Carbon dating cannot be used on most fossils, not only because they are almost always too old, but also because they rarely contain the original carbon of the organism. Also, many fossils are contaminated with carbon from the environment during collection or preservation proceedures.

Why is carbon-14 not used to date materials that are millions of years old?

Carbon-14 has a half life of 5730 years, meaning that 5730 years after an organism dies, half of its carbon-14 atoms have decayed to nitrogen atoms. Because of the short length of the carbon-14 half-life, carbon dating is only accurate for items that are thousands to tens of thousands of years old.