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The dinosaur Triceratops ate plants and stood approximately 8 feet tall. Scientists inferred its shape, size, and behavior from fossils, preserved remains or imprints, such as footprints, bones, and teeth.

Most organisms do not fossilize after death. Decomposers, such as bacteria and fungi,  and scavengers, such as flies and beetles, typically destroy tissues before fossilization can occur. Soft tissues, such as muscles, feathers, and fur, often disappear, while hard tissues—including bones, teeth, and shells—more commonly remain.

Fossils, like a mammoth encased in ice or a spider preserved in amber, offer an extremely rare glimpse of intact, early life-forms.

Sedimentary rocks yield the most fossils, particularly as fragments rather than complete organisms.

Sedimentary fossils form when sediment, such as sand or mud, buries an organism or its tracks. Pressure and heat transform the layers into rock, creating a fossil. These sedimentary rock layers, or strata, accumulate fossils over time.

Stratigraphy—the study of strata deposited through time and space—allows scientists to determine the relative age of fossils.

For example, strata form horizontally, with newer layers forming above the older layers. Thus, scientists regard fossils resting in more recently deposited strata as younger than fossils that occur at greater depths.

Radiometric dating allows scientists to determine a fossil’s actual age by using the known half-life of radioactive isotopes.

For instance, all living organisms accumulate both carbon-12 and the radioactive isotope carbon-14. When an organism dies, the amount of carbon-12 remains constant, while the amount of carbon-14 declines. Scientists can measure a fossil’s carbon-14 to carbon-12 ratio to estimate its actual age.

The overall fossil collection, or fossil record, documents the history of life and provides evidence of evolution. Significant events such as the mass extinction of dinosaurs, the subsequent explosion in mammalian diversity, and even human evolution are reflected in the fossil record.

The fossil record is, however, incomplete and biased as short-lived, scarce, and soft-bodied species are poorly represented. Fossils represent only a small fraction of all species that have ever inhabited Earth.