How Fossils Work


A fossil of a Microraptor from a 130-million year old forest that existed in what is now Liaoning Province, China is displayed at the American Museum of Natural History in New York City. See more dinosaur pictures.
Getty Images

Imagine the scene of a crime that happened years ago. There's not much to go on -- a few bones, an ex­posed rock face and a couple of bullets. But to a crime-scene investigator, these clues tell a story. The size and shape of the bones reveal whether the victim was a man or a woman. Chipped bones indicate the bullet's point of entry. A gouge on the rock face traces the path of a bullet that missed its target.

You can think about fossils in much the same way. They tell a story, just like the clues at the scene of a crime. It may seem odd to imagine Earth as a giant crime scene, but in a way, that's exactly what paleontologists are doing when they search for and study fossils. First, researchers are looking for evidence, much like crime-scene investigators do. Second, paleontologists and forensics experts alike study this evidence to answer, in one way or another, one basic question: What happened?

Advertisement

This question has a lot of angles. One of the most obvious -- what happened to the dinosaurs -- is really one of the smallest. By studying fossils, you can explore questions like:

  • What were the first life forms on our planet?
  • Where did these life forms come from? What happened to them?
  • How has life on Earth changed over time?
  • How has the Earth's climate changed over time?
  • Where have new species of plants and animals come from, and how do they relate to species that died out?

These questions are complex, and to a lot of people, they're extremely important. They attempt to address the history of our planet, including where life came from and how it developed or evolved. Because they provide a physical record of life on Earth, fossils are a great source of insight when studying questions like these.

But fossils don't tell the whole story. They usually preserve only part of an organism -- the part that was hard and sturdy to begin with. There are also trace fossils, which preserve evidence that an organism existed, like tooth marks or footprints, but don't preserve the organism itself. Fossils also form only in very specific conditions, regardless of whether they're petrified, carbonized, mummified, frozen or encased in a substance like tar or amber. For this reason, only a fraction of the organisms that have existed on Earth appear in the fossil record, or the combined total of the fossils discovered on Earth and the information learned from them. On top of that, there are gaps in the fossil record, although the ongoing discovery of new fossils and fossil beds that may one day fill these gaps.

To understand all this, you need to know a little about geology, decomposition and the fossilization process itself. In this article, we'll explore each of these, and we'll delve into exactly what goes on at a fossil dig. We'll start with a look at two of the processes that are central to fossil formation -- building up the Earth's layers and breaking down its waste.

Building Up and Breaking Down: Geology and Decomposition

In Lake Mead National Recreation Area, you can see exposed layers of sedimentary rock from several periods of the Paleozoic area. The Paleozoic era ended before dinosaurs and mammals appeared on Earth.
In Lake Mead National Recreation Area, you can see exposed layers of sedimentary rock from several periods of the Paleozoic area. The Paleozoic era ended before dinosaurs and mammals appeared on Earth.
Photo courtesy U.S. Geological Survey

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].

Advertisement

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:

  • Aerobic bacteria, or bacteria that grow in the presence of oxygen, consume and break down organisms' soft tissues.
  • Scavengers, such as vultures, consume the bodies of dead animals.
  • Insects, like cockroaches and ants, eat and digest plant and animal waste, returning it to the soil as a type of fertilizer.
  • Weather and erosion physically break down waste, but bacteria and other life forms play a larger role in decomposition.

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.

Bone to Stone: Building Fossils

Fossilized eggs on display at the Inner Mongolia Museum in the regional capital of Hohhot
Fossilized eggs on display at the Inner Mongolia Museum in the regional capital of Hohhot

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.

Advertisement

STR/AFP/AFP/Getty Images An 8-million-year-old

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.

Mummies, Tar and Amber

A man holds an insect fossilized in amber during
A man holds an insect fossilized in amber during
BANARAS KHAN/AFP/Getty Images

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.

Advertisement

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.

The Search for Fossils

Paleontologists excavate a 30,000-plus year-old mastodon and the remains of several other extinct animals at the Eastside Reservoir Project near Hemet, Calif.
Paleontologists excavate a 30,000-plus year-old mastodon and the remains of several other extinct animals at the Eastside Reservoir Project near Hemet, Calif.
Getty Images

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.

Advertisement

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.

