Geological Time

The time line that describes the history of the earth has been divided into large blocks of time, but each large block is normally subdivided, and subdivided again, for convenience (Fig. 16). The generally accepted divisions are eon, era, period, epoch, and age. The names given the block of time often have historical significance, and may be associated with occurrence of different fossils. For example, the Phanerozoic eon also consists of three major divisions: the Cenozoic, the Mesozoic, and the Paleozoic eras. The “zoic” part of the word comes from the root “zoo,” meaning animal. “Cen” means recent, “Meso” means middle, and “Paleo” means ancient. These divisions reflect major changes in the composition of ancient faunas, with each era associated with domination by a particular group of animals. The Cenozoic has sometimes been called the “Age of Mammals,” the Mesozoic, the “Age of Dinosaurs,” and the Paleozoic the “Age of Fishes.” This is not entirely accurate, though there is some...