How Fossils Form: From Death to Discovery

The existence of any dinosaur fossil is a minor miracle. The chain of events required to transform a living animal into a stone record preserved for millions of years involves an improbable series of coincidences, and the vast majority of animals that have ever lived left no fossil trace whatsoever. Understanding how fossils form — the science of taphonomy — is essential for interpreting what they mean.

Fossilization begins at death. For a dinosaur to have any chance of preservation, its remains must be protected from the most common destroyers: scavengers, bacteria, weathering, and erosion. The fastest route to preservation is rapid burial under sediment. River deltas, floodplains, lake beds, and volcanic ash falls are among the most favorable environments. A dinosaur that dies on an exposed hillside will be scattered and weathered to nothing within decades. One that dies on a floodplain and is quickly covered by a flash flood deposit has a chance, however slim, of entering the fossil record.

Once buried, fossilization most commonly proceeds through permineralization. Mineral-rich groundwater percolates through the sediment and into the porous spaces of bone. Over millions of years, minerals — typically silica, calcite, or iron compounds — precipitate out of solution and replace or infill the original organic material. The result is a stone replica of the original bone, often preserving microscopic details of cellular structure. The process is slow, occurring over thousands to millions of years.

Not everything that fossilizes is a body part. Trace fossils — the preserved evidence of an organism's behavior rather than its body — are enormously informative. Dinosaur footprints are trace fossils, preserving information about locomotion, gait, group behavior, and body size that skeletal remains cannot provide. Nests, eggs, burrows, tooth marks on bone, and even fossilized dung (coprolites) are trace fossils. In some cases we have extensive records of trace fossils from animals whose body fossils are extremely rare.

Some fossil sites are so extraordinarily well-preserved that they are given the German term Lagerstätten (singular Lagerstätte), meaning storage places. These exceptional sites preserve soft tissues — skin, feathers, stomach contents, even color patterns — that are otherwise almost never fossilized. The Solnhofen Limestone of Bavaria preserves Archaeopteryx and dozens of other Late Jurassic animals with extraordinary completeness, because animals that fell into the oxygen-poor, hypersaline lagoon were buried rapidly and protected from decay. The Yixian Formation of Liaoning, China, preserved the feathered dinosaurs that revolutionized our understanding of bird origins.

Chemical processes can produce remarkable preservation. Amber — fossilized tree resin — can trap insects, plants, and occasionally small vertebrates in exquisite detail. Several specimens of dinosaur-era feathers and even small dinosaur tails have been found preserved in Cretaceous amber from Myanmar. Permafrost preserves soft tissue in woolly mammoths; though no dinosaur remains have been found in permafrost (they predate the ice ages), similar principles of low-temperature preservation have been found to protect proteins in exceptionally preserved bone.

What makes a good fossil site? Geology is the primary determinant. Sedimentary rocks — formed from accumulated sediments — are the only type that preserve fossils; igneous and metamorphic rocks form under conditions that destroy organic material. Among sedimentary rocks, those deposited in continental environments (floodplains, river channels, lake basins) are most likely to contain dinosaurs. Erosion must subsequently expose these rocks at the surface, which is why badlands — arid landscapes where wind and water rapidly strip away rock — are prime fossil hunting territory. The combination of the right geology, right climate for erosion, and accessible terrain makes places like the Hell Creek Formation of Montana and the Gobi Desert of Mongolia so productive.