Fossil
A fossil is any preserved remain, impression, or trace of a once-living organism from a past geologic age, usually found in sedimentary rock.
Definition
A fossil is the preserved remains or traces of a plant or animal that lived long ago: a bone, a shell, a leaf print, or even a footprint in rock. Fossils are evidence of past life preserved in rock, usually sedimentary rock, and generally older than about $10{,}000$ years. Body fossils preserve parts of the organism (bones, shells, teeth, petrified wood) and trace fossils preserve activity (tracks, burrows, coprolites); preservation requires rapid burial and hard parts, so the record favors shelled marine organisms. Fossilization proceeds through modes such as permineralization (pore spaces filled by silica, calcite, or pyrite), replacement, carbonization (leaving a carbon film), molds and casts, and rare unaltered preservation in amber, ice, or tar. Taphonomy, the study of what happens between death and discovery, explains the strong biases in the record: soft-bodied organisms, upland habitats, and small populations are underrepresented. Index fossils, which are widespread, abundant, and short-lived, allow correlation of rock layers across continents.
Example
Shark teeth found on North Carolina beaches are fossils from sharks that lived millions of years ago, and dinosaur footprints pressed into old mud are fossils too. The Aurora phosphate mine in eastern North Carolina has yielded whale bones and huge megalodon shark teeth from Miocene seas, and fossil ferns and dinosaur bones from the Triassic occur in the Deep River basin near Sanford. The Burgess Shale in British Columbia preserves soft-bodied Cambrian animals because rapid burial in anoxic mud stopped scavenging and decay. Microfossils such as foraminifera and conodonts are the workhorses of petroleum biostratigraphy.
Key Insight
Most fossils are not bones turned to stone; many are just the shape an organism left behind after it rotted away. Fossils show that environments change, and marine fossils high in the Appalachians prove those rocks formed on a sea floor before being lifted into mountains. The incompleteness of the fossil record is quantifiable, not just an excuse. Comparing fossil first appearances with molecular clock estimates lets paleontologists estimate how much history is missing.