The Handy Dinosaur Answer Book. Patricia Barnes-Svarney

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СКАЧАТЬ is that as more animals evolved, there would have been a higher demand for a better food source. By adapting to land life—and the “new” food sources on land—these organisms would have a better chance of survival.

      When did the first primitive dinosaurs appear?

      The first primitive dinosaurs appeared about 230 million years ago. They were much smaller, and less fierce, than the Tyrannosaurus rex we often think of when someone mentions the word “dinosaur.”

      How long did it take for dinosaurs to evolve from the first land animals?

      The first larger land animals that would eventually lead to the appearance of dinosaurs evolved around 440 million years ago. Dinosaurs then evolved around 250 million years ago. Thus, it took about 190 million years for dinosaurs to appear after the first land animals. Remember, these numbers are based on the currently known fossil record, and could change if new fossils are found.

GEOLOGIC TIME

      What is geologic time?

      Geologic time is the immense span of time that has elapsed since Earth first formed—almost 4.5 billion years ago—to recent times.

      What is the geologic time scale?

      The geologic time scale is a way of putting Earth’s vast history into an orderly fashion, giving a better perspective of events. At the turn of the nineteenth century, William Smith (1769–1839), an English canal engineer, observed that certain types of rocks, along with certain groups of fossils, always occurred in a predictable order in relation to each other. In 1815, he published a map of England and Wales geology, establishing a practical system of stratigraphy, or the study of geologic history layer-by-layer. Simply put, Smith proposed that the lowest rocks in a cliff or quarry are the oldest, while the highest are the youngest.

      By observing fossils and rock type in the various layers, it was possible to correlate the rocks at one location with those at other locations. Smith’s work, combined with the first discoveries of dinosaur fossils in the early 1800s, led to a framework that scientists still use today to divide Earth’s long history into the geologic time scale, with its various, arbitrary divisions of time including eras, periods, and epochs. Established between 1820 and 1870, the time divisions are a relative means of dating; that is, rocks and fossils are dated relative to each other as to which are older and younger. It was not until radiometric dating was invented in the 1920s that absolute dates were applied to rocks and fossils—and to the geologic time scale.

      What are the divisions of the geologic time scale?

      The geologic time scale divisions have changed significantly over time, mainly because of new fossil discoveries and better radiometric dating techniques—and it will no doubt continue to change. The following table is a general listing of the geologic time table based on current interpretations of rocks and fossils.

      How are the divisions on the geologic time scale named?

      Most of the major divisions on the geologic time scale are based on Latin names, or areas in which the rocks were first found. For example, the Carboniferous period gets its name from the Latin words for “carbon-bearing,” in reference to the coal-rich rocks found in England; the Jurassic period is named after the Jura Mountains along the border of France and Switzerland. The names of the stages or ages most often depend on city and regions where the rocks were found; this is why division names frequently vary on geologic time scale charts from different countries.

      What are the major time units used in the geologic time scale?

      There are five major time units on the geologic time scale. The units are—in order of descending size—eons, eras, periods, epochs, and stages (although some list this division as ages and subages). The eon represents the longest geologic unit on the scale; an era is a division of time smaller than the eon, and is normally subdivided into two or more periods. An epoch is a subdivision of a period; a stage is a subdivision of an epoch.

      What do the divisions on the geologic time scale represent?

      The geologic time scale is not an arbitrary listing of Earth’s natural history, nor are the divisions merely fanciful. Each boundary between divisions represents a change or an event that delineates it from the other divisions. In most cases, a boundary is drawn to represent a time when a major catastrophe or evolutionary change in animals or plants (including the evolution of specific species) occurred.

      Natural erosion clearly reveals the layers of Earth’s crust, such as seen here in Badlands National Park in South Dakota. Observing these layers is like taking a trip back in time, with each lower level representing a different time period in the planet’s history (iStock).

      What is relative time in relationship to geologic time?

      Relative time is a way to establish the relative age of rocks and fossils. It is based on the location of a rock layer in comparison to the location of other rock layers; that is, it is only relative, not absolute, time. In many cases, rock layers are laid down in order, the older layers being below the younger layers. For example, a fossil found in a higher rock layer is usually younger than a fossil found in a rock layer below it. During the nineteenth century, scientists used this method to date rock layers relative to each other and to establish and construct the first geologic time scale.

      What is absolute time in relationship to geologic time?

      Absolute geologic time is the (approximate) true age of the rock; that is, the absolute time that the rock layer formed. Typically, radiometric techniques, which measure the amount of radioactive decay in rocks, are used to determine absolute time.

      When were radiometric dating techniques discovered?

      The basic principles and techniques of radiometric dating were not discovered until the turn of the twentieth century. In 1896, French physicist Antoine-Henri Becquerel (1852–1908) accidentally discovered radioactivity when a photographic plate left next to some uranium-containing mineral salts blackened, proving that uranium gave off its own energy. In 1902, British physicist Lord Ernest Rutherford (1871–1937) collaborated with British chemist Frederic Soddy (1877–1966) to discover that the atoms of radioactive elements are unstable, giving off particles and decaying to more stable forms. These findings led United States chemist Bertram Borden Boltwood (1870–1927) to argue that, by knowing the decay rate of uranium and thorium into lead, the dating of rock would be possible. In 1905, Boltwood and John William Strutt dated various rocks, obtaining ages of 400 to 2,000 million years for various rock samples and proving such dating could be done.

       Why do some dates differ on the various geologic time scale charts?

      Determining the true age divisions of the past СКАЧАТЬ