Fossilization and Types of Fossils

A fossil is evidence of life that once existed on earth. The fossil can be of the actual organism (or parts of the organism) OR evidence that the organism existed (burrows, footprints, excrement, etc.) Few organisms fossilize due to their soft tissue and/or not being in an environment to facilitate fossilization. When an organism dies, their remains must be in an environment where sediments cover the remains and preserve the organic material from completely decomposing. Over time, as the organic remains are buried deeper and deeper, the sediments and/or minerals present will preserve the organisms remains to become a fossil. This process ranges in time from a few days to millions of years.

Bones and shells are most commonly fossilized evidence or ancient organisms. These remains take longer to decompose and therefore are more likely to be preserved as a fossil. Soft tissue (leaves, fur, skin cells, or insects) can be preserved as a fossil, but in more unique circumstances. The common fossilization processes include preserved remains, permineralization, impressions and compression, and mold and cast.

Preserved Remains

When an organism dies and is in an environment where it can be completely preserved, these are considered preserved remains. These environments include drying, freezing, being trapped in amber, and being trapped in natural asphalt. A familiar example of this type of fossilization would be an insect that has been preserved in tree sap that over time becomes amber. Pop culture highlighted this fossilization process in Michael Crichton’s novel Jurassic Park, which of course was made into a blockbuster movie.

Permineralization

The permineralization process is also known as petrification. Petrified wood is a common example of a permineralized fossil. During permineralization, minerals replace the organic material and turn the remains into stone. These minerals come from groundwater that is moving through the remains and the minerals fill in the pores of the organic remains. Bones, shells, and wood are most commonly permineralized.

Impression and Compression

Organisms’ remains can be preserved by leaving an impression, being compressed, or the carbon atoms being altered to become more stable. With these fossilization processes, the fossil is a two-dimensional preservation of the organism.

Impression fossils do not contain organic material. An analogy for this type of fossil would be using a stamp. The image of the stamp is left behind on the paper. Impression fossils are created in fine-grained sediments like clay or silt sediments.

Compression fossils are different from impression fossils in that these fossils retain the carbon in the film left behind, but the organism’s remains will look distorted. The larger the organism, the more distortion there can be. Leaves, flowers, insects, and other smaller, flat remains tend to have less distortion. Carbonization can take place with this type of fossilization, which involves gases and fluids being released during decomposition. This leaves behind a dark residue that preserves the organism’s compressed remains.

Mold and Cast

Mold and Cast fossils are a three-dimensional preservation of ancient organisms. When an organism dies in an environment where sediments encapsulate the organism, it creates a mold of the remains. As the remains decompose, sediments and minerals fill the void in the mold, creating a cast of the organism’s remains.