Fossil is the mark or remains of an organism that lived thousands or millions of years ago. Some of the best-known fossils include leaves, shells, or skeletons that were preserved after a plant or animal died. Others include tracks, trails, or burrows left by moving animals.
Most fossils occur in sedimentary rocks. Such fossils formed from plant or animal remains that were quickly buried in sediments the mud or sand that collects at the bottom of rivers, lakes, swamps, and oceans. Over time, these sediments became buried under other sediments. The upper sediments pressed down on the layers of mud and squeezed them into compact rock layers. Water that traveled slowly through the layers of sand deposited mineral cement around these particles, cementing the layers together to form rocks (see Sedimentary rock).
A few fossils formed in other ways. For example, whole plants or animals became preserved in ice, tar, or hardened sap.
The oldest fossils are microscopic traces of bacteria that probably lived about 3 1/2 billion years ago. The oldest animal fossils are remains of invertebrates (animals without a backbone) about 600 million years old. The oldest fossils of vertebrates (animals with a backbone) are fossil fish about 450 million years old.
Fossils occur more commonly than many people realize. Even so, only a small portion of the countless organisms that have lived on Earth have been preserved as fossils. Many species (kinds) of organisms lived and died without leaving any trace in the fossil record.
Although the fossil record is incomplete, many important groups of organisms have left fossil remains. These fossils help scientists discover what forms of life existed at various periods in the past and how these prehistoric species lived. Fossils also indicate how life on Earth has gradually changed over time.
