Fossilization

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Fossilization

The only way we know anything at all about prehistoric life is through fossils. Some people refer to specimens of dinosaurs as "dinosaur bones", but in fact, they are not. No organic material can remain unchanged for millions of years. That is why, the only pieces of the past that survive to be looked upon by human eyes, do so as rocks, or fossils, as they are called when they came from living organisms. So really, there are no dinosaur bones left anywhere, just the ones that have been turned to stone.

How do things turn to stone, or become fossilized? First of all, a very small amount of prehistoric life got fossilized. In order for this phenomenon to take place, conditions had to be exactly right. It was just like winning the prehistoric lottery.
Only the hard parts of an organism can become fossilized, such as teeth, claws, shells, and bones. The soft body parts are usually lost, except for in very special conditions.

There are many ways for an organism to get preserved, but I will explain the general way in which most fossils form. First of all, fossils only occur in sedimentary rock, no others. Here is a basic example of what happens when fossilization occurs. An organism, let's just say, a dinosaur, dies. Their flesh and other tissues is probably eaten by carnivorous animals, leaving just the bones.

Let's say, just by chance, that this whole ordeal took place in an area with lots of sand, very near to a river. Let's also say that the bones were left undisturbed just long enough for the wind to blow sand and sediments over top of them, causing them to lie a little bit underground. Over the years, and I mean thousands of years, the sediments slowly pile up over top of the bones, until they are buried far underneath the ground. Let's say the river floods or changes it's course too, and the land over top gets covered in water.

While this is going on, the minerals in the bones, calciums and hydroxyapatite, get replaced, one by one, with the minerals in the sand. Due to the great pressure over top, the lower levels of sediment get pressed together to form sedimentary rock, with the bones still in it. Eventually, millions of years pass by, and there is no organic material left in the bones, they are now solid rock, and are buried deep below the surface, incased in sedimentary rocks. One day, someone is digging deep into a quarry, and notices these bones, now fossils.

At this point I should explain that there are two main types of fossils, organic fossils, and trace fossils. Organic fossils were once part of a living organism, and trace fossils are the prints they left behind, such as footprints, and worm holes.

There are other ways of preservation, too. One of them being petrification. Most people have seen petrified wood. This is how it happened: Long ago, dead logs were washed into a river and buried in the sand. Water with alkaline and dissolved silica went down through the sediments, and contacted the logs. The logs decayed, releasing carbon dioxide, which dissolved in the water and formed carbonic acid. The alkaline water was then neutralized, and the silica precipitated out of the solution. Very slowly, the cellulose of the wood is replaced, molecule by molecule, by the silica. Eventually, the wood is replaced in perfect detail by minerals. If other minerals are there, also, the wood could be stained pretty colours.

Organisms can also be preserved by carbonization. If a leaf falls into a stagnant, oxygen-poor swamp, it may not decay. If it gets covered in silt and subjected to heat and pressure, most of the leaf's organic material is released as methane, water, and carbon dioxide. The remainder is a thin film of carbon, showing the imprint of the leaf. Insects and fish can be preserved in this way too.

Sometimes, specimens came be preserved unaltered. Wooly Mammoths have been frozen in ice for 25,000 years, and when they were thawed, their flesh was still red, and one time it was eaten by dogs. They were preserved in perfect detail, even their last lunch was still in their stomachs.

The remains of thousands of Pleistocene mammals has been found in Los Angeles, California, in tar pits. Apparently, millions of animals had wandered into oil that had seeped up from the ground, and been trapped. Predators had tried to attack them, but they, too, had been trapped. They all met their death in the sticky ooze.

The perfect way for insects to be preserved, is in amber. Amber is the hardened tree sap of ancient trees. Sometimes, insects were trapped in the sap, and when it hardened, they were no longer exposed to air, so their bodies couldn't decompose. They have been preserved this way in such detail that scientists are able to examine them under microscopes.

Like I said before, very few organisms are lucky enough to get fossilized and even less make it to the hands of humans. Most are destroyed by earthquakes, weather, and the molten magma of the Earth. Still, even with the chances being so slim, there are lots of organisms on Earth at any given time, and a very large number of years, so we are still able to find enough fossils to base our research on.

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