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For most fossils there are three basic things that are needed: sediment, pressure, and time.To create a mold and cast fossil, the animal has to die in a place where it will be covered with dirt, or sediment, fairly quickly, like an animal that dies in the ocean and sinks to the sea floor.Sign up for INFObytes and receive an MP3 (audio presentation) called Genesis, The Gospel and the Creation/Evolution Issue by Dr Emil Silvestru—free for you to download! Fossils are the remains of animals or plants that lived a long time ago.
Bogs are another place where animal remains can be well preserved.
The mold is an empty cavity, and can be filled with minerals that become a cast for the long dissolved bone.
There are some rare places where a whole animal can be preserved.
The remains are then covered in sediment, or layers of sand and mud.
Over time the soft parts of the animal decay, leaving only the harder parts like teeth and bones.
Petrification happens when the organic material is slowly replaced by minerals. Making a fossil is tricky and the conditions have to be just right.