All About Fossils

Fossil Formation

Silurian Cephalopods possessed long, hard, segmented shells like the one shown in this fossil. They also possessed soft bodies with tentacles. These softer parts of their bodies were seldom, if not never, been preserved in fossils.

Fossils formation happens over long period of time, from hundreds of thousands to millions of years. The process can occur in a few different ways, each resulting in a different final product.

The most familiar type of fossils are cast fossils. These fossils typically form in an underwater environment when hard tissues, like teeth, shells, bones or sometimes cartilage, left behind from a decaying organism are covered in a layer of sediment which is compacted over time by heat and pressure from the environment and hardened into sedimentary rock. Over time, minerals replace the trapped tissues in a process called petrification, by which the tissues are eventually turned to stone. The result is a three dimensional copy or "cast" of the organism's hard tissues.

Mold fossils, unlike cast fossils, form when an organism has fully decomposed leaving behind no hard or soft tissues, but leaves behind a case in the shape of its body.

Another type of fossil is trace fossils. These fossils indicate the presence of an organism, but do not include any fossilized tissues of the organism. The most common example of trace fossils are trackways, or footprints left behind by an organism that are preserved as sediment fills and hardens into the shape of the footprints.

Finally, imprint fossils show the exterior impression of an animal, like mold casts, however these fossils are two-dimensional renderings of an organism's exterior rather than three-dimensional cases.

Use the next annotation activity to draw out your learnings about the different types of fossils discussed in this activity.