Paleontology is the study of the history of life on Earth as based on fossils. Fossils are the remains of plants, animals, fungi, bacteria, and single-celled living things that have been replaced by rock material or impressions of organisms preserved in rock. Paleontologists use fossil remains to understand different aspects of extinct and living organisms.

Individual fossils may contain information about an organism's life and environment. Much like the rings of a tree, for example, each ring on the surface of an oyster shell denotes one year of its life. Studying oyster fossils can help paleontologists discover how long the oyster lived, and in what conditions. If the climate was favorable for the oyster, the oyster probably grew more quickly and the rings would be thicker. If the oyster struggled for survival, the rings would be thinner. Thinner rings would indicate an environment not favorable to organisms like the oyster—too warm or too cold for the oyster, for example, or lacking nutrients necessary for them to grow.

Some fossils show how an organism lived. Amber, for instance, is hardened, fossilized tree resin. At times, the sticky resin has dripped down a tree trunk, it trapped air bubbles, as well as small insects and some organisms as large as frogs and lizards. Paleontologists study amber, called "fossil resin," to observe these complete specimens. Amber can preserve tissue as delicate as dragonfly wings. Some ants were trapped in amber while eating leaves, allowing scientists to know exactly what they ate, and how they ate it. Even the air bubbles trapped in amber are valuable to paleontologists. By analyzing the chemistry of the air, scientists can tell if there was a volcanic eruption or other atmospheric changes nearby.

The behavior of organisms can also be deduced from fossil evidence. Paleontologists suggest that hadrosaurs, duck-billed dinosaurs, lived in large herds, for instance. They made this hypothesis after observing evidence of social behavior, including a single site with approximately 10,000 skeletons.

Fossils can also provide evidence of the evolutionary history of organisms. Paleontologists infer that whales evolved from land-dwelling animals, for instance. Fossils of extinct animals closely related to whales have front limbs like paddles, similar to front legs. They even have tiny back limbs. Although the front limbs of these fossil animals are in some ways similar to legs, in other ways they also show strong similarities to the fins of modern whales.

Subdisciplines of Paleontology

The field of paleontology has many subdisciplines. A subdiscipline is a specialized field of study within a broader subject or discipline. In the case of paleontology, subdisciplines can focus on a specific fossil type or a specific aspect of the planet, such as its climate.

Vertebrate Paleontology

One important subdiscipline is vertebrate paleontology, the study of fossils of animals with backbones. Vertebrate paleontologists have discovered and reconstructed the skeletons of dinosaurs, turtles, cats, and many other animals to show how they lived and their evolutionary history.

Using fossil evidence, vertebrate paleontologists deduced that pterosaurs, a group of flying reptiles, could fly by flapping their wings, as opposed to just gliding. Reconstructed skeletons of pterosaurs have hollow and light bones like modern birds.

One type of pterosaur, Quetzalcoatlus, is considered one of the largest flying creatures in history. It had a wingspan of 11 meters (36 feet). Paleontologists have competing theories about if and how Quetzalcoatlus flew. Some paleontologists argue it was too heavy to fly at all. Others maintain it could distribute its weight well enough to soar slowly. Still, other scientists say Quetzalcoatlus was muscular enough to fly quickly over short distances. These theories demonstrate how vertebrate paleontologists can interpret fossil evidence differently.

Invertebrate Paleontology

Invertebrate paleontologists examine the fossils of animals without backbones—mollusks, corals, arthropods like crabs and shrimp, echinoderms like sand dollars and sea stars, sponges, and worms. Unlike vertebrates, invertebrates do not have bones—they do leave behind evidence of their existence in the form of fossilized shells and exoskeletons, impressions of their soft body parts, and tracks from their movement along the ground or ocean floor.

Invertebrate fossils are especially important to the study and reconstruction of prehistoric aquatic environments. For example, large communities of 200-million-year-old invertebrate marine fossils found in the deserts of the Nevada, in the United States, tell us that certain areas of the state were covered by water during that period of time.

