Types of Fossils With Examples

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What are Fossils?

Fossils​ are the remains or traces of organisms that lived long ago and have been preserved. Both paleontologists and geologists use fossils to help determine what life and geology were like in the distant past. Through the remains of plants and animals from long ago, a picture of life at that time emerges. The fossilized organism might retain features such as leaves, stems, bones, teeth or other remains that can help categorize it and its family tree.

Geologists benefit from fossil discoveries by being able to use them for dating the rocks around them. Paleobotanists have learned about ancient forests that no longer exist in many parts of the world from the evidence that carbon film fossils, in particular, have left behind in coal deposits.

Characteristics of Fossils

  • Fossils are made from once-living organisms. When an organism dies, its body becomes buried by layers of Earth over time. Pressure from these layers and heat from Earth’s core creates a hardened fossil of the once-living organism. An organism will not become a fossil if it is dispersed in water or if it gets eaten by another organism.
  • Fossils are made of rock that replaced the body of the organism or filled in an impression left by the organism. When the body of an organism decays, it leaves behind an impression in one of the layers of rock it was buried in. Over time, new layers of rock fill in the impression, creating the fossil image in the rock.
  • Fossils are typically made from sedimentary rock rather than igneous or metamorphic rock. Metamorphic rock and igneous rock are found beneath Earth’s surface. Since organisms die on Earth’s surface, they are more likely to be covered by sediment, therefore creating a fossil out of sedimentary rock.
  • Fossils are formed from plant matter or bones of an organism because soft tissue decays before a fossil is formed. Soft tissue, like the skin, brain, and organs, is quick to decay. This leaves behind the hard parts like bones, horns, or shells to become fossilized. If an organism is entirely soft-bodied it is unlikely that it will become fossilized.

List of Types of Fossils

  • Petrified Fossils
  • Mold and Cast Fossils
  • Carbon Film Fossils
  • Trace Fossils
  • Preserved Remains/True-form fossils
  • Compression Fossils
  • Impression Fossils
  • Pseudo Fossils
  • Micro fossils 

Petrified fossils

Petrification (petros means stone) occurs when the organic matter is completely replaced by minerals and the fossil is turned to stone. This generally occurs by filling the pores of the tissue, and inter and intra cellular spaces with minerals, then dissolving the organic matter and replacing it with minerals. This method reproduces the original tissue in every detail. This kind of fossilization occurs in both hard and soft tissues. An example of this kind of fossilization is petrified wood.

Petrifying begins with the quick burial of plant or animal material. Burial slows the decomposition rate enough to allow the replacement to happen. Water containing dissolved minerals circulates through the sediments. Over time, these mineral-rich solutions seep into and saturate the buried remains. As the water evaporates, the minerals remain. The dissolved minerals in the solution crystallize between the cells of the organism.

As the cells slowly decay, the solution fills in the gaps left behind. Eventually the deposited minerals replace all of the organic material. Shells, bones and plants, especially trees, are particularly suited to permineralization because the natural structures of the cells maintain their shape during burial and the replacement process.

Mold and Cast Fossils

Molds and casts are three-dimensional impressions in which the surface contours of an organism are preserved. Sometimes when an animal dies and its body decays, it can leave an imprint in the sediment. If this imprint fills in with minerals from sediment and groundwater, it can harden to form a fossil. This fossil is called a cast fossil. The fossilized imprint is called a mold fossil. Although the fossil may exhibit characteristics of the original organism, normally no organic material remains.

Usually molds and casts exhibit a distinct three-dimensional character. Occasionally, inorganic material replaces the shell of an organism, leaving an impression of the interior surface called an internal mold. When this mold fills with soluble minerals, it forms an internal cast, called a steinkern, which means “stone cast” in German.

Traces of extinct organisms, such as burrows, shells, plants, trails and tracks, represent a type of fossil mold or cast if the three-dimensional integrity is preserved. Molds and casts that faithfully replicate the external form of an organism provide paleontologists clues about the surface anatomy and behavior of an ancient organism.

Carbon film fossils

Carbon film fossils are also called ​compression fossils​ or ​carbonized fossils​. Carbonized fossils can often be found near coal seams. The term ​carbonization​ refers to the process of something being changed into carbon.

