The Handy Dinosaur Answer Book. Patricia Barnes-Svarney

(iStock).

      Most fossils are found in sedimentary rocks—those rocks produced by the accumulation of sediment such as sand or mud. Wind and other weathering conditions wash away sediment on land, depositing it in bodies of water. For this reason, fossils of sea creatures are more common than those of land creatures. Land animals and plants that have been preserved are found mostly in sediment of calm lakes, rivers, and estuaries.

      A fossil may also consist of unaltered original material. Bones and teeth are commonly preserved in this way. However, far more often the pores of bone and teeth are filled with minerals in a process called permineralization (what many have called petrifying). Circulating ground water carries silica or calcium carbonate (and sometimes other minerals, such as pyrite) that fill the pores. What remains is, in essence, a duplicate of the original bone or other organic material.

      How likely is it that an organism becomes a fossil?

      Not all organisms survive to become fossils, and the chance of a living organism becoming a fossil is generally very low. Many organisms completely decay away or are chewed apart by other animals. Because of this, some scientists estimate that although billions of flora and fauna have lived on Earth, very few survived into fossil form. The fossils we do find represent only a fraction of the animals and plants that ever lived.

      An organism has the best chance to become a fossil if it is quickly covered by moist sediment after death, protecting the decaying organisms from predators, scavengers, and bacteria. The soft parts of the organisms (such as skin, membranes, tissues, and organs) quickly decay, leaving behind bones and teeth. The majority of found fossils date back no farther than almost 500 million years ago, when organisms first began to develop skeletons and other hard parts.

      The following are the steps to fossilization, using a dinosaur as an example. This outline shows how difficult it is for a dinosaur to become a fossil:

      Scavenging and decay—When a dinosaur died, it did not take long for scavengers to remove the soft flesh parts of its body. Those parts that were not eaten decayed at a fast or slow rate, depending on the prevailing climate. In any case, within a short amount of time only a skeleton would remain. But even the remaining hard body parts were not impervious to change. They were often weathered by the action of wind, water, sunlight, and chemicals in the surroundings, rounding the bones or reducing them to small pieces.

      Location—If the dinosaur’s skeleton was in an area in which rapid burial did not take place, then the chances of fossilization were slim. The bones would break and scatter, often moved by the action of changing river courses or flash floods. But occasionally, this transport increased the chance of fossilization, moving the bones to a better area for preservation, such as a sandbank in a river.

      Burial—The most crucial step in becoming a fossil is burial. The sooner the burial of the dinosaur bones, the better the chance of a good fossil being created. If the bones were covered by mud or sand, whether before or after transport, then the amount of further damage would have lessened; in addition, the exposure to oxygen was less, thus reducing additional decay of the dinosaur bones. Some damage might still have occurred, however, primarily from the pressure created by the increasing amount of sediment on top of the bones, or even from acidic chemicals that dissolved into the sediment.

      Fossilization—The fourth step is the actual process of fossilization itself. Here, the sediments surrounding the fossil slowly turn to stone by the action of pressure of the overlying sediment layers and loss of water. Eventually, the grains become cemented together into the hard structure we call rock. The dinosaur bones fossilized, as the spaces in the bone structures fill with minerals, such as calcite (calcium carbonate), or other iron-containing minerals; or the actual mineral component of the bone itself, apatite (calcium phosphate), may have recrystallized.

      Exposure—Lastly, deeply buried dinosaur bones must be exposed on the surface where they can be discovered. This involves the uplift of the bone-containing sedimentary rock to the surface, where erosion by wind and water expose the fossilized skeleton. If the bones are not found in time, the action of the wind and water can destroy the precious record of the ancient species.

       Why are there gaps in the fossil records?

      Gaps in the fossil records—eras or evolutionary stages that are “missing” from the known collection of fossils—are most often the result of erosion. This geologic process erodes away layers of rock and embedded fossils, usually by the action of wind, water, and ice. Gaps in fossil records can also be caused by mountain uplift, which destroys fossils, and volcanic activity, which can bury fossil evidence with hot magma rock that physically changes the rock, and thus fossils.

      How do scientists determine the age of fossils?

      A number of methods are used today to date fossils. Most of the methods are indirect—meaning that the age of the soil or rock in which the fossils are found are dated, not the fossils themselves. The most common way to ascertain the age of a fossil is by determining where it is found in rock layers. In many cases, the age of the rock can be determined by other fossils within that rock. If this is not possible, certain analytical techniques are often used to determine the date of the rock layer.

      One of the basic ways to determine the age of rock is through the use of radioactivity. For example, radioactivity within Earth continuously bombards the atoms in minerals, exciting electrons that become trapped in the crystals’ structures. Using this knowledge, scientists use certain radiometric techniques to determine the age of the minerals, including electron spin resonance and thermoluminescence. By determining the number of excited electrons present in the minerals—and comparing it to known data that represents the actual rate of increase of similar excited electrons—the time it took for the amount of excited electrons to accumulate can be calculated. In turn, this data can be used to determine the age of the rock and the fossils within the rock.

      There are other methods for determining fossil age. For example, uranium-series dating measures the amount of thorium-230 present in limestone deposits. Limestone deposits form with uranium present and almost no thorium. Because scientists know the decay rate of uranium into thorium-230, the age of the limestone rocks, and the fossils found in them, can be calculated from the amount of thorium-230 found within a particular limestone rock.

      What are molds and casts?

      Molds and casts are types of fossils. After burial, a plant or animal often decays, leaving only an impression of its hard parts (and less often, soft parts) as a hollow mold in the rock. If the mold is filled with sediment, it can often harden, forming a corresponding cast.

      Fossils are not always bones. These fish fossils are not actually bones, but rather imprints the fish made in the soil. (iStock).

      What are trace fossils?

      Not all fossils are hardened bones and teeth, or molds and casts. There are also fossils that are merely evidence that creatures once crawled, walked, hopped, burrowed, or ran across the land. Trace fossils are just that: the traces of a creature left behind, usually in soft sediment like sand or mud. For example, small animals bored branching tunnels in the mud of a lake bed in search of food; and dinosaurs hunted for meals along a river bank, leaving their footprints in the soft sand. Similar to the fossil formation of hard parts, the footprints and tunnels were filled in by sediment, then buried by layers of more sediment over millions of years, eventually solidifying. Today we see the results of this long-ago activity as trace fossils. Many originators of trace fossils are unidentifiable—in other words, there are no hard fossils of the creatures left in the area, just their tracks. Some of the most famous trace fossils are those of dinosaurs tracks (for example, in Culpepper, Virginia, and near Golden, Colorado), and human-like footprints (for example, in east Africa), which were all found in hardened sediment.

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