Fossils: Formation, Importance, and Different Types

Introduction:

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.

Fig. no.1: 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. Ammonites, for example, are excellent stratigraphic guide fossils; they can be used to determine the relative age of two or more layers of rock, or strata, that are in different locations within the same country or elsewhere in the world.

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

Fig. no.2: Fossil

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.

When living organisms die, not all of them become fossils. In fact, only a very small number do because it takes special circumstances—one or more of the three listed below helps to produce a fossil and allow it to remain alive:

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.

Fig. no. 3: Fossil Formation

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. Just about 30% of the organisms in a typical modern-day shallow-marine environment have a high preservation potential.

However, only a few of these organisms die in a depositional setting where they become fossilized, so fossilization is the exception instead of the rule.

Various Types of Fossils

Perhaps you visualize a dinosaur bone or the imprint of a seashell in rock when you think of a fossil. In fact, 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.

Fig. no.4: 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.

Fig. no.5: 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.

Fig. no.6: 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.

Fig. no.7: Mold and cast fossil

Carbonized Impressions of Bodies:

Impressions are flat molds formed when soft or semisoft organisms or their parts (leaves, insects, invertebrates, sponges, feathers, jellyfish) are pressed between sediment layers. Chemical reactions eventually eliminate the majority of the organic material, leaving only a thin layer of carbon on the impression’s surface.

Fig. no.8: 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.

Fig. no.9: Petrified wood

Trace Fossils:

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

Fig. no.10: 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.

Fig. no.11: Microfossils and macrofossils

The Concept of Extinction

Paleontologists recognized in the 18th century that not all fossils represented the traces of observed living species. However, they implicitly assumed that because the world had not been thoroughly explored, all fossils represented species living somewhere on the planet.

By the nineteenth century, it was clear that this understanding could not be correct because no giant animals, such as mastodons or dinosaurs, had been discovered anywhere. Based on this realization, the French paleontologist Georges Cuvier (1769-1832) contended that some fossil species had become extinct, which meant that all individuals of those species had died. We now take the phenomenon of extinction for granted, having seen numerous animals become extinct throughout history, but Cuvier’s proposal was revolutionary in his day.

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 required 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, 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.

Fig. No.12 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 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.

Evidence for Evolution

Fossils show a progression of evolution and provide concrete proof that organisms from the past are not the same as those found today.

The morphological, or anatomical, record is made up of fossils and the comparative anatomy of modern organisms.

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.

The resulting 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.

The gradual changes observed within an animal group are used to describe the group’s evolution.

Fossils also allow geologists to quickly and easily assign an age to the strata in which they occur.

Furthermore, fossil organisms may reveal information about the climate and environment of the site where they were deposited and preserved.

Summary

• A fossil is a remnant, impression, or trace of an animal or plant from a previous geologic age that has been preserved in the earth’s crust.
• Fossils are commonly found in sedimentary rocks and, on rare occasions, in fine-grained, low-grade metamorphic rocks.
• Permineralization takes place when mineral matter from the ground, lakes, or ocean enters the pores of plant materials, bones, and shells.
• Trace fossils are organisms’ footprints, feeding traces, burrows, and dung (coprolites) left behind in sediment.
• A chemical fossil or biomarker is a distinct chemical derived from an organism and preserved in rock.
• Macrofossils are fossils that are visible to the naked eye.
• Some rocks and sediments are rich in microfossils that can only be seen through a microscope.
• Microfossils are plankton, bacteria, and pollen fragments.
• 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).
• We can learn how long life has existed on Earth and how different plants and animals are related to one another by studying the fossil record.
• Fossil organisms may reveal information about the climate and environment of the site where they were deposited and preserved.