaded land later in the era began to expand rapidly.
Relative Time Scale
After the first great extinction, in which many forms of marine life had perished, the Mesozoic Era began with a new radiation of marine life and the dominance of reptiles on land. This era also closed with a number of extinction, in which the reptiles, such as the dinosaurs, and some marine groups disappeared.
Finally, the Cenozoic, or Present, Era, is characterized by the presence of mammals, insects, and flowering plants on land, and still another breed of marine life.
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