Excavation and Preparation

Paleontologist Michael Henderson of the Burpee Museum of Natural History in Rockford, Ill., cleans debris away from the jaw bone
Paleontologist Michael Henderson of the Burpee Museum of Natural History in Rockford, Ill., cleans debris away from the jaw bone
Getty Images

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:

Advertisement

  • Removing a fossil from its surroundings takes away its context -- you lose any knowledge of other plant or animal species that were fossilized nearby.
  • Even though they're essentially made of rock, it's easy to damage fossils during excavation. Since some fossil beds contain the bones of lots of animals, it can also be hard to tell which bones belong to which species.
Experts protect a tooth of a Palaeoloxodon excavated
Getty Images

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.

Knowledge from Stone: Studying Fossils

Professor Fernando Novas demonstrates his theory of how birds are directly descended from dinosaurs during a press conference at the National Geographic Society in Washington, D.C.
Professor Fernando Novas demonstrates his theory of how birds are directly descended from dinosaurs during a press conference at the National Geographic Society in Washington, D.C.

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:

Advertisement

  • A sudden increase in the number of fossilized algae can relate to a change in climate and available food sources.
  • Fossilized pollen can reveal the types of trees and other plants that grew during specific periods, even if the plants themselves haven't been fossilized.
  • Differences in the sizes of rings in petrified wood can correspond to changes in climate.
Museum Victoria Research Associate and Monash University PhD student Erich Fitzgerald inspects the skull of a 25-million-year-old fossil from southeast Australia identifying a new family of small, highly predatory, toothed baleen whales with enormous eyes.

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.

This replica fossil of a 90-million-year-old leaf specimen was used to help identify the Woollemi pine, an old, rare species of pine tree.
Getty Images

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.

A collection of fossils believed to be the evolutionary series of man from his earliest existence millions of years ago is pictured in Nairobi's 75-year-old National Museum.

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

Related HowStuffWorks Articles

More Great Links

Sources

  • Edwards, Lucy E. and John Pojeta, Jr. "Fossils, Rocks and Time." USGS. (12/14/2007) http://pubs.usgs.gov/gip/fossils/contents.html
  • Fields, Helen. "Dinosaur Shocker." Smithsonian Magazine. May 2006. (12/14/2007) http://www.smithsonianmag.com/science-nature/10021606.html
  • Hecht, Jeff. "Tyrannosaurus Rex fossil gives up precious protein." New Scientist. 4/12/2007. (12/14/2007) http://www.smithsonianmag.com/science-nature/10021606.html
  • Minnesota Department of Natural Resources. "Fossils - Clues to the Past." (12/14/2007) http://www.dnr.state.mn.us/education/geology/digging/fossils.html
  • Palomar Community College. "Interpreting the Fossil Record." (12/14/2007) http://anthro.palomar.edu/time/time_1.htm
  • Penney, David. "Fossils in Amber: Unlocking the Secrets of the Past." Biologist. Vol. 53, no. 5. October 2006.
  • Protho, Donald R. "Fossils Say Yes." Natural History. Vol.114, issue 9. November 2005.
  • Tarpy, Cliff. "Jewels in the Ash." National Geographic. Vol. 208 issue 2. August 2005.
  • University of California Museum of Paleontology. "Fossil Record of Bacteria." (12/14/2007) http://www.ucmp.berkeley.edu/bacteria/bacteriafr.html
  • University of California Museum of Paleontology. "Fossils: Windows to the Past." (12/14/2007) http://www.ucmp.berkeley.edu/paleo/fossils/
  • University of California Museum of Paleontology. "Learning About the Vendian Animals." (12/14/2007) http://www.ucmp.berkeley.edu/vendian/critters.html
  • University of California Museum of Paleontology. "Plant Fossils and their Preservation." (12/14/2007) http://www.ucmp.berkeley.edu/IB181/VPL/Pres/PresTitle.html
  • University of Florida. "Fossil Preparation and Conservation." (12/14/2007) http://www.flmnh.ufl.edu/natsci/vertpaleo/resources/prep.htm
  • University of Wisconsin-Madison. "Brief Guide to Preparing Fossils with Dental Picks." (12/14/2007) http://www.geology.wisc.edu/~museum/hughes/DentalPicks.html