How Fossils Work

If you've read How the Earth Works, you know that the Earth's physical structure has several distinct layers. There's a solid inner core, a molten outer core, a malleable mantle and a solid crust. The crust, the thinnest layer, forms the Earth's surface, and it's where fossils are both formed and found.

The majority of the rocks found in and on the Earth's crust are sedimentary rocks. They form when sediments, like silt and sand, collect and harden. Over the course of millions of years, this process results in thick layers of sedimentary rock. In some parts of the world, such as the Grand Canyon, you can see these layers. Each layer is younger than the one below it and older than the one above it, a concept first described in the 1600s by geologist Nicholas Steno [source: University of California Museum of Paleontology].

This may seem like a tidy, orderly process, but the Earth is dynamic. Its continents rest on plates, which move very slowly in relation to one another. Plates can collide or spread apart, or the edge of one plate can slip under the edge of another. All of this activity can push older layers of rock to the surface while burying others. This is why some rock formations have layers, or strata, that appear as vertical stripes or swirls instead of horizontal layers. It's also why rocks of the same age can be found in vastly different parts of the world -- the movement of the planet's surface has carried these geological formations from place to place. You can learn more about the process in How Earthquakes Work. Environmental effects, like weathering and erosion, can also reveal very old layers of sedimentary rock.

This is a very simplified view of the process, but it demonstrates two key points that are necessary to understanding fossils. One is that sedimentary rock forms the surface of the Earth. The other is that the Earth's movement has a big impact on how and where these rocks appear.

The other thing you need to know is that the Earth is very efficient at getting rid of waste. Living organisms decompose after they die. Although some people imagine decomposition as a natural process that takes place without outside influences, there are a lot of factors at work that keep the planet from being buried in waste. Here are some of the components of the Earth's cleanup crew:

But how does all this apply to fossils? First, although there are exceptions, most types of fossils form in sedimentary rock. Second, all types of fossilization involve protecting a dead organism from agents of decomposition. Next, we'll take a closer look at how these two factors work together and why fossilization is more likely to happen in the water than on land.

Most of the dinosaur skeletons you see in museums exist because of sedimentary rocks. These fossils got their start when a dinosaur died in an environment that had lots of moving sediment, like an ocean, riverbed or lake. One such place is the benthic zone -- the deepest part of a body of water. This sediment quickly buried the dinosaur, offering its body some protection it from decomposition. While the dinosaur's soft parts still eventually decomposed, its hard parts -- bones, teeth and claws -- remained.

But a buried bone isn't the same thing as a fossil -- to become a fossil, the bone has to become rock. The organic parts of the bone, like blood cells, collagen (a protein), and fat, eventually break down. But the inorganic parts of the bone, or the parts made from minerals like calcium, have more staying power. They remain after the organic materials have disappeared, creating a fragile, porous mineral in the shape of the original bone.

Other minerals reinforce this bone, burning into a fossil. Water gradually makes its way into the bone, carrying minerals like iron and calcium carbonate picked up from the surrounding sediment. As the water penetrates the dinosaur's bones, some of these minerals precipitate into their microscopic pores. As this process continues, the bone becomes more and more rocklike. It's like filling a sponge with glue -- the sponge's physical structure stays the same, and the pores and pockets within it fill up. The glue makes the sponge sturdier and more resistant to damage. Large, thick bones, which have more room for mineral glue, make better fossils than small, flat bones.

Over the course of millions of years, the sediment around these reinforced bones becomes sedimentary rock. Erosion, tides and other natural processes continue to deposit more sediment, and this sediment becomes rock, too. As long as they can withstand the pressure from the surrounding rock, the bones remain safely hidden and preserved. After millions of years, some natural process, like the gradual shifting of the planet's surface, can reveal these layers of rock and the fossils they contain.

Sedimentary rock can also hold trace fossils, which record an organism's behavior. Some of the most well-known trace fossils are trackways, or the tracks of extinct animals. These form when an animal leaves its prints in soft but sturdy soil, which creates a mold. This mold fills with sediment, and both the mold and its filling harden over millions of years. Forces like erosion remove the upper layers of rock, revealing the preserved footprints underneath.

Sediment can also fill the mold and harden into a cast, or a reproduction of the foot that made the print. This can happen with other traces, too, like burrows and tunnels. Some other trace fossils include coprolites (fossilized dung) tooth marks on bones or wood, and nests. Sediment can even preserve plant life. Plants can make impressions in hardening sediment or become petrified wood after going through much the same process as fossilized dinosaur bones do.

