9 Different Types of Fossils

Fossil imprint in sediment

Did you know that a single preserved bone from ancient civilizations can give you quite an insight about almost the entirety of that particular civilization? Along with genetics, fossils have proven to be one of the most useful ways to learn about the great history of life on earth. Common examples of fossils include tracks, footprints, bones, teeth, dung, and skin, to name a few.

Simply put, fossils are records of organic matters and organisms that consist of intricate details about their shape, size, and features of the body parts. They are basically conducive to conserving and preserving a variety of different types of organisms. This has greatly helped scientists, archaeologists, and paleontologist learn a great deal about how the world and life on earth looked like some millions and billions of years ago.

Where are Fossils Found?

Remains of a Fish

There is no denying the fact that fossils are found in the Earth’s crust and surface, however, if you were to narrow it down, they are commonly found in all those areas or spaces where sedimentary rocks may be exposed.

These rocks are defined as those that are formed when multiple layers of clay, sand, mud, and silt combine together and then harden over a span of a billion years or so. They harden so much that eventually, they end up adopting a rocky formation, hence the name sedimentary rocks.

These rock-like layers settle down in water bodies like oceans, lakes, and swamps and basically create a kind of a settlement there. This is one of the main reasons why you will notice that most of the fossils found till today are remains of those animals that lived either near the water or in the water.

Types of Fossils

Fossils are remnants or traces of organisms from prehistoric times that belonged to the past geological ages. These traces could be anything from a leaf imprint to the skeletal remains of living organisms that are preserved in the Earth’s crust.

There are different types of fossils, each of which has been incredibly beneficial in telling us about the life on earth that existed billions of years ago.

Mold Fossils

Mold fossils from the past

These are fossilized imprints that are made within a substrate. The substrate, in this case, is usually a rock or sediment. The fossil makes use of these substrate components in order to leave their mark on them. A mold fossil is a kind of like an impression that is made in the substrate. They are quite similar to the negative impressions left by an animal or a plant.

Mold fossils are primarily formed through a process called ‘leaching.’ In this process, the shell material of rock, for instance, is removed, leaving behind a hollow or gap in the rock. This rock basically sports some qualities or features of the original shell. Throughout the whole process of leaching, some of these end up retaining the complete image of the original animal or plant.

Unlike cast fossils, mold fossils are more towards the hollow end, which is one of the reasons why the end or resulting image is more like a negative image or impression of the organism’s body part.

In easier or simpler terms, mold fossils are created shells, and other similar structures are buried in sediment, after which they end up being dissolved by the underground water. The resulting mold that is created afterward is a reflection of the surface markings and also the shape of the organism.

The hard part of the organism completely dissolves over time, and all that is left behind is an empty space or area of its body shape. Once dissolved underwater, layers and layers of silt start building up throughout the years, eventually resulting in the production of rocks and stones.

Cast Fossils

Cast fossil shaped like a shell

These are a result of the mold fossils when the latter gets filled with some kind of minerals. What happens here is that when mold fossils are formed, there are times when their hollowness is filled with certain kinds of minerals. Over time, these minerals begin to harden, and in doing so, they tend to form an exact replica or model of the original fossil that was there in the first place. This replica is what we call a cast fossil.

In other words, when a plant, animal, or any other organism dies, certain chemical reactions take place in its remains that lead to a deterioration of bones and great flesh decay. Both these things create a gap or hollow in the organism, more like a cavity that is eventually filled up minerals found in the underground. When these minerals harden over the passage of time, they create a ‘cast’ which is what gave these fossils their respective name.

Cast fossils leave an imprint in the rock or sediment and go a single step further than the mold fossils. So, if you were to draw a comparison of the two, mold fossils take up all the negative space in an organism while cast fossils hold all the positive space. Some examples of cast fossils include embryos, skin, teeth, leaves, etc.

Trace Fossils

Dinosaur footprint from old times

These fossils are also called ‘ichnofossils,’ and they contain any potential information or traces left behind by the organism. They are basically glimpses that let us know how the particular organism behaved and what its activities were like when it was alive. The term ichno’ comes from the Greek word “ikhnos,” which translates to mean ‘trace or track’ in the English Language.

Trace fossils are probably the most common types of fossils, and they are more helpful than fossilized body parts in terms of providing us with key information on the organism how it lived, hunted and rested, what it ate, etc.

These fossils are more like indirect evidence of the existence of the prehistoric life. There are different types of trace fossils that can be grouped together into various categories. These types include coprolites, tracks and trails, gastroliths and burrows, and borings. All these types of trace fossils help archeologists decipher the happenings and the activities of the past life.

