The History of Crystals

What Are Fossils?

Fossils are the preserved remains or traces of ancient organisms, such as plants and animals. They can be parts of the organism, such as shells, bones, teeth, or feathers; or they can be evidence of the organism's behavior, such as footprints, burrows, or dung.

Fossils are important because they provide valuable insight into the history of life on our planet. They can help us better understand what the climate was like, what creatures were like and when they lived on the planet, as well as give us indications of what caused significant changes in their evolution.

How Fossils Are Formed

Fossils are formed in many different ways, but most begin their development when a living organism, such as a plant or animal, dies and is quickly buried by sediment, such as mud, sand, or volcanic ash. Soft tissues like the skin and organs will most often decompose, leaving the hard bones or shells behind. After the organism has been buried, layers of sediment can continue to blanket the remains until, eventually, all of the layers harden into rock, becoming what is called ‘lithified’.

Types of Fossils

Most fossils can be categorized into three primary groups – impression fossils, trace fossils, and replacement fossils. Amber is also often considered a fourth type of fossil; Although, technically, it is still categorized as a gemstone.

Impression fossils are a common type of fossil that is found in sedimentary rocks, such as limestone, sandstone, and shale. They are created when an organism is pressed into sediment, which over time hardens, leaving behind an imprint of the organism. In Western Pennsylvania, these types of fossils are abundant, with numerous species of carboniferous plant remains being commonly found within compressed layers of shale.

Trace fossils are the left behind biproducts of an organism’s functions, such as tracks and trails, burrows and bores, gastroliths (stomach stones), and coprolites (fossilized dung).

Coprolites are an incredible resource for paleontologists! They help us to understand the bodily functions and eating habits of long extinct creatures of our planet.

Replacement fossils are a type of fossil where the original organic material of an organism is replaced by minerals, essentially creating a detailed replica of the organism's hard or soft parts. Examples of replacement fossils include petrified wood, where the original plant tissue is replaced by silica; and fossilized bones, where the original bone material is replaced by minerals, such as calcite or quartz. 

Think of these fossils like a sponge: If you dip a sponge into paint and allow it to dry, what will happen? It will harden and change the physical structure of the sponge, but it will also retain its original shape.