The remains of an organism were exposed to tremendous pressure over time, such as being buried under sediments beneath a seafloor. This process is called ​compression​, which, in addition to pressing something together, also means its size and volume are reduced. Once the remains are pressed, eventually, many of its components are siphoned away, such as hydrogen, nitrogen and oxygen. This leaves a carbon film or residue behind in a thin layer on a rock that is two-dimensional.

A carbon film imprint of an organism in flattened form remains with carbon film fossils, yet many features of that original organism can be seen on this form of fossil. For example, researchers can easily discern the stems and leaves of plants and even glean cellular information from the fossils.

Trace fossils

Ichnofossils, also known as trace fossils, are geological records of the activities and behaviors of past life. Some examples include rock evidence of nests, feces, burrows, footprints, and scat. Trace fossils contrast with body fossils, which are the fossilized remains of parts of organisms’ bodies such as shells or bones, usually altered by later chemical activity or mineralization. The study of such trace fossils is ichnology and is the work of ichnologists.

Trace fossils offer indirect evidence of ancient plants or animals. Footprints can help a paleontologist determine how fast an animal moved. Nests can illuminate how extinct species raised their young. And feces can give insight into an animal’s diet. 

There are several different types of trace fossils, but most can be grouped into three categories: tracks and trails, burrows and borings, and gastroliths and coprolites. Tracks and trails can include a single footprint, a number of footprints made during a single trip, or an impression made by an organism without legs. Burrows and borings can provide insight into an animal’s motions or nesting habits. Burrows are holes that an animal dug into loose sediments, like mud. Borings are holes that an animal dug into a hard substrate, like rock or wood. Gastroliths, also known as stomach stones, are rocks that are held inside an animal’s intestinal tract.

True-form fossils

True-form fossils are organisms that are preserved entirely in their natural form. This can happen a few ways, but it typically involves the organism becoming entrapped and preserved. Amber is the resin from a coniferous tree from the early Tertiary period. Insects fall into the tree resin and remain stuck there because of its stickiness. Over time, more resin falls on top of them. Over millions of years, the resin hardens and changes its molecular structure in a process called polymerization until it becomes amber. Entrapment in the hardening resin protects the fossilized insect from scavengers and decomposition.

Desiccation is another type of true-form fossil. It is also called mummification. Some animals crawled into caves in the southwest deserts of North America during the ice age and died. Their bodies were dried by the desert air and were preserved perfectly for thousands of years. Mummified remains are so well preserved that hair color and clothing are still visible, but these fossils often fall apart at the slightest touch.

Freezing is one of the best-preserved processes of fossilization. The organism’s soft tissues remain entirely intact. The circumstance that leads to a frozen fossil is often the sudden entrapment of an animal in a location that is freezing. This was not uncommon for large mammals in Siberia and Alaska during the late ice age, particularly woolly mammoths.

Micro-fossils

Micro fossils are plant or animal remains that are microscopic in size, generally less than 1 millimeter in length. They may be either small organisms, such as viruses or bacteria, or small bits of larger plants or animals. They’re considered to be the most important group of fossils since they are useful in dating surrounding rocks and other fossils and are the most numerous and accessible of all fossils.

Pseudo Fossils 

Pseudo fossils are objects that resemble fossils but are not actually fossils. Pseudo fossils are created from impressions of inorganic materials such as minerals. Cracks in the rock or imprints left by raindrops are often mistaken for fossils, but they are not fossils by definition.

Impression fossil

An imprint fossil, also known as an impression fossil or fossilized impression, is a fossil that doesn’t contain organic material. They are a form of trace fossil – a fossil that leaves evidence of an organism’s movements or activity. Examples of trace fossils include footprints, eggshells, nests and more.

For an impression fossil to form, the conditions have to be just right. The type of ground dictates if an impression fossil remains or fades away. Most impression fossils form in sediments with very fine grains, such as clay or silt.

Compression Fossils

Compressions are the most common fossil form, especially in plants, where some or the entire original organism is left behind as an imprint as the organism is slowly compressed between sediment layers. This means compression fossils are often distorted. Coal, a fossil fuel, is an example of a compression, where the combination of fallen vegetation on hypoxic swampland formed a sludge which was slowly compressed under the vertical pressures of swampland silts, and over a course of approximately 300 million years.