Tracks and trails refer to animal footprints, for instance, that were initially made of soft sediment after which they later transformed into hard sedimentary rock. Coprolites are fossils that are formed from the stomach content of organisms as well as their dung. These greatly help in identifying the food and dietary habits of various organisms.

Burrows, on the other hand, are holes made by organisms in rock, wood or sediment. These burrows are later filled with mineral matter found in the underground that helps preserve them for many years to come. Lastly, gastroliths are stomach stones that are basically found in the stomach region of many reptiles, mammals, and birds. These stones were once popularly used to grind food by several extinct reptiles.

The thing with trace fossils is that they often end up separating themselves from the organism that originally formed or created them. This makes it really difficult for researchers and scientists to figure out which specific organism and its habitat and activities resulted in the formation of a particular trace fossil.

What further makes the study of trace fossils a tad more challenging is that there are also other natural events and instances that lead to the creation of similar patterns; however, they are not made by any living creature.

True Form Fossils

Fossil of a dragonfly

There are often times when different plants and animals get trapped or stuck within the ice, tar, or tree sap, and in most cases, they end up being trapped for hundreds of years. As they remain there for years, the substance they’re stuck in ends up keeping their original features and characteristics fully intact and whole.

These features or hard parts remain just the way they began and do not decay or rot for hundreds of thousands of years. Eventually, after being frozen and jammed between these substances, the organisms are found thousands of years later and hold great historical evidence within them.

The true form fossils that we may have found today are probably from millions of years in the past and chances are that they will give us evidence of a past that wasn’t just a hundred or thousand years old, but, in fact, millions and millions of years old.

Some common examples of true form fossils include fingers, limbs, heads, and torsos. These are large body parts of any organism that were preserved through the process of petrification. Through this process, these body parts were actually replaced by minerals.

Unlike mold fossils and cast fossils, true form fossils are not created with the help of an impression but are formed as a result of being displaced by the minerals that eventually hardened to transform into rocks.

Body Fossils

Fish fossil on sandstone

These are easily the most common and popular types of fossils found all over the world. As the name evidently suggests, body fossils are formed from the remains of dead plants and animals. These are usually hard parts of an organism including shells, bones, teeth, woody trunk, stems, and branches.

The interesting bit about body fossils is that they are found in a variety of different sizes, ranging from tiny living things that can only be looked at with the help of a microscope to massive and gigantic dinosaurs that are highly visible to the naked eye even from far away.

Whole-body fossils are the complete remains of organisms belonging to the prehistoric times. These also often include the soft tissue of organisms, for example, insects that get trapped inside tree sap and then harden down to create amber. Soft tissue like skin, organs, and muscle usually fall to pieces once the organism dies, leaving behind only the bone skeleton and the hard shells of all those organisms.

The most common types of body fossils include teeth, bones, and claws. Bones have by far been the most vital body fossil to learn about dinosaurs. The fossilized bones of numerous dinosaur species have been found since ancient times and in 1818, the first-ever dinosaur bone was discovered.

These are the remains of actual organisms while their hard skeleton is preserved and kept intact even after millions of years. The soft tissues decay and deteriorate soon after the organisms die. Keeping this phenomenon in mind, it is important to note that animals like shrimps and insects that have weak skeletons are less likely to be preserved. Also, those animals that don’t even have a skeleton, to begin with, have very thin and rare chances of being fossilized.

Fossilized Feces

Although archeologists typically look for things like bones and tools when trying to dig for clues about ancient civilizations, collaborations with microbiologists have allowed them to get information from another very important source, which is fossilized feces.

Also known as coprolites, fossilized feces are the best clues of finding out where certain animals and organisms lived and the things that they ate. This is a very rare type of fossil primarily because feces tends to decay and decompose really quickly. Some of the most common fossilized feces belong to reptiles, fish and numerous other sea organisms.

These are usually preserved through the process of petrifaction, or even cast and mold. What really makes them able to be preserved is the fact that feces of organisms often consist of indigestible remains like bones, teeth, shells, and pieces of scales, all which basically come from the organism’s food.

Numerous scientists and microbiologists have gone on to explain the importance of how studying millions of years old feces samples can help us find out clues about the past civilizations and how the different ancient groups stand out from each other.

Coprolites have long been studied as a subject of fossils, but it is only recently that they have been used as highly important tools to draw a clear, defining line that distinguishes two extinct cultures and also separates them on the basis of ethnicity.

Various groups of scientists and archaeologists have tried to examine the DNA present in fossilized feces from various ancient indigenous cultures. They were eventually able to find out the kind of fungal and bacterial pieces of evidence present in each culture along with the different components that could have possibly been a part of their diets.

