Source: Produced for Teachers' Domain
Paleontologists seldom have the good fortune to find a complete set of remains of an ancient organism that is wholly intact. For instance, the discovery of a frozen woolly mammoth carcass, preserved hair and all, was a truly rare event. More common are discoveries of incomplete remains, such as bones, teeth, or hair, and trace fossils, such as footprints or leaf impressions, which indicate an organism once existed even though its actual remains have not been found. This still collage produced for Teachers' Domain reveals the variety of forms that fossils take, as well as examples of the kinds of life whose remains have been preserved.
Fossils form in different environments, including on land and in the sea, in one of three basic ways. First, an impression left in soft sediment, such as a footprint or worm trail, may be preserved if younger sediment fills it in and lithifies, or hardens to stone, under great compression. Second, water may infiltrate tiny air spaces in buried bone and shell and deposit minerals. Reinforced by such mineral deposits, bone and shell can survive for millions of years. Third, an organism may become trapped in an air-tight substance that preserves its entire body, including its DNA. Tree resin, ice, and tar have all successfully preserved numerous species of plants and animals.
When an organism dies, it may be eaten, be left to decompose, or be buried. Oxygen speeds the decomposition of an organism, so quick burial of the organism or its trace means the remains are more likely to be preserved. On land, sudden burial happens in mudslides, volcanic ash falls, and other flash events. Creatures that die in a marine environment and settle on the ocean floor may also be preserved because there is little oxygen in deep water. By contrast, the rainforest floor, where large amounts of precipitation speed the decay of dead organisms, fossil preservation is unlikely to occur.
Temperature and moisture are two other important environmental factors that influence the fossilization process. If bodies freeze or dry out, they have a greater chance of being preserved. Sometimes, frozen or desiccated bodies are discovered largely unchanged from the time of their death, thousands or even millions of years ago.
Because so many factors work against fossil preservation, the discovery of any kind of fossil is a rare occurrence. Even if an organism is buried, its remains could be destroyed by biological or physical factors. For example, bones -- in their original state or fossilized -- can be crushed under the weight of large amounts of overlying sediment or ground up in a landslide. And even if they survive all this, some remains may never be found. Their chance discovery happens only if the layer in which they are buried has been forced to the surface or excavated.
Academic standards correlations on Teachers' Domain use the Achievement Standards Network (ASN) database of state and national standards, provided to NSDL projects courtesy of JES & Co.
We assign reference terms to each statement within a standards document and to each media resource, and correlations are based upon matches of these terms for a given grade band. If a particular standards document of interest to you is not displayed yet, it most likely has not yet been processed by ASN or by Teachers' Domain. We will be adding social studies and arts correlations over the coming year, and also will be increasing the specificity of alignment.