Paleobotany

Paleobotanists study the fossils of ancient plants. These fossils can be impressions of plants left on rock surfaces, or they can be parts of the plants themselves, such as leaves and seeds, that have been preserved by rock material. These fossils help us understand the evolution and diversity of plants, in addition to being a key part of the reconstruction of ancient environments and climates, subdisciplines known as paleoecology (the study of ancient environments) and paleoclimatology (the study of ancient climates).

At a small site in the Patagonia region of Argentina, paleobotanists discovered the fossils of more than 100 plant species that date back about 52 million years. Prior to this discovery, many scientists said South America's biological diversity is a result of glaciers breaking up the continent into isolated ecosystem "islands" two million years ago. The Patagonia leaf fossils may disprove this theory. Paleobotanists now have evidence that the continent's diversity of plant species was present 50 million years before the end of the last Ice Age.

Some plant fossils are found in hard lumps called coal balls. Coal, a fossil fuel, is formed from the remains of decomposed plants. Coal balls are also formed from the plant remains of forests and swamps, but these materials did not turn into coal. They slowly petrified, or were replaced by rock. Coal balls, found in or near coal deposits, preserve evidence of the different plants that formed the coal, making them important for studying ancient environments, and for understanding a major energy source.

Micropaleontology

Micropaleontology is the study of fossils of microscopic organisms, such as protists, algae, tiny crustaceans, and pollen. Micropaleontologists use powerful electron microscopes to study microfossils that are generally smaller than four millimeters (0.16 inches). Microfossil species tend to be short-lived and abundant where they are found, which makes them helpful for identifying rock layers that are the same age, a process known as biostratigraphy. The chemical makeup of some microfossils can be used to learn about the environment when the organism was alive, making them important for paleoclimatology.

Micropaleontologists study shells from deep-sea microorganisms in order to understand how Earth's climate has changed. Shells accumulate on the ocean floor after the organisms die. Because the organisms draw the elements for their shells from the ocean water around them, the composition of the shells reflects the current composition of the ocean. By chemically analyzing the shells, paleontologists can determine the amount of oxygen, carbon, and other life-sustaining nutrients in the ocean when the shells developed. They can then compare shells from one period of time to another, or from one geographic area to another. Differences in the chemical composition of the ocean can be good indicators of differences in climate.

Micropaleontologists often study the oldest fossils on Earth. The oldest fossils are of cyanobacteria, sometimes called blue-green algae or pond scum. Cyanobacteria grew in shallow oceans when Earth was still cooling, billions of years ago. Fossils formed by cyanobacteria are called stromatolites. The oldest fossils on Earth are stromatolites discovered in western Australia that are 3.5 billion years old.

History of Paleontology

Throughout human history, fossils have been used, studied, and understood in different ways. Early civilizations used fossils for decorative or religious purposes, but did not always understand where they came from.

Although some ancient Greek and Roman scientists recognized that fossils were the remains of life forms, many early scholars believed fossils were evidence of mythological creatures such as dragons. From the Middle Ages until the early 1700s, fossils were widely regarded as works of the devil or of a higher power. Many people believed the remains had special curative or destructive powers. Many scholars also believed that fossils were remains left by Noah's flood and other disasters from the Hebrew holy book.

Some ancient scientists did understand what fossils were, and were able to formulate complex hypotheses based on fossil evidence. Greek biologist Xenophanes discovered seashells on land, and deduced that the land was once a seafloor. Remarkably, Chinese scientist Shen Kuo was able to use fossilized bamboo to form a theory of climate change.

The formal science of paleontology—fossil collection and description—began in the 1700s, a period of time known as the Age of Enlightenment. Scientists began to describe and map rock formations and classify fossils. Geologists discovered that rock layers were the product of long periods of sediment buildup, rather than the result of single events or catastrophes. In the early 1800s, Georges Cuvier and William Smith, considered the pioneers of paleontology, found that rock layers in different areas could be compared and matched on the basis of their fossils.

Later that century, the works of Charles Lyell and Charles Darwin strongly influenced how society understood the history of Earth and its organisms. Lyell's Principles of Geology stated that the fossils in one rock layer were similar, but fossils in other rock layers were different. This sequence could be used to show relationships between similar rock layers separated by great distances. Fossils discovered in South America may have more in common with fossils from Africa than fossils from different rock layers nearby.

