Fossilization is a topic that is often poorly understood among the rockhounding community. At nearly every club meeting or field trip, there is always somebody picking up a random rock with quartz veins running through it, and insisting it is a fossilized animal of some sort. Or finding concretions and wondering if they are petrified dinosaur eggs.
I hope that by reading this, you will come away with a better idea of just what fossils are, where to look for them, and how to identify if you have actually found something special.
What is a fossil, exactly?
A fossil refers to the remnants of living organisms which have been preserved in the form of stone. Usually, it refers to the preserved organism itself, though it can also be applied to traces left behind, such as fossilized footprints, burrows, or coprolites.
Fossils are found in sedimentary rocks. Ancient seafloor deposits, sandstone, mudstone, limestone, or volcanic ash deposits -- this is where you want to search. Very VERY rarely, a fossil will be found in metamorphic stone, but only if that stone was originally sedimentary to start with. Usually, when rocks metamorphize, any fossils contained will be so deformed as to be unrecognizable, but occasionally one will retain enough shape to still be identified as such.
You will never find fossils in igneous rocks. [Volcanic ash deposits are considered sedimentary].
Types of fossilization
So how do fossils form? Well, that depends a bit on exactly what type of fossilization has occurred.
- Petrifaction: The original material is slowly replaced by minerals which gradually seep in from the surrounding matrix. The majority of fossil bones and petrified wood are preserved in this manner.
- Molds: These occur when the remains of the organism decays or dissolves away, leaving only an empty cavity in the rock. These can retain a surprising amount of surface detail imprinted on the rock.
- Casts: These are when a mold becomes filled with a mineral such as pyrite, calcite, or agate, which takes the form of the mold.
- Steinkerns: The interior of a shell becomes filled with mud, which hardens, and eventually becomes free of the matrix. Basically, a cast of the inside of a shell.
- Trace Fossils: These are not fossils of the organism itself, but rather some trace of it -- preserved footprints in mud, for example, or nests/burrows, etc.
How to tell if it's a fossil
Does it have clearly identifiable bones/teeth/wood grain/shells? Always remember that hard parts are MUCH more common than soft part fossils. If you think you've found a tooth, you're more likely right than if you think you found a petrified eyeball.
The fossilization process has to race against decomposition. And fossil-forming can be a very long process, sometimes taking thousands of years. The harder and more rot-resistant parts of the organism are what is likely to actually be left, by the time fossilization has fully set in.
Be sure to take a look at the type of rock you found it in, and the geologic setting nearby. Is it sedimentary? Are there other types of fossils found in the area?
And, of course, if you pick it up and have to wonder, "Is this a fossil?" instead of immediately going "Wow, a fossil clam!" ....then it may not be a very good specimen anyway!
If you'd like to learn more about fossils and how to identify them, I highly recommend the following books:
National Audubon Society Field Guide to North American Fossils
Smithsonian Handbooks: Fossils