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225 Dinosaurs and Geologic Time James I. Kirkland and James O. Farlow 12 The histories of paleontology and geology are intertwined because developments in both sciences have been dependent on each other since their beginnings (Albritton 1986; Berry 1987). Geology gives paleontology context, and therefore geological discoveries have been critical in developing our understanding of dinosaurs. Nowhere is this more evident than in understanding the distribution of dinosaurs through geologic time. In 1699, Danish geologist Nicholaus Steno established some basic principles by which the relative timing of geological events could be determined . These are (1) the principle of superposition, in which each rock layer was laid down one upon another, (2) the principle of original horizontality, in which rock layers were originally laid down horizontally and were inclined only if secondarily tilted, and (3) the principle of original continuity, in which rock layers were assumed to be continuous over great distances. Together with Scottish geologist Charles Lyell’s principle of crosscutting relationships, where any feature cutting across another must have formed after the layer it crosses, these four principles gave geology its initial foundation as a science (Dott and Prothero 2003; Stanley 2009). Archbishop Usher’s 1654 pronouncement that the genealogy recorded within the Bible indicated that the Earth was created at 9:00 am on October 26, 4004 bc, reflected a literal view of Genesis. This restricted, 6,000-year chronology strengthened a catastrophic view of Earth history to account for the development of Earth’s geological features in such a short amount of time. This view of the Earth laid the grounds for developing the overall framework for geologic time, where divisions were made at distinct breaks in the geologic and fossil records. Thus, the geologic time intervals we continue to use today are variable in their duration and are based on abrupt changes in the European rock record caused by gaps in the rock record and, in a few cases, mass extinctions. In 1795, an alternative view of geologic time was proposed by Scottish geologist James Hutton, who recognized that the processes shaping the Earth today are the same processes responsible for forming older geological features on the Earth. This principle of uniformitarianism led Hutton to propose, “No vestige of a beginning, no prospect of an end.” From this conceptual framework developed by Steno, Lyell, and Hutton, the subdivisions of a relative geologic timescale were gradually established over the next 50 years. Geologists divide Earth history into extremely long time intervals known as eons, and eons are in turn subdivided into eras, eras into periods, periods into epochs, and epochs into ages. Chronostratigraphic rock units are the rocks deposited during these intervals of geologic time. Thus, rocks The Relative Framework of Geologic Time Kirkland and Farlow 226 deposited during an eon are referred to as an eonothem, those during an era are an erathem, those during a period are a system, those during an epoch are a series, and those during an age are a stage. The names used for both geologic time units and chronostratigraphic rock units are the same and are derived from names used in developing a relative geologic timescale in Europe during the eighteenth and early part of the nineteenth centuries. The most recent geologic timescale (Gradstein et al. 2004) divides the Earth’s history into four eons (Fig. 12.1). Earth history prior to the oldest rocks preserved at the Earth’s surface is referred to the Hadean. The Hadean can be visualized by gazing up at the moon, which still preserves the scars of meteor impacts that occurred during part of the Hadean Eon. The oldest rocks on earth date back to about 4 billion (4,000 million) years and mark the beginning of the Archean Eon. The earliest fossils, representing bacteria or similar organisms, are found in rocks of just slightly younger age. The Archean Eon ended 2.5 billion (2,500 million) years ago, followed by the Proterozoic Eon. The Proterozoic Eon ended at about 542 million years ago. During this long interval of time, living things became gradually more diverse and complex, with the first true animals appearing toward the end of the Proterozoic Eon. The first appearance of animals with skeletal hard parts marks the beginning of the Phanerozoic Eon (time of evident life), which began at the close of the Proterozoic and continues to the present. Eons prior to the Phanerozoic are commonly lumped together as Precambrian as in the time before the Cambrian Period at...

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