Evidence and observation are the building blocks of all scientific inquiry. To formulate theories, scientists rely on tangible evidence, such as physical remains, together with inference about things that can't be observed directly.

Physical remains and traces of ancient life, including dinosaurs, are preserved in rock as fossils. Fossils form in one of three basic ways, depending on the environment. First, an impression left in soft sediment, such as a footprint, may be preserved if younger sediment fills it in and hardens to stone under compression. Second, water may seep into tiny air spaces in buried bone and shell, and minerals in the water may be deposited there. Reinforced by such mineral deposits, bone and shell can survive for millions of years. Third, an organism may become trapped in an air-tight substance that preserves its entire body. Tree resin, ice, and tar have all successfully preserved numerous species of plants and animals.

A variety of factors usually prevent the fossilization of organic remains. These include biotic factors, such as scavengers, which destroy the organism, and abiotic factors, including weathering, erosion, and tectonic processes, which can wear away at the remains — even after they are fossilized — and may obliterate them.

Paleontologists seldom find complete sets of fossilized remains. More common are discoveries of incomplete remains, such as bones, teeth, or hair, and trace fossils, such as footprints or leaf impressions, which indicate an organism once existed even though its actual remains have not been found.

Paleontologists focus their discovery efforts in areas of sedimentary rock, the rock type most likely to contain fossils. Sedimentary rock is formed by the deposition of successive layers of sediment over time. In general, deeper rock layers are older, so fossils discovered in one layer can be dated relative to those found in another. More advanced dating techniques involve measuring the amount of certain elements, such as carbon and uranium, present in a fossil or in the rock in which a fossil is found. Scientists use these measurements to calculate the precise age of a fossil or rock layer, even if it is millions — or billions — of years old.

Dating of the rock layers in which fossilized dinosaur remains have been found has given scientists a pretty clear idea of when the dinosaurs lived, and when they died. Still, no conclusive determination has been made as to why dinosaurs appear to have suddenly died off about 65 million years ago.

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