The following statements summarize and describe many of the key terms and concepts presented
in the chapter.
- The Precambrian spans about 88 percent of Earth history, beginning with the formation of Earth about 4.5 billion years ago and ending 540 million years ago with the diversification of life that marks the start of the Paleozoic era. It is the least understood span of Earth's history because most Precambrian rocks are buried from view. However, on each continent there is a "core area" of Precambrian rocks called the shield. The iron-ore deposits of Precambrian age represent the time when oxygen became abundant in the atmosphere and combined with iron to form iron oxide.
- Earth's primitive atmosphere consisted of such gases as water vapor, carbon dioxide, nitrogen, and several trace gases that were released in volcanic emissions, a process called outgassing. The first life forms on Earth, probably anaerobic bacteria, did not require oxygen. As life evolved, plants, through the process of photosynthesis, used carbon dioxide and water and released oxygen into the atmosphere. Once the available iron on Earth was oxidized (combined with oxygen), substantial quantities of oxygen accumulated in the atmosphere. About 4 billion years into Earth's existence, the fossil record reveals abundant ocean-dwelling organisms that require oxygen to live.
- The most common middle Precambrian fossils are stromatolites. Microfossils of bacteria and blue-green algae, both primitive prokaryotes whose cells lack organized nuclei, have been found in chert, a hard, dense, chemical sedimentary rock in southern Africa (3.1 billion years of age) and near Lake Superior (1.7 billion years of age). Eukaryotes, with cells containing organized nuclei, are among billion-year-old fossils discovered in Australia. Plant fossils date from the middle Precambrian, but animal fossils came a bit later, in the late Precambrian. Many of these fossils are trace fossils, and not of the animals themselves.
- The Paleozoic era extends from 540 million years ago to about 248 million years ago. The beginning of the Paleozoic is marked by the appearance of the first life forms with hard parts, such as shells. Therefore, abundant Paleozoic fossils occur, and a far more detailed record of Paleozoic events can be constructed. During the early Paleozoic (the Cambrian, Ordovician, and Silurian periods) the vast southern continent of Gondwana existed. Seas inundated and receded from North America several times, leaving thick evaporite beds of rock salt and gypsum. Life in the early Paleozoic was restricted to the seas and consisted of several invertebrate groups. During the late Paleozoic (the Devonian, Mississippian, Pennsylvanian, and Permian periods), ancestral North America collided with Africa to produce the original northern Appalachian Mountains, and the northern continent of Laurasia formed. By the close of the Paleozoic, all the continents had fused into the supercontinent of Pangaea. During most of the Paleozoic, organisms diversified dramatically. Insects and plants moved onto the land, and amphibians evolved and diversified quickly. By the Pennsylvanian period, large tropical swamps, which became the major coal deposits of today, extended across North America, Europe, and Siberia. At the close of the Paleozoic, altered climatic conditions caused one of the most dramatic biological declines in all of Earth history.
- The Mesozoic era, often called the "age of dinosaurs," began about 248 million years ago and ended approximately 65 million years ago. Early in the Mesozoic much of the land was above sea level. However, by the middle Mesozoic, seas invaded western North America. As Pangaea began to break up, the westward-moving North American plate began to override the Pacific plate, causing crustal deformation along the entire western margin of the continent. Organisms that had survived extinction at the end of the Paleozoic began to diversify in spectacular ways. Gymnosperms (cycads, conifers, and ginkgoes) quickly became the dominant trees of the Mesozoic because they could adapt to the drier climates. Reptiles quickly became the dominant land animals, with one group eventually becoming the birds. The most awesome of the Mesozoic reptiles were the dinosaurs. At the close of the Mesozoic, many reptile groups, including the dinosaurs, became extinct.
- The Cenozoic era, or "era of recent life," began approximately 65 million years ago and continues today. It is the time of mammals, including humans. The widespread, less disturbed rock formations of the Cenozoic provide a rich geologic record. Most of North America was above sea level throughout the Cenozoic. Because of their different relations with tectonic plate boundaries, the eastern and western margins of the North American continent experienced contrasting events. The stable eastern margin was the site of abundant sedimentation as isostatic adjustment raised the eroded Appalachians, causing the streams to downcut with renewed vigor and to deposit their sediment along the continental margin. In the west, building of the Rocky Mountains was coming to an end, the Basin and Range Province was forming, and volcanic activity was extensive. The Cenozoic is often called "the age of mammals" because these animals replaced the reptiles as the dominant land life. Two groups of mammals, the marsupials and the placentals, evolved and expanded to dominate the era. One tendency was for some mammal groups to become very large. However, a wave of late Pleistocene extinctions rapidly eliminated these animals from the landscape. Some scientists believe that humans hastened the decline of these animals by selectively hunting the larger species. The Cenozoic could also be called the "age of flowering plants." As a source of food, flowering plants strongly influenced the evolution of both birds and herbivorous (plant-eating) mammals throughout the Cenozoic era.