This type of fossilization creates sturdy fossils, but it can only take place in specific conditions. Next, we'll look at the other ways life forms can be buried, encased or otherwise protected so their remains last millions of years.

Burying a body in sediment isn't the only way to make a fossil. In fact, some of the world's most dramatic fossil finds haven't involved sedimentary rock at all. Here's a look at some of the other natural methods for preserving the remains of living things.

If an animal dies in a dry, protected location, like an arid cave, its remains can dry out, or desiccate. These fossils are sometimes known as mummified fossils, although they haven't gone through the sort of process used to preserve Egyptian mummies. Instead, it's a little like dehydrating fruit or meat -- removing the water from the body makes it inhospitable to bacteria, so the remains last longer. Desiccation can preserve an organism's skin and soft tissues, which fossilization in sediment usually can't.

Another form of fossilization that can preserve an animal's entire body is freezing. As with desiccation, freezing temperatures can slow down the rate at which bacteria can invade and break down a body. A thick layer of ice or frozen soil can also deter predators. Researchers have discovered well-preserved mammoth bodies in frozen tundra and icy crevasses. Sometimes, these bodies still have their skin, hair and organs intact, giving paleontologists a more complete idea of the animal's appearance and its physiology. Freezing can preserve specimens well, but often not quite as well as desiccation.

When an animal becomes trapped in naturally-occurring tar or paraffin, its whole body can be preserved. While paraffin and other waxes can preserve an animal's soft tissue, substances like tar preserve only hard parts. A good example of this is the mammals and plants preserved in the La Brea Tar Pits in Los Angeles, Calif. The bones excavated from tar pits are often dark brown -- they've absorbed the tar through their pores. Tar and paraffin can preserve plants as well. Some life forms, including humans, have also been preserved in peat, which is composed mostly of decomposing mosses.

When an insect lands in tree resin, insects, plant debris and pollen can become encased in tree resin. The volatile components of the resin evaporate over thousands of years. First, it becomes a hard substance known as copal, and as all of the volatile compounds disappear, it turns into a hard, inert material called amber. These specimens are very useful, since they preserve the fossil's entire physical structure. Amber can also contain bubbles of water, air and gas.

All of these types of fossils, and the bones preserved in sedimentary rock, can give scientists a lot of insight into how life has developed on the planet. But paleontologists can only study what they can find. Read on to learn about how scientists find and recover fossils.

If you wanted to study an animal in the wild, you'd start by finding its habitat -- you'd travel to Australia to study kangaroos or to China to study koalas. The same is true for fossils. If you were looking for frozen mammoths or other mammals, you'd search glaciers, icy crevasses and tundra. To find fossils in sedimentary rock, you'd hunt for layers of rock that are the same age as the fossils you want to study. To do this, you'd consult a geologic map, which displays the locations, features and ages of rock formations.

Cartographers use data sources like aerial photographs and surveys to determine the locations and features. The rocks' ages come from radiometric dating. You may have heard of one type of radiometric dating -- carbon-14 dating, which scientists often use to determine the age of archaeological artifacts. Like all methods of radiometric dating, carbon-14 dating determines the age of a sample by evaluating the radioactive decay of specific atoms in the sample. The atoms being measured are isotopes -- atoms that are identical except for the number of neutrons in their nuclei. You can read more about this process in How Nuclear Radiation Works. The isotopes used to determine the age of rocks shed their extra neutrons until they become stable, and scientists measure the proportions of the two isotopes.

Carbon-14 dating isn't used to measure the age of fossils because its half-life -- the amount of time it takes for half of the atoms in the sample to decay -- is too short. Carbon-14 dating can determine the age of samples up to about 60,000 years old, but many layers of rock and the fossils they contain are millions or billions of years old. To determine the ages of these samples, scientists measure other radioactive isotopes, like potassium-40 and uranium-238, which are found in neighboring igneous rock. Each of these isotopes has a half-life of more than a billion years, compared to carbon-14's half-life of only 5,730 years.

So if you wanted to look for the bones of a Tyrannosaurus rex, you'd look for exposed rock that's around 65 million years old. If you wanted to find a trilobite, like the ones pictured above, you'd need much older rock -- it would have to be more than 245 million years old. And if you wanted to study the cyanobacteria that make up some of the oldest known life on Earth, you'd need to find rock that's about 3.5 billion years old.

