Types of Fossils & How They're Formed
Have you ever wondered how fossils are formed? You might have seen fossils in museums in the form of dinosaur skeletons and in their souvenir shops as "dinosaur bug" trilobites that remind you of those bugs that Fred Flintstone used to play table tennis with.
You may have even found small fossils in playground pebbles (where I often find fossils known as crinoids.)
So how do plants and animals go from a living thing to something preserved in rock as a fossil?
Types of Fossils
After they die, most plants and animals decompose due to bacteria or scavengers. Because of this, usually only the bones and shells of living creatures are preserved (and thus most fossils are only made up of skeletal remains.)
However, fossils can often be more than just bones and rock. What's found in a fossil depends on how the fossil was formed. In order to best understand how fossilization works, it's important to understand learn the different types of fossils (and what causes each type to occur.)
The process of permineralization is one of a number of ways a fossil can form. A permineralized fossil has been buried by sediment that hardens into rock.
The original material that made up the bones and shells have been slowly leeched out over a long period of time and replaced by minerals in the surrounding rock. The result is an impression of the original living parts.
Fossils of small creatures (like insects) are often found preserved in amber. Insects flying into sap became trapped and, over time, the sap dried and hardened into amber.
The tree may have fallen and eventually turned into coal. The coal bed might have been submerged in water or sediment for many years and, over time, was eroded away (or even mined) to reveal the amber with the small creature inside, almost perfectly preserved.
Tar Pit Fossils
Tar pits have been known to preserve fossils for thousands of years. Tar pits slowly seep asphalt up to the surface where it gets covered in leaves. Creatures who have wandered onto the surface, died there, and were preserved by the tar.
Several incredibly intact fossils from the Pleistocene era, such as mammoths and saber-toothed cats, have been pulled from asphalt deposits such as the La Brea Tar Pit in Los Angeles.
What's particularly noteworthy about tar pits is that they often contain more fossils of predatory animals than prey.
Some fossils are formed through a process known as carbonization.
This is where the carbon-based compounds in a plant or animal slowly decompose while trapped in sediment. This leaves an impression of the original plant or animal. These types of fossils are often highly detailed.
Mummification, Frozen Fossils, and Peat Bogs
Mummification can occur in a number of different environments, most commonly in deserts where it's very dry, extremely cold areas where plants and animals can be frozen in ice, and in peat bogs whose pH level causes a "pickling" effect.
Mummified specimens of various animals have been found with soft tissues like skin and fur preserved through drying. Human mummies are a well-known example of this process.
In colder climates, frozen bodies of mammoths and other Ice Age creatures have been found. This is not a common form of fossilization because the climate can change over thousands or millions of years.
Many animals (and even people) have been found in peat bogs. Since peat bogs have no drainage and high acidity, it doesn't have the conditions necessary for bacteria (which aids in decomposition) to live, so many specimens taken from bogs are incredibly intact.