Fossil

A fossil is any preserved remains, impression, or trace of a living thing from a past geological age. This can include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, hair, petrified wood, and even DNA remnants. The study of fossils, called paleontology, helps scientists understand how life on Earth has changed over millions of years.

Fossils can vary greatly in size, from tiny one-micrometre bacteria to huge dinosaurs and trees. They provide important clues about the history of life, showing how different species evolved and lived together. By studying where fossils are found and their age, scientists have been able to create a geological timescale that helps us understand the order in which events happened in Earth's past.

Many processes can lead to the formation of fossils, including permineralization, casts and molds, and carbonization. Fossils are not always the actual remains of an organism; sometimes they are marks left behind, such as animal tracks or feces known as coprolites. These are called trace fossils or ichnofossils, and they give scientists valuable information about how ancient animals lived and behaved.

History of study

Main article: History of paleontology

For a chronological guide, see Timeline of paleontology.

People have been finding and studying fossils for thousands of years. Fossils help us understand how life on Earth has changed over time and are important for learning about evolution. Paleontologists look at fossils to learn how species have evolved and developed.

Ancient people used fossils in many interesting ways. For example, some early humans in Europe used fossil shells as beads for jewelry. The ancient Egyptians kept fossils that looked like bones of animals they admired in temples. In China, fossils were sometimes thought to be dragon bones and used in medicine.

As time passed, people began to understand more about fossils. Scientists like Aristotle realized that fossils found in rocks were actually remains of ancient sea creatures. Later, scholars used fossils to help understand the history of the Earth and its living things. Today, fossils continue to be a key part of learning about the history of life on our planet.

Dating/Age

Main articles: Geochronology and Relative dating

Fossils help scientists understand how life changed over millions of years, but figuring out their exact ages can be tricky. One method, called radiometric dating, works well for rocks older than 50 million years. It measures the decay of radioactive elements to determine a rock’s age. However, this method only works for certain rocks, like volcanic ash.

Scientists often use a technique called stratigraphy, which studies layers of rock. Since newer layers sit on top of older ones, fossils found between dated layers give clues about their age. Special fossils called index fossils, which lived for short periods and are found worldwide, help match rock layers from different places. Even with these tools, dating fossils isn’t always easy, especially when rock layers are broken or eroded.

The fossil record is incomplete because only a small fraction of organisms become fossils. Most fossils are from animals with hard shells or bones, while soft-bodied creatures are rarely preserved. This means our picture of ancient life is missing many pieces.

Sites

Main article: Lagerstätte

Further information: List of fossil sites

Some fossil sites are special because they preserve details of ancient life that usually disappear over time. These sites are called Lagerstätten, which means "storage places" in German. They often form when animals are buried quickly in environments without much oxygen, which helps keep their remains intact. Examples of these amazing sites include the Cambrian Maotianshan Shales and Burgess Shale, the Devonian Hunsrück Slates, the Jurassic Solnhofen Limestone, and the Carboniferous Mazon Creek areas. These places allow scientists to study ancient life in incredible detail.

Fossilization processes

Fossilization is the process by which parts of an organism are preserved in rock over time. One common way this happens is through recrystallization, where original materials like shell compounds change into different crystal forms, such as from aragonite to calcite.

Another process is replacement, where the original material of a shell or bone is slowly replaced by minerals. This can preserve tiny details of the ancient organism. Permineralization is a special type of replacement where minerals fill in empty spaces inside the organism, creating very detailed fossils. Other replacement processes include phosphatization, where organic matter turns into dense, dark-colored minerals, and pyritization, where sulfur and iron create a fossil preservation involving pyrite.

Casts and molds form when the original organism dissolves away, leaving a shape in the rock. If this space later fills with sediment, it creates a cast that looks like the original organism. Adpression refers to fossils formed by compression, where the remains are flattened but still show details of the original organism. Some fossils preserve even soft tissues or cellular structures, which is rare but has been found in some dinosaur fossils.

Types

Index

Main article: Index fossil

Index fossils are fossils used to identify geologic periods. They work because different sediments may contain the same species of fossil. The shorter the time range of the species, the more precisely different sediments can be matched. Rapidly evolving species are especially useful as index fossils.

