What is a fossil?

A fossil is any naturally preserved evidence of life

Fossils can be divided into two categories, fossilized body parts (bones, claws, teeth, skin, embryos, etc.) and fossilized traces, called ichnofossils (which are footprints, nests, dung, toothmarks, etc.), that record the movements and behaviors of the organism.

The four types of fossils are:

mold fossils (a fossilized impression made in the substrate - a negative image of the organism)

cast fossils (formed when a mold is filled in)

trace fossils (fossilized nests, gastroliths, burrows, footprints, etc.)

true form fossils (fossils of the actual animal or animal part)

How are fossils formed?

unaltered preservation (like insects or plant parts trapped in amber, a hardened form of tree sap) 

permineralization=petrification (rock-like minerals seep in slowly and replace the original organic tissues with silica, calcite or pyrite, forming a rock-like fossil - can preserve hard and soft parts - most bone and wood fossils are permineralized)

authigenic preservation (molds and casts of organisms that have been destroyed or dissolved).