• trace of an organism that once lived

• Body fossil: remains of animals and plants

Bones

shells

leaves

imprints

preserved specimen

• fossilized biological activity

footprints

teeth marks

burrows

coprolites

• material that settles to the bottom of a liquid (mud, sand,clay)
• Plants and animals must be buried in sediment to fossilize (to prevent or slow decomposition)

• Over time, pressure squeezes water out of sediment and minerals fill in the spaces forming sedimentary rock.
• Fossils are often uplifted due to the activity of tectonic plates and exposed by erosion

• sediments are added to existing layers over time
• a fossil from a deeper layer is older than a fossil from a layer close to the surface
• Relative dating technique:  only allows us to age one fossil relative to another

• radiometric dating measures the exact age of a fossil by measuring the amount of radioactive elements.
• A radioactive element will decay at a steady rate.  The amount of time it takes for half of the remaining amount of radioactive atoms to decay is called a half-life.  Fossil ages are determined by half lives 

variation