Once you find the right rock, finding a fossil requires luck and a good eye. Fossilization is a relatively rare occurrence, so you can spend lots of time scouring a likely formation without finding any bones, prints or impressions. If you do find a fossil, the next step is excavation and preparation -- we'll look at what it takes to separate bone from rock on the next page.

In order to become a fossil, an organism has to die in an environment that encourages fossilization. This doesn't happen very often, and it happens only in certain environments. For this reason, the total number of fossils is extremely small compared to the number of plants and animals that have ever lived. Fossils are also far less diverse than plant and animal life -- only a small percentage of species ever become fossils. In addition to that, fossil specimens have to survive for millions of years, withstanding earthquakes, volcanic activity and the immense pressure of surrounding layers of rock.

For this reason, each fossil specimen can be important -- it has the potential to add to scientific knowledge about life on Earth. This is especially true if the fossil is of a vertebrate animal, or one with a spine. When amateur paleontologists find fossils of vertebrates, their best bet is to contact a museum or research facility for assistance. There are several reasons for this:

For paleontologists, excavating a fossil is a slow, careful process. Although an excavation team can use large tools and cranes to remove an entire skeleton in one large slab, removing the bones from the surrounding rock takes time and patience. For this reason, when working with large skeletons or bones, paleontologists often remove large specimens, encase them in plaster and ship them to a research facility for easier study.

Working from the exposed bone surfaces to the unexposed surfaces, paleontologists slowly flake away the rock matrix that surrounds the bone. This may sound difficult, but there's a plane of weakness between the bone and the rock. The rock will tend to break along this plane with the help of tools, like paintbrushes and dental picks. Paleontologists can also mist the rock with water to soften the sediment.

Sometimes, the fossilized bone is brittle -- so brittle that the removal process could cause it to shatter or break. When this happens, researchers will reinforce the bone with a thin glue or resin. This liquid soaks into the bone, reinforcing its structure. This step requires lots of care, since the glue can permanently attach flakes of sediment or dust to the bone.

Once the fossil is removed from the rock, scientists can determine its age using a mass spectrometer, measuring isotopes for radiometric dating. Another technique is to compare the fossil to other samples with known ages. Other tools include computerized tomography (CAT) scans and computer models. When it comes to vertebrate animals, paleontologists can also approach the skeleton like a giant jigsaw puzzle, trying to figure out exactly how the bones fit together to determine how the animal lived and moved.

The overall goal for all of this is to learn something about life on Earth. Next, we'll explore a few of the things that the study of fossils can reveal.

The study of fossils is as much about relationships as it is about the study of specific plants and animals. When researchers look at a layer of rock, they look at all the fossils contained there, determining which species lived at the same time. By looking at neighboring rock layers, researchers can eventually determine how life has developed over the billions of years of the Earth's history. All these discoveries add up to create the fossil record -- the total collection of all the known fossils on Earth.

These relationships can give scientists a lot of clues about how life has changed over time. Here are some examples:

Other relationships can be more controversial. Scientists can use the relationships between fossils of different periods to support the theory of evolution. For example, a paleontologist might study the fossilized remains of prehistoric horses to determine how they relate to modern horses. The similarities between some dinosaur bones and the bones of today's birds suggest that some dinosaurs eventually evolved into birds.

Transitional fossils, or fossils that display the characteristics of more than one type of animal, can also support the theory of evolution. For example, the skull shown above is a 25-million-year-old fossil from a baleen whale. But unlike today's baleen whales, this one had sharp teeth. It appears to be an intermediate stage between extinct whales, which had legs and teeth, and today's whales.

Scientists can also use fossils to identify species of plants and animals that exist today. Researchers identified the species of pine tree shown at the right with the help of a 90-million-year-old fossil impression.

And fossils can even help researchers understand human life. Fossil specimens reveal many human-like ancestors that lived millions of years ago. The skulls pictured below are from a variety of ancient human ancestors, and they demonstrate how the shape of the skull, which relates to the size and structure of the brain, may have changed as humans developed.

These and other fossils have made massive contributions to life on Earth, and scientists continue to make new discoveries. Some of the newest finds come from newly-excavated fossil beds in China. One such bed is in Liaoning Province in northeast China. By 2005, researchers had excavated samples from 90 vertebrate species, 300 invertebrate species and 60 plant species. Some of these discoveries are filling holes in the fossil record, while others are supporting scientists' existing theories -- such as the idea that some dinosaurs had feathers.

On the next page, you'll find lots of links on fossils, dinosaurs, paleontology and related subjects.