Science‎ > ‎Science Help‎ > ‎

Describe how fossils are formed

        Fossils teach us about plants and animals that lived on Earth long ago.  Scientists study these fossils to learn what happened on Earth millions of years ago.  Fossils are usually found in sedimentary rocks.  Hard parts of an organism such as bones or shells can become a fossil, but soft parts rot too quickly.  The plant or animal must be buried quickly by sediments and stay untouched for a long period of time.

        Sometimes a fossil is just a mark left behind by an organism when it was alive such as a footprint or a burrow.  Sometimes a dead plant or animal sinks into mud leavaing its shape when it decays.  When the sediment hardens, it becomes an impression or trace fossil.

        Some organisms are preserved without changing.  This might happen when an animal falls through ice and is frozen.  An animal might also be trapped in a tar pit.  Some organisms have been preserved in amber (the fossilized resin from ancient trees and plants).  These are called preserved organisms.

        Mineral replacement fossils are made when water dissolves part of the dead plant or animal and washes it away.  Minerals fill in the tiny holes left in the imprint of the plant or animal.  These minerals harden into stone.  The fossil is the same shape and size as the original plant or animal.   Sometimes you can see very detailed parts of the once living organism.  Replacement fossils can be very colorful because the minerals which fill in the holes may be different colors.

Source: Utah State Office of Education/Utah State University