Darwin's On The Origin of Species observed somewhat similar sequencing in the living world. Darwin suggested that new species evolve over time. New fossil discoveries supported Darwin's theory that creatures living in the distant past were different from, yet sometimes interconnected with, those living today. This theory allowed paleontologists to study living organisms for clues to understanding fossil evidence. The Archaeopteryx, for example, had wings like a bird, but had other features (such as teeth) typical of a type of dinosaur called a theropod. Now regarded as a very early bird, Archaeopteryx retains more similarities to theropods than does any modern bird. Studying the physical features of Archaeopteryx is an example of how paleontologists and other scientists establish a sequence, or ordering, of when one species evolved relative to another.

The dating of rock layers and fossils was revolutionized after the discovery of radioactivity in the late 1800s. Using a process known as radiometric dating, scientists can determine the age of a rock layer by examining how certain atoms in the rock have changed since the rock formed. As atoms change, they emit different levels of radioactivity. Changes in radioactivity are standard and can be accurately measured in units of time.

By measuring radioactive material in an ancient sample and comparing it to a current sample, scientists can calculate how much time has passed. Radiometric dating allows ages to be assigned to rock layers, which can then be used to determine the ages of fossils.

Paleontologists used radiometric dating to study the fossilized eggshells of Genyornis, an extinct bird from Australia. They discovered that Genyornis became extinct between 40,000 and 50,000 years ago. Fossil evidence from plants and other organisms in the region shows that there was abundant food for the large, flightless bird at the time of its extinction. Climate changes were too slow to explain the relatively quick extinction.

By studying human fossils and ancient Australian cave paintings that were dated to the same time period, paleontologists hypothesized that human beings—the earliest people to inhabit Australia—may have contributed to the extinction of Genyornis.

Paleontology Today

Modern paleontologists have a variety of tools that help them discover, examine, and describe fossils. Electron microscopes allow paleontologists to study the tiniest details of the smallest fossils. X-ray machines and CT scanners reveal fossils' internal structures. Advanced computer programs can analyze fossil data, reconstruct skeletons, and visualize the bodies and movements of extinct organisms.

Paleontologists and biologists used a CT scan to study the preserved body of a baby mammoth discovered in Siberia in 2007. A CT scanner allows scientists to construct 3-D representations of the bones and tissue of the organism. Using this technology, scientists were able to see that the baby mammoth had healthy teeth, bones, and muscle tissue. However, the animal's lungs and trunk were full of mud and debris. This suggested to scientists that the animal was healthy, but most likely suffocated in a muddy river or lake.

Scientists can even extract genetic material from bones and tissues.

Paleontologists made a remarkable genetic discovery when the bones of a Tyrannosaurus rex were broken during an excavation in the 1990s. Soft tissue was discovered inside the bones. Soft tissue is the actual connective tissue of an organism, such as muscle, fat, and blood. Soft tissue is rarely preserved during fossilization. Paleontologists usually must rely on fossilized remains—rocks. Paleontologists now hope to use this rare discovery of 68-million-year-old tissue to study the biology and possibly even the DNA of the T. rex.

Even with all these advancements, paleontologists still make important discoveries by using simple tools and basic techniques in the field.

The National Geographic Society supports fieldwork in paleontology throughout the world. Emerging Explorer Zeresenay "Zeray" Alemseged conducts studies in northern Ethiopia. There, Alemseged and his colleagues unearth and study fossils that contribute to the understanding of human evolution.

Emerging Explorer Bolortsetseg Minjin is a paleontologist who has found fossils of dinosaurs, ancient mammals, and even corals in the Gobi Desert of Mongolia. She also works to teach Mongolian students about the dinosaurs in their backyard, and is hoping to establish a paleontology museum in the country.

Many dig sites offer visitors the chance to watch paleontologists work in the field, including the following U.S. sites: the Gray Fossil Site in Gray, Tennessee; the La Brea Tar Pits in Los Angeles, California; and the Ashfall Fossil Beds in Royal, Nebraska.