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How Fossils Form

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Fossil Dragonfly (Cordulagomphus fenestratus) Lower Cretaceous, 125 million years old, Araripe basin, Brazil, South America

Fossil Dragonfly (Cordulagomphus fenestratus) Lower Cretaceous, 125 million years old, Araripe basin, Brazil, South America

Photo © John Cancalosi / Getty Images.

Fossils are the remains of organisms that lived in the distant past. For a specimen to be considered a fossil, it must be of a specified minimum age (often designated as greater than 10,000 years). Together, all fossils—when considered in the context of the rocks and sediments in which they are found—form what is referred to as the fossil record. The fossil record provides the foundation for understanding the evolution of life on Earth.

Fossilization is a rare occurrence in the history of life. Most animals die and leave no trace; their remains are scavanged soon after their death or they decompose quickly. But occasionally, an animal's remains are preserved under special circumstances and a fossil is produced.

There are six different ways an organism might become fossilized. These include:

  • Permineralization (petrification)
  • Casts and molds (authigenic preservation)
  • Compression fossils (carbonization or coalification)
  • Recrystalization
  • Bioimmuration
  • Unaltered preservation
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