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E RECKON the history of Urantia as beginning about one billion years ago and extending through five major eras:

The prelife era extends over the initial four hundred and fifty million years, from about the time the planet attained its present size to the time of life establishment. Your students have designated this period as the Archeozoic. The life-dawn era extends over the next one hundred and fifty million years. This epoch intervenes between the preceding prelife or cataclysmic age and the following period of more highly developed marine life. This era is known to your researchers as the Proterozoic. The marine-life era covers the next two hundred and fifty million years and is best known to you as the Paleozoic. The early land-life era extends over the next one hundred million years and is known as the Mesozoic. The mammalian era occupies the last fifty million years. This recent-times era is known as the Cenozoic.

The marine-life era thus covers about one quarter of your planetary history. It may be subdivided into six long periods, each characterized by certain well-defined developments in both the geologic realms and the biologic domains.

As this era begins, the sea bottoms, the extensive continental shelves, and the numerous shallow near-shore basins are covered with prolific vegetation. The more simple and primitive forms of animal life have already developed from preceding vegetable organisms, and the early animal organisms have gradually made their way along the extensive coast lines of the various land masses until the many inland seas are teeming with primitive marine life. Since so few of these early organisms had shells, not many have been preserved as fossils. Nevertheless the stage is set for the opening chapters of that great "stone book" of the life-record preservation which was so methodically laid down during the succeeding ages.

The continent of North America is wonderfully rich in the fossil-bearing deposits of the entire marine-life era. The very first and oldest layers are separated from the later strata of the preceding period by extensive erosion deposits which clearly segregate these two stages of planetary development.