Fossils are often thought to be just preserved remains of animals from prehistoric time periods. However, the term 'fossil' also includes impressions, traces, droppings, footprints and various other biological evidence/records that animals have left behind that have been preserved over time. All over the world fossils have been found in abundance, and they have helped us shape what the world might have been during the time of our ancestors and even millions of years before the first humanoid creature walked on Earth.
Fossils are very important in understanding the geological and biological past of our planet, and ultimately evolution. The theory of evolution would have never come into existence if fossils were not found. Just like the bio diversity of today, fossils comprise of lifeforms ranging from micro-organism to large trees and dinosaurs. But compared to the the zillions of lifeforms that had ever lived, the lifeform to fossil ratio is very, very small. Why so? Let's take a look at the various ways fossils and just why they are so rare compared to the innumerable lifeforms that have lived over time.
Freezing/Refrigeration: One of the best but rarest methods of fossilization is through freezing. Many mammoths, woolly rhinoceros, and other creatures from Pliocene Era to as recent as 4500 BCE have been found perfectly preserved in permafrost layers. Although organisms found frozen are preserved almost perfectly, i.e. almost no cellular disintegration (some have been found with food still in their mouth and digestive system which would indicate flash freezing), this is still one of the rarest incidents of fossilization. Furthermore, specimens would need to be constantly frozen over a long period of time to retain their cellular properties.
One very important factor that decides how well preserved a frozen fossil is found, however, is at what time after the death of the animal did the freezing process begin. The more time lapse between the death and the beginning of the freezing process, the less of the animal will be frozen and thus preserved.
Sedimentation/Perminerilization: Perhaps the most common process of fossilization, organisms would need to be near soft, mineral rich soil sources for this to take place. It usually occurs after an organism is buried under mud or soil (a dinosaur buried under a mud-slide would be a good candidate). Minerals fill empty spaces inside the organisms such as parts of the body filled or high in gas or water content and crytalize. Due to this very small scale fossilization can be possible using this process such as within the cell wall of a plant call and can result in very detailed fossils.
The composition of sediments present have a huge influence on how the fossil would ultimately turn out. Fine grained particles, such as clay allow more detail, while larger, more coarse particles such as sand, don't allow as such. The chemical composition of the sediments also decide what color the fossil will ultimately turn out to be.
It is important for the organism to be covered by sediments soon after its death or atleast after the initial decaying process. The ultimate result of fossilization depend on which parts are decayed initially before being covered by sediment. Most of the time, only bones, teeth, shell and other hard parts are found, however, feathers, skin or even soft tissue is not uncommon either. However, over millions of years as the layers of sediments pile up over the fossil, due to the pressure, and mineral content, they might even ultimately dissolve away the hard parts of the animal itself. Ultimately what we will have is a rock in an exact shape of the original lifeform. However, finding original parts just covered in hardened rocks is common as well.
Carbonization: All life forms on earth are carbon based. This is why when a lifeform dies, it gives out various gases while decomposing, including carbon. When a fossil is formed through carbonization, almost all of the original creature decomposes, leaving behind carbon traces and this creates an impression of the organism in the surrounding rock, often in great detail.
Resin/Amber: Fossil resin is a type of polymer found globally in many different layers of rock. Certain plants excrete resin as an evolutionary protective measure against fungi, bacteria, and other things that might damage the plant. Animals (usually small, mostly invertebrates) unlucky enough to get stuck in the sticky resin get engulfed and die. These animals are often found almost perfectly preserved in amber, often with traces of their DNA still intact. The sticky resin eventually hardens and when the plant dies and gets fossilized, it is preserved with or without the original plant it was a part of.
Now that we have looked at a few possible ways fossilization occur, let's take a look at why fossilization is rare. A very simple rule of nature is, it has a way of recycling all organisms. Fossilization is a process which itself goes against natural recycling. When a animal dies, every part of it is consumed by another organism. Larger predators consume larger, edible parts, and whatever is left is consumed by smaller organism, such as maggots and even bacteria. Infact bacteria is able to decompose the sturdiest of bones.
On top of that, a dying organism would have to be very near a possible fossil forming site, like swamps, mud-water, or in a sub-zero temperature. The odds of everything falling into place, that is, escape from possible predators, and being located at such a site is slim, Most of Earth was not, and still is not covered with ice or soft, mineral rich sediments.
But regardless, it is still a natural miracle for an animal or its remains to be preserved over the millions of years for us to see today. If it weren't for fossils we would never have known about evolution, or about how the world was long before we walked on it. Even though so many fossils have been found worldwide, they are nothing compared to the the sheer number of animals in the billions and billions that have lived over time and lets us appreciate the flora and fauna of planet Earth, and ofcourse Earth itself, as we know it.