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.