Sedimentary rocks make up around 75% of the earths surface, found both in and on the Earth’s crust they are formed when sediments like sand and silt collect and harden.
Over the course of millions of years, this process results in thick layers of sedimentary rock. We can see this for ourselves, the Grand Canyon being a great example. Each layer you can see is younger than the one below it and older than the one above it.
As simple as this seems, because the earth is dynamic with our continents resting on plates, which collide, spread apart or slip under one another it can push or bury the varying layers of sedimentary rock. This process will cause some rock formations to have strata. Strata is a layer of sedimentary rock or soil with internally consistent characteristics, distinguishing it from other layers, these may display themselves as vertical stripes or swirls as opposed to the horizontal layers we may expect to see.
Most of the fossils we have discovered to date, including dinosaur skeletons exist because of sedimentary rocks. To become a fossil, bone essentially has to become rock.
Although decomposition will breakdown the organic parts of the bone, inorganic elements or parts made from minerals like calcium have more staying power, remaining long after the organic materials have disappeared, creating a fragile, porous mineral in the shape of the original bone. Although the fossil has the same shape as the original object, chemically it is more like a rock!
Fossils are less diverse than animal or plant life, with only a small percentage of species ever becoming fossils. Because of it only being possible for fossilisation to occur in certain environments the total number of them in existent compared to the number of plants and animals that have ever lived is very small, this can also be attributed in part to specimens having to survive for millions of years through earthquakes, volcanic activity and vast amounts of pressure from surrounding layers of rock.
The above is a very simplified view of the process and burying a body in sediment is far from being the only way for fossils to be created, but it does provide a glimpse into how fossils are formed, providing us with an insight into ancient life often long since passed.