Just as with the living species, fossils too are studied under sections referred to as "kingdoms." In the 19th century, fossils were grouped together under two basic categories: either plants or animals. Subsequent research and discoveries made it necessary for other main fossil groups to be established, including for life forms such as fungi and bacteria. Under the fossil classification developed in 1963, fossils began to be studied in the form of five separate kingdoms:
1. Animalia – fossils from the animal kingdom, of which the oldest known specimens date back 600 million years.
2. Plantae – fossils from the plant kingdom, of which the oldest known specimens date back 500 million years.
3. Monera – fossils of bacteria with no nucleus, the oldest known specimens dating back 3.9 billion years.
4. Protoctista– fossils of single-celled organisms. The oldest known specimens date back 1.7 billion years.
5. Fungi – fossils of multi-celled organisms, of which the oldest known specimens date back 550 million years.
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