All About Fossils

How are fossils created and preserved?

Did every animal or plant that ever lived become a fossil, or leave a footprint, or draw a detailed map to its house? No! Most were eaten by someone bigger, or sometimes even by someone much littler, like bacteria. Sometimes chemicals or even just the wind and rain did the trick. So how come we find any fossils at all?

Luck in finding fossils = how the critters (or plants, or anything else) were deposited and preserved in the first place. It's that simple.

This section is still under development. Thank you for your patience.

  Step One: Preservation

GETTING BURIED: Only preservation - how a fossil is protected in the earth - lets us find any fossils at all. When a Triceratops died in the Cretaceous period, it might have been well preserved if it fell into a creek and became buried by sediment. And if the sediment sealed out big and little scavengers, and if it had the perfect mix of chemicals. And if no flood washed everything away or broke the carcass into little pieces. And if no passing T. rex wandered by and had a little snack. Basically, if conditions were just right, the evidence would be preserved.

After that, years would pass, and then centuries, and then millions of years. Enough rain would fall, washing more and more sediment on top, and burying the fossil deeper and deeper. Over all that time, the sediments around the Triceratops finally would harden into rock.

GETTING UN-BURIED: Over millions of years more, exactly the opposite process would have to happen for anyone to be able to find this fossil. Glaciers or rain, or perhaps a river, would have to cut through those outer rock layers, until one day some edge or tip or corner of a bone, maybe a horn or snout or tail, would poke out of the ground. Even with good preservation, a Triceratops horn you find today may be chipped and cracked and broken. It's almost too incredible to believe, but it is still there. When you think about it, it's absolutely amazing that so many fossils are preserved!

Good preservation is not the only requirement needed for our Triceratops to be collected and studied. Someone like you has to find it first. If no one ever sees it, eventually the same elements that uncovered that horn or snout or tail will break it down to dust.

It's always a surprise and a mystery when you see a bit of bone poking out of the ground. Is this piece the last bit, the only tiny morsel left of a huge animal? Or is it the first bit - just hinting at the rest of the fossil lying safely underground, still waiting for you?

  Step Two: Fossilization

Most of the time, any fossil bone we find is actually much heavier than it was when the dinosaur was alive. Why? Because of something called fossilization, the process that turns a plant or animal into a fossil after it's been buried.

When our Triceratops waws sandwiched underground, he was not alone. Bacteria and creepy-crawlies wandered through his carcass, consuming his flesh and leaving just his skeleton. Then, water filled with heavy minerals flowed through the ground and seeped into his bones. Minerals love to grow inside any open spaces in bone, filling them and making the bones heavy. The bones are not actually turned to stone, or petrified, because they still contain leftovers that were in the living animal.

Pathologies are how a scientific detective learns about the life and times of a dead creature. They are healed injuries whose effects on bones provide a map of an event. A broken bone. A bite mark. A muscle scar. From these pieces of evidence, scientists can recreate events, measure strength, identify killers, and sketch out hypotheses about behavior.

  Ancient Life and Environment

Scientists find clues to the past, and then put together a "picture" of what animals and plants lived at various times. By collecting carefully all associated specimens, they are able to not only describe each "resident" of an area, but also how everyone stood in the food chain.

Sometimes the associations seem clear: in Mongolia, a Velociraptor and Protoceratops were killed together, maybe in a sudden sandstorm, right in the middle of a fight. The Protoceratops is biting with its beak-like jaws on the Velociraptor's right hand, and the raptor's hind claw is slicing where the Protoceratops's stomach would have been. At first, we might assume that the Velociraptor was picking a fight, maybe just catching some prey; but it might not be so simple. Maybe either creature was caught sneaking up to eat the other's eggs. Or maybe one surprised the other, and it got scared and attacked. It looks like a dinner date, but we can't be positive.

  Geologic Timeline
 Simplified Geologic Time Scale

*MYA=Millions of Years Ago


Tracing the path of dinosaurs through the millennia is one of paleontology's biggest contributions to science. Evolution says that animal species change through time, and eventually change into entire new species. Whether slowly or suddenly, all species today came from other species from the past, and all species are linked together this way. Animal One leads to Animal Two, and then to Animal Three, and so on.

In 1859, Charles Darwin came up with the basic ideas of evolution, but he wasn't the only one who ever talked about how animals lined up after one another on the family tree. Stephen Jay Gould and Niles Eldredge also did it, and by introducing the concept of Punctuated Equilibrium, they explained why the fossil record wasn't as neat and tidy as Darwin thought it would be.

Darwin said that life evolved from one animal to the next through time, in small, smooth steps. Punctuated Equilibrium stated that while sometimes animals evolved gradually, other times changes happened quickly. In fact, sometimes a descendant lived at the same time as its ancestor. The fossil record also showed several times in geologic history where big chunks of animal and plant life on Earth just stopped - and new, different life began.

The geologic ages, like the Cretaceous and the Jurassic, are defined based on these sudden changes, which are called 'extinction events.' Scientists thought extinctions occurred because of natural disasters or other global changes in ecology. Each time lots of animals died, there was room for new animals to develop. Extinction is probably one of the biggest driving forces of evolution.


Dinosaur extinction has caused plenty of arguments in paleontology. Over one hundred ideas and hypotheses have been discussed. Some of them were impossible to prove, like what if strange, new plants didn't agree with dinosaur guts—causing all the dinosaurs to fill with gas and explode.

Another idea was that mammals ate all the dinosaur eggs. That's another one that's hard to prove, but we can say that so far no mammal has been found in a dinosaur nest. No dinosaur eggs - drained of their contents - have been found with mammal teeth marks. It's not impossible, but so far, no evidence. If we did find evidence like this, the next question for scientists would be: What would have caused those mammals to suddenly change their diets to dinosaur eggs only?

There's one hypothesis that many scientists think has the most evidence. It would explain why everybody who weighed more than one hundred pounds died. The Chicxulub (pronounced CHICK-sha-lube) crater is half in and half out of the Gulf of Mexico, in the Yucatan Peninsula. It's 110 miles across, and would have been formed when an asteroid collided with Earth. This means a six-mile-wide Cosmic Cannonball plowed into Earth at 40,000 miles per hour, driving seven or eight miles deep into the Earth's crust. It was so big that when it first touched down, its nose was in the dirt, and its rear end was still out in space—outside our atmosphere.

The impact turned the Cannonball into pure energy and spit out all kinds of heat, molten rock, dust, and debris. Scientists have found debris splattered from Mexico to Texas to Haiti to Cuba, and preserved asteroid dust all over what were then all the oceans and on what was then the land in North America. The explosion would have been 50 billion times larger than the atomic bomb that destroyed Hiroshima, Japan, during World War II.

After such a tremendous blast, the sky was black with smoke and dust, and the sun didn't reach the earth for perhaps a year or more. There were earthquakes and forest fires, acid rain and giant tsunamis. Plants died, plant-eaters died, and then predators died. We think that the only creatures that could live through this were animals that didn't have to eat very often, like crocodiles, or small creatures who could find shelter and live off dried plants, buried roots, and the carcasses of everybody else—like small mammals and birds.

When everything calmed down again, and the sun came back out, and plants began to grow, there were no large dinosaurs left. This allowed mammals to grow larger than rats and cats—which were the largest mammals got during the 160-million-year reign of the dinosaurs. This made room for us.