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History of Earth – The Distribution of Fossils and Rocks

Grade 8
Jun 3, 2023

The Distribution of Fossils and Rocks

In this article, we’ll learn about the history of the earth with the distribution of fossils and rocks. Let’s begin with the introduction to fossils.

Introduction to Fossils

Fossils are physical remnants of prehistoric organisms, both plant and animal. The preserved skeletal remains of animals are the most common and visible fossils. Leaf impressions, tracks and trails, burrows, droppings, and root casts are all examples of fossilized organisms.

Microfossils are microscopic skeletons of previously existing plants or animals that require close examination with an optical or electron microscope.

Fossils are the preserved remains of plants and animals whose bodies were buried under ancient seas, lakes, and rivers in sediments such as sand and mud. Any preserved trace of life that is more than 10,000 years old is also considered a fossil.

Soft body parts decompose quickly after death, but hard body parts like bones, shells, and teeth can be replaced by minerals that harden into rock. Soft parts such as feathers, plant ferns, or other proof of life, such as footprints or dung, may be preserved in exceptional cases.


Body fossils are preserved evidence of ancient animals, plants, and other life forms’ body parts. Trace fossils are organisms’ evidence left in sediments such as footprints, burrows, and plant roots.

Body fossil and Trace fossil

Why Do We Explore Fossils?

Fossils provide valuable information about the evolution of life on Earth. They can teach us about the origins of life and humans, how the Earth and our environment have changed over geological time, and how continents that are now widely separated were once connected.

Fossils help in providing important evidence for evolution and plant and animal adaptation to their environments. Fossil evidence documents how creatures evolved and how this process can be represented by a ‘tree of life,’ demonstrating that all species are related to one another.

Rocks can also be dated using fossils. Different types of fossils occur in rocks of varying ages as a result of evolution, allowing geologists to use fossils to understand geological history. Fossils are one of the most important tools for geologists when it comes to age correlation.


Fossils can be used to reinvent various worlds, such as those inhabited by dinosaurs or dragonflies with a two-meter wing span.


Fossil Formation

When an animal or plant dies, its remains typically decay to nothing. However, when the conditions are ideal, and its remains can be buried quickly, they may become fossilized. There are several ways for fossils to form.

Fossils are commonly found in sedimentary rocks and, on rare occasions, in fine-grained, low-grade metamorphic rocks. The fossils were sometimes removed, leaving molds in the surrounding rock, or the molds were later filled with other materials, forming casts of the original fossils.

Steps Involved in Fossil Formation

Let’s follow the fate of an old dinosaur as it searches for food along a muddy riverbank to see how a typical fossil develops in sedimentary rock.

On a hot summer day, the dinosaur succumbs to the heat and dies in the mud. Scavengers soon strip the skeleton of meat and may scatter the bones.

However, before the bones can weather away, the river floods and buries the bones, along with the dinosaur’s footprints, beneath a layer of silt.

More sediment buries the bones and prints even deeper until the sediment containing the bones and prints eventually turns to rock (siltstone and shale).

The buried bones and footprints have been preserved as fossils.

Uplift and erosion exposed the dinosaur’s grave one hundred million years later, allowing a lucky paleontologist to excavate them. The dinosaur appears once more, but this time in a museum.

Fossil Formation

Conditions Necessary for Fossil Formation

  • Death in an Oxygen-deficient Environment:

A dead squirrel by the side of the road will not become a fossil in an anoxic environment.

Scavengers come along and eat the carcass over time, and if that doesn’t happen, microbes infest the carcass and slowly digest it, or oxidation (chemical reaction with oxygen) breaks it down into gases.

A carcass has a greater chance of surviving if it settles in an anoxic (oxygen-deficient) environment where oxidation is slow, scavenging organisms are scarce, and microbial metabolism is slow.

  • Quick Burial:

If an organism dies in a depositional environment where sediment accumulates quickly, it is more likely to be buried before disintegrating.

  • The Presence of Hard Parts:

Organisms that lack durable shells, skeletons, or other hard parts are unlikely to fossilize because soft flesh decays much faster than hard parts under most depositional conditions. As a result, paleontologists have discovered far more oyster fossils than spider fossils.

