What is a fossil?

A fossil is the preserved remains or trace of a once living organism of a past geological age.
Fossils of preserved remains can be:

• Skeletons - such as bones of dinosaurs
• Imprints - such as those of plant leaves on shale
• Fossil Wood - in which the internal structure and external bark are well preserved due to
  permineralization
• Microfossils - fossils of organisms that can not be easily analyzed with the naked eye. Generally defined as fossils < 1mm
• Resin - fossilized sap from trees which can trap and encase an organism. Often times referred to as "amber"
• Casts - a hole left by a decomposed organism this has been filled in with another mineral,
  taking the general shape of the past organism
• Molds - a rock in which an organism-shaped hole (made from a decomposed organism) is
  visible

Trace fossils are evidence left of past animal behavior. Some examples include:

• Tracks - left by the movement of an animal; such as dinosaur
  footprints
• Burrows - tunnels left by the movement of a digging
  organism
• Coprolites - fossil feces used to study the dietary
  habits of a past organism

Index fossils (sometimes called guide fossils) are a special category of fossils that only occur in a specific location(s), therefore are often used to identify and define specific geologic periods (or faunal stages).

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