Fossils generally form either as mould fossils or as cast fossils and are either considered a trace fossil or a body fossil. An imprint of a footprint in rock is an example of a mould fossil and a trace fossil, while a mineral deposit in the shape of a shell is an example of a cast fossil and a body fossil. In rare cases, organisms, or parts of organisms, are entirely preserved. Fossils help scientists to understand the behaviour, movement, diet, habitat, and anatomy of prehistoric organisms.
Mold fossils come from a process called authigenic preservation; a process which leaves a negative impression, or indent, of an organism in rock after the organism itself has deteriorated. Sand or mud covers the deceased organism and, over time, that sand or mud hardens into rock, encasing the organism. The organism continues to decay, eventually leaving only an imprint. Entire organisms, partial organisms, or even traces of the passing of organisms can leave mould fossils.
Cast fossils occur when mould fossils are filled in with minerals that harden over time, creating a fossilised replica of the original organism. Water seeps through the rock surrounding the mould fossil, leaving behind minerals which fill the mould. The minerals harden, taking the shape of the mould fossil. Any mould fossil can potentially form a cast mould. The water seepage, strength of the mould fossil and available minerals in the area are the determining factors.
Trace fossils, also known as ichnofossils, are fossils created by the passing of an organism, rather than fossils of the organism itself. Trace fossils include footprints, toothmarks, fossilised faeces, burrows, and nests. Footprints provide knowledge about speed, length of stride, how many legs the organism walks on and how the organism holds its tail, hunting behaviour and herd behaviour. Coprolites, or fossilised faeces, and toothmarks provide knowledge about the diet of organisms. Burrows and nests provide knowledge about habitat, predators, and mating and young -raising habits. Trace fossils can be either mould or cast fossils.
Body fossils are fossils that include part of, or the entire body of, an organism. Bones, teeth, claws, eggs, skin and soft tissues are all examples of body fossils. Bones, teeth, and fossilised eggs are the most common body fossils.
Skin, muscles, tendons, and organs decay quickly and thus are rarely preserved, although rare imprints have been discovered. Body fossils provide information about an organism's diet, reproduction, anatomy, and adaptations. Like trace fossils, body fossils can be either mould or cast fossils.
Petrification occurs when minerals permeate and harden an organism, or part of an organism, or when an organism it is encased in a substance that does not allow decomposition. A piece of petrified wood and an insect trapped in amber are two examples of petrification. Although mould fossils and cast fossils involve petrification, petrified fossils are different in that the original organism has not decayed or disintegrated.