How Fossils Form

We’ve talked a lot about fossils on EarthDate, but we’ve never talked about how they form.Normally, when a plant or animal dies, it decays or is consumed. But occasionally its remains are preserved as a fossil.This usually happens when the organism is buried quickly in sediment. The sediment layer protects it from the elements, scavengers, even oxygen. Often soft parts decompose, leaving bones, teeth, shells, or exoskeletons.As the sediment gradually hardens into rock, mineralized water is absorbed into the pores of the remains, gradually replacing the original material with rock.Fossils are often skeletons or seashells, but other materials can be fossilized: feathers, trees, leaves and seeds—dinosaur eggs, even animal poop, called coprolites.Amber is lithified tree sap that may trap and preserve small organisms within it, like mosquitos.The footprints of animals can be covered in sediment and preserved as fossil trackways, allowing us to study the way creatures moved, even their social structures.But the most common and numerous fossils are microscopic. In some places where ancient plankton rained down to the sea floor for millions of years, their exoskeletons have compacted together to form thick chalk deposits.Fossils provide records of the ancient world for us to read today, informing science and underpinning many of the stories you’ve heard on EarthDate.