Trace

Main article: Trace fossil

Trace fossils show evidence of lifeforms through tracks, burrows, and other marks, rather than the remains of the organism itself. They are important because they show animal behaviors and can date back earlier than body fossils of those animals.

Transitional

Main article: Transitional fossil

Further information: List of transitional fossils

Transitional fossils show traits from both an ancestral group and its descendant group. They remind us that taxonomic divisions are human-made and that there is a continuum of variation in life forms.

Microfossils

Main article: Microfossil

See also: Micropaleontology and Protists in the fossil record

Microfossils are tiny fossils that can be seen only with a microscope. They include small organisms or parts of larger ones, such as teeth or spores. Microfossils help scientists understand past climates and are used to date rock layers.

Resin

Main article: Amber

Fossil resin, known as amber, is a natural polymer that can trap and preserve other tiny organisms. These trapped fossils, called inclusions, can include insects, spiders, and even small fragments of DNA, giving scientists a detailed look at ancient life.

Derived or reworked

See also: Zombie taxon

Derived or reworked fossils are fossils that were originally formed long ago but were later moved and redeposited in newer rock layers by erosion.

Wood

Fossil wood is wood preserved in the fossil record. It can be petrified, meaning its structure is turned to stone, or it may just be preserved as wood. Petrified wood often shows details like growth rings.

Subfossil

The term subfossil refers to remains that are not fully fossilized. These can include bones or shells that are thousands of years old but not old enough to be considered true fossils. Subfossils can contain organic material that helps scientists study past environments and climates.

Chemical fossils

See also: Biosignature

Chemical fossils are chemicals found in rocks that show signs of ancient life. These include certain molecules and isotope ratios. Some of the oldest evidence of life on Earth comes from chemical fossils.

Stromatolites

Main article: Stromatolite

Further information: Earliest known life forms

Stromatolites are layered structures created in shallow water by tiny organisms, especially cyanobacteria, trapping and binding bits of sediment. They are some of the oldest fossils we know, dating back more than 3.5 billion years. These structures tell us about the very first life on Earth.

Stromatolites were far more common in the past, especially before complex animals appeared. Their numbers dropped when grazing animals began to eat them, showing that more advanced life had developed over a billion years ago. Even today, similar structures can be found in some places, helping scientists understand how life began and changed on our planet.

Pseudofossils

Main article: Pseudofossils

Pseudofossils are patterns in rocks that look like fossils but are made by natural processes, not by living things. For example, some pseudofossils, like geological dendrite crystals, form when cracks in rocks fill with minerals. Others, such as kidney ore or moss agates, might look like round shapes or plant leaves. Concretions, round nodules found in layers of sediment, were once thought to be dinosaur eggs but are really just natural rock formations.

Astrobiology

Scientists think that special minerals made by living things could help us find life beyond Earth, especially on planets like Mars. These minerals and other signs of life, called biosignatures, are important for understanding how life might start and grow.

In 2014, NASA sent robots called Curiosity and Opportunity to Mars to look for clues about past life. They searched for signs of tiny living things and places where water once existed, which could have been good homes for life. Finding these clues helps scientists learn if Mars ever had or still has life.

Art

Some very old pieces of art may show fossils. One idea is that a vase from long ago might show a fossil of a giraffe, but another study says this is not correct. The shape on the vase looks more like a type of lizard that people in ancient times would have known about.

Trading and collecting

Fossil trading is the buying and selling of fossils, which sometimes happens illegally with fossils taken from important research sites. This can cause the loss of valuable scientific specimens. Fossil collecting, also known as fossil hunting, is when people gather fossils for study, fun, or profit. Both professionals and hobbyists collect fossils because they are important for learning about life in the past.

As medicine

Some cultures have used fossils to help with health problems, often because the fossil looked like the part of the body that needed healing. For example, dinosaur bones were sometimes called "dragon bones" and used in traditional medicine. While these treatments may have made people feel better, they usually did not actually cure anything. However, fossils might have provided some minerals that our bodies need.