Organisms usually need to be covered by mud, sand, tar or some other sediment as soon as possible or frozen or dessicated (dried out) for fossilization to occur.
Material older than about 50,000 years can’t be dated with radiocarbon techniques because too little of the original radioactive material remains.
Modern labs can measure samples as small as 100 mg (0.003 oz) to a precision of ±16 years (Radiocarbon Web Info) Most fossil are dated by decay of isotopes such as Uranium 235, Potassium 40, and Rubidium 87 that have much longer half-lives than carbon 14.
These isotopes aren’t found in the fossils themselves, but in the rock encasing the fossils Geologists in the International Commission on Stratigraphy make proposals based on the most recent studies to constantly update the geologic time scale of the world’s rocks.
Scientist Olivia Judson provides this good example of what happens to an adult male gorilla in the tropical rainforests of the Congo; "An adult male gorilla— all 330 pounds of him—will be reduced to a pile of bones and hair within 10 days of his death.
What scavengers like vultures and hyenas leave behind, flies, ants, worms, and bacteria quickly consume.
Within three weeks, there will be nothing left but a few small bones." A fossil normally preserves only a portion of an organism, usually that portion that was partially mineralized during life, such as bones and teeth.
Trace fossils are the marks left by a living organism, such as feces, footprints or impressions of feathers or leaves.
The oldest uncontested fossils on earth are 2 billion year-old stromatolites in Minnesota, Wisconsin, and Ontario.
Composed of layers of sediments laid down by colonies of cyanobacteria, stromatolites still exist, but are quite rare today.