Petrified Fossils

Fossil of a dinosaur

Petrifaction is a process in which things turn into stones, and petrified fossils are those that are formed when minerals from the underground replace certain or all parts of numerous organisms.

Since water is often fully enriched with dissolved minerals, it easily reaches the dead organisms by seeping through the layers and cracks of rocks or sediments. Once this mineral-rich water evaporates, it leaves behind the stone, hardened materials and minerals.

Petrifaction turns living organic materials into stone, and this process essentially takes place when the molecules found within an organism are properly replaced by those molecules of the water minerals.

The petrified fossils that are created as a result of this process are often three-dimensional or kind of in raised manner, and they are typically found in sedimentary rock. Different types of petrified fossils might also be found in volcanic deposits in several locations worldwide.

There are numerous types of petrified fossils, among which petrified wood is the most common type. Another type is teeth and bone, but these are less common as compared to petrified wood mainly because they end up decomposing really quickly and easily.

Although petrified fossils are a very rare form of fossilization, they have been greatly used by a number of paleontologists through which they have been able to explain the whole phenomenon behind the transformation of relatively complex life forms.  They have also linked the theory of biological evolution to this concept and have explained specific adaptations and the diversity of life.

Petrified fossils have served far greater purposes than just in terms of their importance and value. Scientists have used information from these fossils and found a link to phylogeny, a study of how organisms are connected to evolution.

This allowed these scientists to determine how some species are related to each other. With respect to the organisms found contained within the petrified fossils, scientists could understand and explain the numerous ways in which the adaptation process has caused so many species to either greatly evolve, or simply become extinct.

Carbon Film Fossils

An organism with carbon film

It is a very common fact that all living organisms contain an element of carbon in their body. So, when a dead organism lays on a rock and is deeply buried inside sediment, a thin layer of this carbon is left behind and deposited onto the rock over the passage of time, and simultaneously, all the material found within the organism gradually breaks down.

This thin carbon layer basically reveals the respective organism’s delicate parts, for instance, leaves from a plant that probably lived millions of years ago. This happens because the oxygen, hydrogen, and nitrogen disappear from the organism’s body and end up either being dissolved or vaporized under the water body. The whole decay process and the carbon being left behind are called by two common names – carbonization or distillation.

Unlike petrified fossils that create a three-dimensional cast, carbon film fossils create a two-dimensional image that is well-imprinted into the rock or sediment. These films also greatly stand out from the rock since they have a very prominent black or brown color.

The most common types of carbon film fossils are leaves, fish and crustaceans, primarily because the specimens that are preserved under the water body are the only ones that leave behind carbon films.

In the case of leaves in particular, their internal components like cell wall structures and membranes are broken down. However, the cells end up being richly filled by water that is enriched with a lot of minerals. The water then becomes solidified which, in turn, helps preserve the delicate and minuscule features of most of these organisms.

Preserved Remains

Amber on a beach in Baltic Sea

The process of fossilization often preserves the whole of an organism; however, there are also times when it preserves only a part of the organism with very little change.  Such organisms are conserved in or very close to their original states. They are the ones that are called preserved remains fossils and are incredibly similar to the true form fossils.

Preserved fossils are a very rare occurrence, considering how most fossils end up suffering a great amount of damage due to various processes like sedimentation and weathering way before these fossils are even discovered.

Fossil preservation happens in two ways, where one is with alteration, and the second type is without alteration. The former is way more common where the original organism is completely or partially changed and then transformed into a whole new material. The alteration takes place through the process of carbonization, petrifaction, and re-crystallization. In all these methods, the hard part of the living organic material is substituted with the help of a new mineral.

On the other hand, preservation without alteration results in the original organic matter remaining intact and completely unchanged. The process through which that happens is called ‘amber.’ In amber, what really happens is the organic matter, be it an insect or a leaf, gets surrounded by a natural tree resin. Over time, this protective cover or layer preserving the matter hardens and becomes stone-like. Amber is also referred to as a gold-colored resin that originally comes from the pine tree sap.

Another way of remains being preserved without alteration includes being frozen in ice. Many organisms die in intensely cold regions, after which their bodies are frozen in ice and even something as small as their strands of hair gets preserved in the ice. A great example of this is the preservation of the wooly mammoth that was extinct for thousands of years. It was discovered in the Siberian Glaciers where it was found frozen under huge blocks of ice.


It is a wonder how remnants and traces that are almost a billion years old can carry the ability to tell us the entire history of a prehistoric civilization. If anything, it is just absolutely fantastic and fascinating how archaeological pieces of evidence preserve the past and reveal the world’s history to the present population!

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