Chapter 33: Fossils: The Record of Life

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are the remains or indication of life of previous geologic periods. Examples: track of a bird preserved in stone; clam shell in a layer of sedimentary rock high in the mountains; skeleton of a creature no longer found on earth. Are fossils rare? Some kinds of fossils are rare and this may give us a misconception about the fossil record. Something like Australopithecus afarensis or Homo erectus are rare, human-like fossils that have been dated to over 3 million years ago (afarensis) and 1.6 million years ago (erectus). But, many kinds of fossils are far from rare. Coal beds are fossil plants. The White Cliffs of Dover are microscopic fossil animals. People have found fossils of fish, sea shells, dinosaurs, giant ground sloths, etc. within short driving distances of Provo. In order for an animal or plant to be preserved as a fossil, two conditions are usually (not always) required: The organism must have (two words?) (bones, shell, or teeth) and, The remains must have a (slow, rapid?) burial to protect them from (element?) to prevent rapid and complete decomposition. Thus, some life forms are common in the fossil record: shelled animals living in shallow seas and fish; while others are uncommon: birds, buffalo, bacteria and worms. Fossils have the following uses: 1) they tell us the nature of past life, 2) they help us to establish ancient , and 3) they help us to establish and correlate (relative, absolute?) time (Principle of Succession.) Geologists also put fossils in their gas tanks to make their trucks go. Fossils tell us the nature of past life: In the oldest sedimentary rock layers (about 3.7 bya) there is no evidence for life. Evidence indicates that there was little or no (element?) in the atmosphere. The first evidence of life in the rocks is microscopic, about 3.5 (kya, mya, bya?) . Life forms become (more, less?) complex as one works upward through the layers toward the present. Fossils help us establish ancient environments: Fossils of life belonging to the sea are common in the mountains surrounding Provo and in southern Utah (shark teeth, turtle shell fragments, clam shells, brachiopods, fish scales, oyster shells). These fossils tell us that when these animals lived here, this area was not mountainous as it is today, but rather was a sea. In a similar way, the kinds of fossils we find in an area tell us what the environment was when the animals or plants actually lived there (sea, swamp, grassland, ocean, prairie, etc.). Some fossils help us keep track of the flow of time: fossils are useful markers of time. Some fossils, once they appear in the record of the rocks, remain in the record unchanged for long periods of time. Some fossils appear only fragmentarily. Index fossils, on the other hand, are useful for keeping track of time because: they are (plentiful, rare?) , with no gaps in the record, and, they (do, do not?) change (in some systematic way) in the fossil record as a function of time. Because the index fossils are plentiful, they are common and relatively easy to find. Because they change in a systematic way with time, one can inspect the fossils to tell where it fits into the Geologic Column and therefore how old the rocks are in which the fossil was found. Examples of index fossils are graptolites, ammonoid cephalopods, and conodonts. Fossils are preserved in a variety of ways. Sometimes the specimen is completely preserved, usually by freezing, such as mammoths that are still sometimes found in Siberia. Sometimes just the hard parts (teeth, bones, etc.) are preserved, but with their original composition unchanged. More often, the original matter is replaced by some new mineral material such as in petrified wood. Sometimes ground water will leach out the remains of an animal or plant, leaving a cavity in the rock that preserves the form of the organism (a "mold"). Sometimes these molds fill with other sediment to become a "cast." Sometimes delicate organisms, which would ordinarily not be preserved by other methods leave a thin residue of carbon on a rock that remains after the other elements have been driven off into the surrounding sediment. Sometimes, all that is left is a track or a filled in burrow of a worm in a former seabed, called a "trace fossil."

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