Curious about Fossils?
A fossil is any preserved prehistoric remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, imprints of animals or microbes, footprints or trackways of animals, objects preserved in amber, hair, petrified wood, oil, coal, and DNA remnants. Fossils reveal a great deal about what the world was like millions or even billions of years ago. (Some people refer to older men like me as “an old fossil” but that is incorrect.)
In 2017, researchers confirmed that the oldest fossils, which were found in a rock in Western Australia, prove life existed on Earth over 3.5 billion years ago.
Fossils can be categorized into five different types:
Body FossilsMolds and CastsPetrification FossilsFootprints and TrackwaysCoprolites
Body Fossils
Whole body fossils are the entire remains of prehistoric organisms including soft tissue, such as insects embalmed in tree sap that hardens to create amber. Typically, soft tissue disintegrates after death, leaving only the hard shell or bone skeleton behind. Animals with weak skeletons, like insects and shrimps, are less likely to be preserved. Two types of body fossils – bones and teeth – are the most common types of fossils.
Molds and Casts
Molds and casts are other types of body fossils. A mold is an imprint left by the shell or a hard skeleton on surrounding rock, such as dinosaur bones buried in layers of sediment. A mold may be internal or external. An internal mold forms when sediment fills the inside of a shell. An external mold is on the outside of the shell. Whenever a shell or bone breaks out of rock, it leaves an external mold behind. Replicas of molds are known as casts, which may be produced naturally when the space left behind after mold removal fills with sediment.
Permineralization and Petrification Fossils
When mineral rich groundwater saturates a plant or animal's remains after it dies, sometimes the organism's materials dissolve away and minerals such as calcite, iron or silica replace them. The fossil’s original shape is retained, but the composition is completely different. This process is known as permineralization. The fossil fish on this website are a product of this process. If you are interested in seeing the actual sediments containing numerous fish, turtle, and insect fossils, visit Fossil Butte National Monument in Wyoming.
Petrification fossils form when the organic matter is gradually replaced by minerals and turns to stone. The original tissue and structure is replicated in every detail. Petrified wood is an example of petrification. The Petrified Forest National Park in Arizona, USA displays wondrous examples of petrified wood in its natural environment.
Footprints & Trackways
Footprints, trackways, trails and burrows in soft sediments sometimes harden and become fossils known as trace fossils. These give information about how animals behaved when they were alive, such as how they moved and how and where they fed. Trackways, which are several footprints together, sometimes include impressions made by another part of the creature, such as its tail dragging behind it. If you are ever in north Texas, you can visit Dinosaur Valley State Park and see dinosaur tracks that were made in what was lime mud at the time. The park is in Glen Rose, Texas, the type locality for the Cretaceous aged Glen Rose formation.
Coprolites
Coprolites are the preserved feces of prehistoric animals. They give clues to where certain animals lived and what they ate. Coprolites are rare because feces usually decay quickly. The most common coprolites are of dinosaurs, and sea organisms such as fish and reptiles. They consist of remains of the organism's food, such as pieces of scale, teeth, shell, bone and plant pollen. Coprolites are preserved by petrification or cast and mold