Paleontologists have been able to estimate organisms’ preservation potential or the likelihood that an organism will be buried and transformed into a fossil by carefully studying modern organisms.

Various Types of Fossils

Perhaps you visualize a dinosaur bone or the imprint of a seashell in rock when you think of a fossil. Paleontologists differentiate many different types of fossils based on how the organisms were fossilized. Let’s take a look at some examples of these categories.

Body fossils that have been frozen or dried: Whole organisms can be preserved in a few environments. The majority of these fossils are relatively young by geologic standards, with ages measured in thousands of years.

Woolly mammoths that became incorporated in Siberian permafrost (permanently frozen ground) or “mummified” fossils preserved in desert caves are two examples.

Woolly mammoth

Body Fossils Maintained in Amber or Tar

Insects landing on tree bark may become entangled in the sticky sap or resin produced by the trees. The golden syrup envelops the insects and hardens into amber over time. Amber can keep insects alive for 40 million years or more. Tar also serves as a preservative.

The insect embedded in Amber

Bones, Teeth, and Shells that have been Preserved or Replaced

Bones, teeth, and shells are made of tough minerals that can withstand immersion in tar or rock. Some minerals in bone, tooth, or shell are not stable and recrystallize over time. Even if this occurs, the original item’s shape may be preserved in the rock.

Fossil skeleton

Mold and Casts

When sediment compresses around a shell or body, it conforms to its shape. If the shell or body eventually disappears due to weathering and dissolution, a cavity known as a mold remains.

If sediment afterward fills the mold, it, too, preserves the shape of the organism. The resulting cast protrudes from the adjacent bed’s surface. Only hard parts are typically used to create molds or casts. Soft part shapes are rarely preserved, resulting in extraordinary fossils.

Mold and Cast fossil

Carbonized Impressions of Bodies

Carbon is found in all living things. When an organism dies and is buried in sediment, the materials that comprise the organism degrade until only carbon remains. The thin layer of carbon left behind can reveal delicate parts of an organism, such as leaves or plants, such as fern fossils 300 million years old.

Carbonized impressions

Permineralized Organisms

Minerals precipitate from groundwater that has seeped into the pores of porous material, such as wood or bone, in the process known as permineralization. Petrified wood, for example, is formed when the wood is permineralized, transforming the wood into chert.

Chert is a fine-grained, hard sedimentary rock made up of microcrystalline or cryptocrystalline quartz, a mineral form of silicon dioxide. (Petrifaction means changing into stones.)

Petrified wood

Trace Fossils

Trace fossils are organisms’ footprints, feeding traces, burrows, and dung (coprolites) left behind in sediment.

Trace fossils

Chemical Fossils

Living things are made up of intricate organic chemicals. Most of these chemicals degrade over geologic time to form different but still distinct chemicals. A chemical fossil or biomarker is a distinct chemical derived from an organism and preserved in rock.

Paleontologists also use size to differentiate between different fossils. Macrofossils are fossils that are visible to the naked eye. However, some rocks and sediments are rich in microfossils that can only be seen through a microscope. Microfossils are plankton, bacteria, and pollen fragments.

Microfossils and Macrofossils

Using Fossils to Find Relative Ages: Fossil Succession

As Britain entered the industrial revolution in the late 18th century, factories required coal to power their steam engines and a low-cost means of transporting raw materials and manufactured goods.

Investors decided to build a canal system and hired an engineer named William Smith (1769-1839) to survey some of the excavations. The excavation of canals revealed previously hidden bedrock. Smith learned to identify distinct layers of sedimentary rock and the fossil assemblage (the group of fossil species) contained in each layer.

Smith’s discovery, which has been replicated in millions of locations around the world, has been codified as the fossil succession principle. Examine the figure, which illustrates a sequence of strata, to see how this principle works.

Bed 1 at the bottom consists of Species A, Bed 2 consists of Species A and B, Bed 3 consists of B and C, Bed 4 consists of C, and so on. We can deduce the range for each species from these data, which is the interval in the sequence in which fossils of that species appear. The sequence of fossils in the figure below, from oldest to youngest, is A, B, C, D, E, and F.

It should be noted that the range of one species may overlap with that of others and that extinct species do not reappear. After determining the relative ages of several fossil species, the fossils can be used to determine the relative ages of the beds that contain them.

