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Review your understanding of fossils in this free article aligned to NGSS standards.

Key points

  • A fossil is the preserved evidence of an organism that lived in the distant past. Some fossils are formed from body parts, such as shells or bones. Others, such as preserved footprints, are from traces of an organism’s behaviors.
  • Fossils are usually found in sedimentary rock. This type of rock forms from sediment, which is made up of pieces of rock, minerals, and organic matter. Over millions of years, sediment solidifies, becoming rock. Organisms trapped in sediment can eventually turn into fossils.
  • Sedimentary rock forms in layers. New layers form on top of old layers. So, fossils found in older, deeper layers are typically older than fossils found in newer, more shallow layers. Scientists can tell the relative ages of fossils based on which rock layers they are found in.
  • Scientists can also use radiometric dating to figure out a fossil's age. Radiometric dating is a way to tell how old something is by studying its chemical properties.
  • All of the fossils that have been found on Earth make up the fossil record.
A three-toed dinosaur footprint preserved in rock.
A fossilized dinosaur footprint. Image credit: “Dinosaur track” by mcdlttx, CC BY 2.0.

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