What are Fossils?

What are Fossils? 

The preserved remains, impression, or traces of
any once-living thing from the past.

Animal Fossils

Plant Fossils

Trace Fossils

How do Fossils Form?

Fossils form in many different ways, including: 


Permineralization is the process in which mineral-rich water seeps into cavities of an organism and forms a crystal cast of the internal structure. 
Petrified Wood, on display at the Lizzadro Museum.
Agatized dinosaur bone

Molds & Casts

Organisms get buried in sediment and decay away, leaving a cavity (mold). If that cavity gets filled in with sediment, a cast is made. These leave 3-dimensional impressions of an organism.

Sometimes a shell gets filled in with sediment first, then the shell is dissolved away. This leaves an internal cast of the creature.

Most molds and casts do not contain the organism’s remains.

Molds: A fossilized impression made in sediment that is turned to stone. The original body parts have been decayed.




Casts: The process of minerals entering the cavity of a mold fossil, creating an image of the organism.


Form when a plant or animal lands in mud, silt, or sand and makes an impression. The organism decays (or walks away) but the impression stays and is filled in with sediment and hardened into rock. The key difference between impressions and molds/casts is that impressions are typically 2-dimensional molds and casts.

  • Imprint of an actual organism
    • Leaves, insect wings, etc.
      Fossilized insect wing from Green River Formation, WY.
    • Mazon Creek Fossils
      Mazon Fossil, IL.
      Cast on left, Mold on right.
  • Trace fossils
    • Footprints
      Dinosaur footprints- Dakota Sandstone, Lower Cretaceous; west of Denver, north-central Colorado, USA
    • Burrows, trails, etc.

      Climactichnites – the trail of a slug-like mollusk moving on top of or through the sediment. From Blackberry Hill, WI, Cambrian age.

Trace Fossils (Ichnofossils)

The geologic records of past animal activities and behaviors. Trace fossils typically do not include the actual organism, only signs of the organisms behavior.





Fossilized feces

Eating habits

Trilobite with bite marks from Anomalocaris.
Artist rendering of Anomalocaris eating a trilobite