For example, geologists can say that a bed containing Fossil A is older than a bed containing Fossil F, even if the two beds do not crop out in the same area.

The principle of Fossil succession

Tracing the History of Life – Fossil Records

Paleontologists have collected more than 250,000 different species of fossils over the last two centuries, according to some estimates.

The fossil record contains only a small percentage of the organisms that have ever lived on Earth. Many lacked skeletons or other hard parts that could be preserved; many did not survive the fossilization process, in which skeletons and tissues are replaced by minerals; and many were subsequently destroyed by chemical or physical processes such as recrystallization, metamorphism, or erosion.

The fossil record refers to the entire collection of fossils, both discovered and undiscovered, and their placement in fossiliferous (fossil-containing) rock formations and sedimentary layers (strata).

The fossil record was one of the first sources of data for the study of evolution, and it is still relevant to the history of life on Earth. The early development of radiometric dating techniques enabled geologists to determine the numerical or “absolute” age of various strata and their associated fossils.

Fossils of all kinds are useful for “reading the rock record” or analyzing the earth’s history. They can assist us in determining the geologic age and environment in which they were deposited (the paleoenvironment).

Finally, if the fossil record is sufficiently complete, studying it can assist us in better understanding the evolution (or progression) of life over

Age of Fossils

There are two methods for determining the age of a fossil: relative dating and absolute dating. Relative dating is used to estimate the age of a fossil by comparing it to similar rocks and fossils of known ages.

Absolute dating is used to determine the exact age of a fossil by using radiometric dating to measure the decay of isotopes within the fossil or, more commonly, the rocks that surround it.

Relative Dating

The majority of the time, relative dating techniques are used to date fossils. The fossil is compared to something whose age is already known using relative dating.

For example, suppose you have a fossil trilobite that was discovered in the Wheeler Formation. The Wheeler Formation has previously been dated to 507 million years ago, so we know the trilobite is also 507 million years old. But how can we tell how old a rock formation is if it hasn’t been previously dated?

Scientists can use index fossils, which are specific types of fossils, to aid in relative dating through correlation.

Index fossils are fossils that are known to occur only within a narrow age range. As index fossils, commonly occurring fossils with a wide geographic distribution, such as brachiopods, trilobites, and ammonites, work best. If the fossil you’re trying to date occurs alongside one of these index fossils, it must be within the age range of the index fossil.

Relative dating

Absolute Dating

Absolute dating employs radiometric dating methods to determine the precise age of a rock or fossil.

This uses radioactive minerals found in rocks and fossils to function as a geological clock. It is frequently much easier to date volcanic rocks than it is to date fossils or the sedimentary rocks in which they are found. As a result, layers of volcanic rocks above and below the fossil-containing layers can often be dated to provide a date range for the fossil-containing rocks.

Relative and Absolute dating

Although most people are familiar with carbon dating, it is rarely applicable to fossils. Carbon-14, the radioactive isotope of carbon used in carbon dating, decays too quickly, with a half-life of 5730 years.

It can only be used to date fossils that are less than 75,000 years old. In contrast, potassium-40 has a half-life of 1.25 billion years and is found in rocks and minerals. This makes it ideal for determining the age of much older rocks and fossils.

Importance of Fossils

  • Fossils show a progression of evolution and provide concrete proof that organisms from the past are not the same as those found today.
  • Paleontologists can infer species lineages by comparing the anatomies of modern and extinct species. This method works best for organisms with hard body parts, such as shells, bones, or teeth.
  • Fossil record tells the story of the past and demonstrates how form evolved over millions of years.
  • Paleontologists, archaeologists, and geologists use the fossil record to place important events and species in the correct geologic era. It is based on the Law of Superposition, which states that the bottom layers in undisturbed rock sequences are older than the top layers. As a result, some newly discovered fossils can be dated using the strata, or distinct layers of rock, in which they were discovered.
  • The study of the fossil record has given valuable information for at least three distinct purposes.
  1. The gradual changes observed within an animal group are used to describe the group’s evolution.
  2. Fossils also allow geologists to quickly and easily assign an age to the strata in which they occur.
  3. Fossil organisms may reveal information about the climate and environment of the site where they were deposited and preserved.


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