What is a fossil?
The vast majority of living organisms leave this world without a trace, never to be seen again.
Every now and then however, under just the right conditions, a once living organism may leave evidence far into the future of its past existence. We call that evidence a fossil. Fossils can take a variety of forms, such as:
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Impressions of once living organisms in sedimentary rock are one of the most common and interesting types of fossils. Many of the fossils in E Joe's collection are of this type. |
How do fossils form in sedimentary rock?
First, the conditions for sedimentary rock formation must be present.
Most land undergoes erosion. The opposite condition, deposition, must be present for a sedimentary rock fossil to form. Where do we find deposition? Most commonly, the particles that are required for deposition are transported by fast moving water. Fast moving water can overcome the force of gravity and transport particles. As water movement slows, the water energy can no longer overcome gravity and particles will be deposited downward (deposition). |
A good example is walking barefoot into the ocean- ever notice that large pebbles are found in the most turbulent surf zone and the farther you walk out into the ocean the finer the sand becomes? That's water and gravity interacting to sift out particles by their size. The largest particles have a heavy pull of gravity, and so require the most water energy to support them. The smaller particles are much more easily transported by water and will only deposit out of the water column in much calmer water that is farther out and away from the surf zone.
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Second, the living organism must escape decomposition after death.
How is it possible for an organism to escape decomposition? It is known that soft-bodied organisms readily decompose after death due to the presence of oxygen and micro-organisms (both of which are abundant and seemingly everywhere). Biological decomposition can be halted if there is an absence of oxygen. Oxygen is abundant in both air and water, so a living organism does not escape decomposition easily. Occasionally a soft bodied organism can be entombed in an oxygen free zone when there is rapid sediment |
deposition, such as during a flood with heavy mud flow.
How about hard bodied components of an organism like bones, teeth and shells? Surely they can escape biological decomposition! Yes, they fare far better in that regard, though the factors that can contribute to sediment deposits (fast moving water) often will lead to the breakdown of these hard bodied components. (Think of a shell worn down from the pounding action of the ocean surf zone.) Scientists call this physical destruction of an organism abiotic decomposition. |
Third, the sedimentary rock formation process is needed to preserve the fossil.
So if an ancient organism can escape decomposition after death and become trapped in sedimentary deposits, there is a possibility that a fossil can be formed. Over an immense passage of time, layer upon layer of sediment will build up and exert extreme pressure on the lower layers. In time, those lower layers will be transformed into sedimentary rock. Now, deep underground, a sedimentary rock fossil has formed. With more passage of time, some of this rock may go through an upheaval |
and be brought back to the surface as part of a new mountain range.
Finally, after a further immense passage of time, erosion may reveal the fossil. If the fossil is discovered when it first emerges, it can be saved. If the fossil is not found, erosion will continue to decompose the fossil until it is no longer recognizable. So if we are observant, persistent and fortunate, an ancient organism can connect with us today. Against all odds, when we hold a fossil in our hand, we see and touch the final parting snapshot of a living organism from millions of years ago. |
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