Source: Produced for Teachers' Domain
Dinosaurs captivate the imagination of children and adults alike. The more we learn about them — their appearance, their habits, their habitats, and suspected reasons for their demise — the more fascinating they become. But how do we know they really existed? And how do we know so precisely when they lived? The fossilized dinosaur bones and tracks depicted in this still collage produced for Teachers' Domain offer physical evidence — the foundation on which all of scientific theory is built — of the dinosaurs' existence.
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.
Academic standards correlations on Teachers' Domain use the Achievement Standards Network (ASN) database of state and national standards, provided to NSDL projects courtesy of JES & Co.
We assign reference terms to each statement within a standards document and to each media resource, and correlations are based upon matches of these terms for a given grade band. If a particular standards document of interest to you is not displayed yet, it most likely has not yet been processed by ASN or by Teachers' Domain. We will be adding social studies and arts correlations over the coming year, and also will be increasing the specificity of alignment.