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
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.
Impressions
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.
- Leaves, insect wings, etc.
- 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.
- Footprints
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.
Nests
Burrows
Footprints
Coprolite
Eating habits
