|At the right you see a fossilized snail shell found in
southwestern Mississippi. My best guess is that it's Platyostoma of Silurian or
Devonian age, which means it may have lived ±360-440 million years ago. It was found
among gravel in the bottom of a deep ravine. That makes sense because geology books tell
me that most of our gravel was washed from atop the Nashville Dome as it rose, and much of
the fossil-bearing rock above the Nashville Dome was of Silurian and Devonian age.
If you're a rank amateur in these matters, probably words like Silurian, and concepts like the Nashville Dome, and gravel washing from atop the Dome as it rose, are very foreign for you. However, maybe you glimpse the fun you can have by mastering these and a few other simple words and concepts. Just think: in a deep gully practically in my backyard, this fossil was found, the fossil of a creature who lived hundreds of millions of years ago... And I can keep that fossil, hold it in my hand when I want to, for it's a common sort of thing. All you have to do is find it and have it...
And such discoveries very often are extremely common! At the left, found in the same general area and under the same circumstances, is a fossil coral from the same time. The fact that it's a coral makes sense because during the Silurian and Devonian ages the area from which our ravine-bottom gravels were washed was covered by a warm, shallow inland sea! These images are about double their natural sizes.
It's not really unusual in many places to find rocks such as the one at the right composed nearly entirely of many kinds of fossils! In that rock, shown about life-size, I think I see crinoid stems, algae, and some other things.
HOW DO FOSSILS FORM?
Several kinds of fossils exist, and they form in different ways. Here is a typical way many but not all fossils are formed:
When a plant or animal dies in the wild, usually it decays and essentially disappears. However, sometimes dead organisms are covered with sediment such as with mud during a flood or when an aquatic organism settles to the water's bottom, or maybe by ash during a volcanic eruption.
The organism thus encased in mineral matter does not decay in the usual way since the decay process requires oxygen, and now air can't get to the dead creature. Over perhaps millions of years then a process takes place by which minerals in the surrounded material seep into the organism's body and gradually replace the tissue. The body's more fragile constituents such as its protein and carbohydrates break down and are largely or completely removed, perhaps dissolved in water. What you end up with then is something that is more rock than dead animal, but it looks somewhat like the living animal did. It's a fossil!
But, again, there are different kinds of fossils, and not all of them are formed like this.
WHAT KINDS OF FOSSILS ARE THERE?
At the right you see "fossil dinosaur footprints" in Utah. Once a dinosaur walked in what appears to be volcanic ash, the ash was buried and eventually hardened to rock, and now we can see the tracks. But, are these tracks really fossils? Many would say that "real fossils" belong to one of the following six categories:
Among the most commonly encountered fossils are pieces of petrified wood such as those shown above, collected from streambeds in southwestern Mississippi. As the picture suggests, petrified wood comes in many colors, textures and forms, depending on the kind of wood and the chemical environment in which the wood was buried. In the picture the largest chunk is a little larger than an outspread hand.
Petrification is another term for permineralization, defined above. Minerals involved in the permineralization of wood include silica, silicates, carbonates, sulfates, sulfides, oxides, and phosphates, though most petrified wood consists predominantly of silica in the form of opal or chert. Most petrified wood becomes permineralized when trees fall and are covered with volcanic material such as ash or tuff, or else the fallen trees are buried beneath sands, silts and muds deposited atop them by rivers and streams.
Often in petrified wood you can clearly see the grain of the former wood.
HOW OLD IS THAT FOSSIL?
To get the most kick from the fossils you find, you need to have some idea what they are, and how long ago they lived. The "what the are" part of that equation can be handled with books and with sites on the Internet. The "how long ago they lived" part can be approached using the geology maps we talk about on our backyards page,
Using a good geology map you can determine the age of the rocks outcropping in the area where you found your fossil, and therefore the age of your fossil (unless your fossil was carried there from someplace else, as was the case with the ones shown above).
Usually when we talk about a fossil's age we don't say "It's 363,000,000 to 409,000,000 years in age," but rather we say something like "It's Devonian" or "It's Cretaceous." Devonian and Cretaceous are names for "periods" in geological history. You can see at a glance geological history's time periods, eras and epochs on our Geological Time Scale Page.
Once you get the hang of finding fossils and using geology maps it's great fun to travel a bit, to areas where rocks of different ages outcrop, and of course those rocks will contain fossils of different ages from the ones around your home. For example, traveling in western Kentucky I found the crinoid stems at the right in rocks deposited during the Mississippian Period, so they are ± 345 to 310 million years old.
Sometimes you find things that look like fossils but they aren't. At the right you see what I'm calling fossilized wormholes, though I may be wrong. I think that maybe worms burrowed through mud, their holes filled with mud of a different texture than the surrounding mud, so now millions of years later the surrounding mud is eroding away faster than the wormhole mud, leaving the wormholes looking like fossils. I know the things shown are not fossil tree roots because the rock is Ordovician limestone in central Kentucky, and our Geological Time Scale Page shows that during that period (490 to 443 million years ago) trees hadn't evolved yet. In fact, no life had moved onto land yet. If you have a better idea for what the image shows, drop me a line.
Sometimes you also find fossilized mudcracks, and ripple marks of the kind made in mud in shallow water. Some concretions look like fossils. In fact, sometimes it's just hard to know what you've found!
You can learn what life on Earth was like during those times at Berkeley University's Web Geological Time Machine.
You can review and buy books about fossils by clicking here.