Fossil
A fossil is the preserved remains, trace, or impression of a once-living organism, typically found in sedimentary rock and dating to more than 10,000 years ago. Fossilization requires rapid burial by sediment, which prevents decay and allows mineral replacement of organic tissues (permineralization), leaving a durable record of past life. Fossils are the primary evidence for reconstructing the history of life on Earth, documenting evolutionary relationships, ancient environments, and the timing of major biological events through the fossil record.
Types of Fossils and Their Formation Methods
| Fossil Type | Formation Process | What Is Preserved | Rarity | Example |
|---|---|---|---|---|
| Permineralized | Minerals fill cellular spaces | Original structure + minerals | Common | Petrified wood |
| Mold | Organism dissolves, cavity remains | Shape/external form | Common | Shell mold in limestone |
| Cast | Mold fills with sediment/minerals | External shape | Common | Ammonite cast |
| Trace fossil | Preserved behavior/activity | Footprints, burrows, coprolites | Common | Dinosaur tracks |
| Amber inclusion | Organism trapped in tree resin | Soft tissue, complete body | Rare | Insects in Baltic amber |
| Compression | Flattening under pressure | Carbon film outline | Moderate | Fern leaf impressions |
Interactive Tools
Paleobiology Database
Global database of fossil occurrences with stratigraphic and taxonomic data for over 1.4 million records.
Open ToolKhan Academy — Fossil Record
Lesson on how the fossil record serves as evidence for evolution and geological time.
Open ToolSmithsonian National Museum of Natural History — Fossil Hall
Teaching resources and fossil identification guides from the world-class paleontology collection.
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Stratigraphic Layer
A stratigraphic layer (stratum, plural strata) is a single bed or layer of sedimentary rock, volcanic ash, or other deposited material that was laid down during a specific interval of geological time, distinguished from adjacent layers by differences in composition, texture, or fossil content. The principle of superposition, established by Nicolas Steno in 1669, states that in an undisturbed sequence, lower layers are older than upper ones, making stratigraphy the primary method for establishing relative geological time. Correlation of strata across regions using index fossils and radiometric dating allows geologists to reconstruct the geological history of continents and oceans.
Geologic Time Scale
The Geologic Time Scale (GTS) is the internationally standardized chronological framework that divides Earth's 4.54-billion-year history into hierarchical time units—eons, eras, periods, epochs, and ages—based on stratigraphic evidence, fossil assemblages, and radiometric dating. The boundaries between units correspond to major geological or biological events such as mass extinctions, sea-level changes, or tectonic reorganizations. The GTS is maintained by the International Commission on Stratigraphy (ICS) and is essential for communicating geological ages, correlating rock formations globally, and understanding Earth's evolutionary and environmental history.
Radiocarbon Dating
Radiocarbon dating (¹⁴C dating) is a radiometric technique used to determine the age of organic materials up to approximately 50,000 years old by measuring the decay of the radioactive carbon isotope ¹⁴C. Living organisms continuously incorporate ¹⁴C from the atmosphere through photosynthesis and respiration; at death, ¹⁴C incorporation ceases and the remaining ¹⁴C decays at a known rate with a half-life of 5,730 years. Developed by Willard Libby in 1949, for which he received the 1960 Nobel Prize in Chemistry, radiocarbon dating revolutionized archaeology, geology, and environmental science by providing absolute ages for recent geological and archaeological materials.
From Latin "fossilis" (dug up), derived from "fodere" (to dig). The term was used broadly for any object dug from the earth until the 17th century when Nicolas Steno and Robert Hooke began distinguishing organic fossils from inorganic minerals, establishing the biological origin of fossils in the 1660s.