What Is A Fossil?

A couple of days ago the weather was nice enough that I walked to my son’s kindergarten to pick him up. As we walked home, he declared that he was “collecting nature” and picked stuff up. Pine cones (which he rejected after a minute as being “squishy”) and sticks, things like that. Oh, and a rock. He picked up a largish, flat rock that got him asking if we could eat off a stone one day. “People used to do that,” he assured me. “Can we?”

“Maybe,” I told him, turning the stone over in my hands. He’d asked me to carry it, so he could climb a retaining wall. “But we’ll have to clean it well first.” Then something caught my eye. “Hey, look,” I said. “Fossils!”

His eyes went big, and he scrambled over to see what I was talking about. Impressions of shells in the stone. “My first fossil!” he declared, taking the stone back. “I found my first fossil!” And suddenly, he had to carry the stone, and he told me he wanted to get a paint brush out.

“Why?” I asked, puzzled.

“That’s what paleontologists use to clean stones!” he told me.

At the end, we settled on using an old toothbrush of his to scrub the dirt away. And that is how we found the thing that will be at least two entries for this blog.

What is a fossil?

If you’re reading a blog like this, you probably have a passing familiarity with what a fossil is. If nothing else, you’ll be thinking about dinosaur bones that have turned to stone and then been dug up and assembled to display in a museum. A fairly succinct definition of the word comes from Biology Online, which states:

noun, plural: fossils

(1) Any preserved evidence of life from a past geological age, such as the impressions and remains of organisms embedded in stratified rocks.

(2) The mineralized remains of an animal or plant.


Having the characteristic, or pertaining to the nature, of a fossil.


Fossils include shells, imprints, burrows, coprolites and organically-produced chemicals. The oldest fossils were bacteria that existed 3.8 billion years old. Fossils are once thought of to be all from extinct species until some were found to belong to species that are still living.

Word origin: from Latin fossilus, (something dug up)

In other words, fossils aren’t just bones, and they aren’t even necessarily pieces of living things. The Sam Noble Museum in Oklahoma follows what appears to be a common practice of breaking fossils into three broad categories: body fossils, molds and casts, and trace fossils.

  • Body Fossils are what you usually think of when you think of a fossil, the fossilized remains of an organism (or part of an organism). Usually, it’s just the bones or other hard parts of the animal because the soft tissues rot and/or get eaten.
  • Molds and casts are a type of body fossil, but they aren’t directly the remains of the organism. A mold is rock that has formed around the body fossil, making an imprint of the fossil’s shape. A cast is what you get if new rock forms in that mold, or more commonly if sand or mud fills the inside of a hollow structure (such as a shell) and becomes rock. Interestingly, sometimes you get molds of the soft tissues of the dead animal. When found, these allow insight into what the animal’s skin was like.
  • Trace fossils are indirect signs of a creature, such as footprints or coprolites (fossilized waste) or burrows or nests.

How Do Fossils Form?

I’ll be honest, and tell you right here and now that I’d assumed that there was a whopping one single way that fossils form. The classic “bones get covered and minerals slowly leach into them over time, replacing the calcium in the bone” routine you probably learned about in elementary school (or, if you’re like me, from reading a ton of dinosaur books in kindergarten). This is certainly one way to do it, but it isn’t the only way. The most common methods are petrification and carbonization, but there are most certainly others.

Petrification is the most common variety of fossilization, and (as you might guess) it’s the classic “bone or shell to stone” form of fossilization. It comes in at least two forms, replacement and permineralization.

  • Replacement is the best known of the best known methods, because it’s the one you always hear about. Water dissolves the original hard structures, replacing them with different mineral matter. What kind of mineral matter depends on the mineral content of the water, but calcite, silica, pyrite, and hematite are most common.
  • Permineralization happens when ground water infiltrates microscopic pores and cavities in the hard structure, depositing minerals. This results in a fossil that still has much of the original hard material left behind, mingled with the deposited mineral.

Carbonization is a form of fossilization most commonly found in plants, although animal can do it as well. Under the right conditions the organic substances of the organism decay but the carbon in the organic molecules is left behind in the shape of the fossilized structure.

There’s also a few types of fossil that you might not think of as “fossils”, because they don’t turn to stone. Plants and animals trapped in amber are fossils, for instance. So are the animals that got trapped in tar pits (La Brea being the famous example here in the United States), and the animals that froze and then freeze-dried in the Arctic (such as mammoths). They’re still preserved evidence of life, even though they didn’t turn to stone.

So, that’s a really high level look at fossils. Tune in next time, when we try to work out what fossils he has. Because trust me, my son has been asking.