Final answer:
The pathways by which a shell becomes a fossil, or not, are referred to as the process of fossilization, which involves alteration, replacement, and the preservation of environmental conditions that allow for the fossil to form.
Step-by-step explanation:
The term used for the pathways by which a shell either becomes a fossil or not is known as the process of fossilization. In this process, the original shell may undergo several changes. It can be buried and dissolved over time, after which the cavity left behind may be filled with sediment to form a cast fossil. Through this process, a shell can become an external mold, or if filled, a cast of the original organism's shape. Fossilization can include the alteration and replacement of the original shell material with minerals like silica or pyrite.
Unaltered fossils contain original minerals that were part of the organism, such as apatite, calcite, aragonite, and opal. Replacement occurs when the original skeletal material is substituted by secondary minerals, allowing for the preservation of some of the organism's details despite the compositional change in the fossil.
In paleontology, these fossils are invaluable for reconstructing the paleoenvironment and understanding the conditions in which the organism lived. The conditions that affect fossilization include temperature, water depth, salinity, pH, and oxygen levels. Trace fossils, like tracks, burrows, and coprolites, provide evidence of the organism's interaction with its environment. These are part of the larger fossil record that helps to document the history of life on Earth.