Usually occurring in layers of sedimentary rock distributed around the world, a fossil is any geological imprint of a once-living life form and the study of fossils is known as paleontology.
Fossils can be found by deliberate searching but they are often discovered as a result of industrial mining, development, natural disasters or weathering. A specimen need not die to leave a fossil, but the best fossils occur when a specimen is buried alive or rapidly after death as in land-slides, tar-pits, volcanic catastrophe and river-bank sediments. This process can occur in land, water or even amber, a hardened form of tree sap, often resulting in unaltered preservation. Water-dwelling creatures comprise the most common fossils, and natural mineral growths are occasionally mistaken as fossilized organisms.
The most common types of fossils are impressions and casts. An impression is any imprint of any organism initially made in soft sediments; a cast occurs when an organism’s hard parts are preserved in rock and subsequently washed out, leaving a cavity that may later be filled with hard minerals.
Although the chemical process of fossilization is rather simple, the particular conditions necessary for this process to occur are accordingly rare, and any given specimen will only fossilize under scant, fixed circumstances. Scientists estimate that archaeopteryx once thrived in a population of 1 million; the number of known extant archaeopteryx fossils is under ten. Intact fossilized bones are rare; full skeletons much more so. If a specimen is not buried rapidly after death, it is usually destroyed by scavengers. Specimens lucky enough to remain intact a few years after death will leach carbon, calcium and other minerals over time, and eventually turn to dust. Arid environments will retard natural decomposition, while acidic environments typical of tropical regions will expedite the decay process.
Museum reconstructions of dinosaurs typically contain very little genuine fossil remains. In the cases where we are lucky enough to have a full skeletal form, fossils can show us the general shape of an organism and reveal details about its skeletal structure, however, it is important to note that in most cases of fossilization, records of any internal structure are seldom discernible.
Additionally, sediments containing fossil specimens must not shift; even light tremors and other tectonic activity can disturb the fossilization process. Natural forces beneath Earth’s surface often twist, bend or bury sedimentary layers, even pushing parts of the sea floor into mountain ranges in a process of recycling carbon dioxide. Fossils usually will not preserve in sediment subjected to extremes in heat or pressure, such as those required to convert sedimentary rock into metamorphic rock.
Provided a specimen is rapidly buried under a sufficient amount of sediment, the resulting pressure typically expels water from that sediment, which will then convert into rock. Water flow through a rock containing a specimen can cause its organic elements to leach out and the specimen will convert into soft minerals, which are gradually replaced by hard minerals such as pyrite, silica, quartz, aragonite, or calcite. This process of remineralization can also reverse itself under certain circumstances, and the road that a living, breathing organism must travel to become a fossil is full of challenges!