posted on Mar, 18 2009 @ 09:50 AM
1. The oldest method is stratigraphy, studying how deeply a fossil is buried. Dinosaur fossils are usually found in sedimentary rock. Sedimentary rock
layers (strata) are formed episodically as earth is deposited horizontally over time. Newer layers are formed on top of older layers, pressurizing
them into rocks. Paleontologists can estimate the amount of time that has passed since the stratum containing the fossil was formed. Generally, deeper
rocks and fossils are older than those found above them.
2. Observations of the fluctuations of the Earth's magnetic field, which leaves different magnetic fields in rocks from different geological eras.
3. Dating a fossil in terms of approximately how many years old it is can be possible using radioisotope-dating of igneous rocks found near the
fossil. Unstable radioactive isotopes of elements, such as Uranium-235, decay at constant, known rates over time (its half-life, which is over 700
million years). An accurate estimate of the rock's age can be determined by examining the ratios of the remaining radioactive element and its
daughters. For example, when lava cools, it has no lead content but it does contain some radioactive Uranium (U-235). Over time, the unstable
radioactive Uranium decays into its daughter, Lead-207, at a constant, known rate (its half-life). By comparing the relative proportion of Uranium-235
and Lead-207, the age of the igneous rock can be determined. Potassium-40 (which decays to argon-40) is also used to date fossils.
The half-life of carbon-14 is 5,568 years. That means that half of the C-14 decays (into nitrogen-14) in 5,568 years. Half of the remaining C-14
decays in the next 5,568 years, etc. This is too short a half-life to date dinosaurs; C-14 dating is useful for dating items up to about 50,000 -
60,000 years ago (useful for dating organiams like Neanderthal man and ice age animals).
Radioisotope dating cannot be used directly on fossils since they don't contain the unstable radioactive isotopes used in the dating process.
To determine a fossil's age, igneous layers (volcanic rock) beneath the fossil (predating the fossil) and above it (representing a time after the
dinosaur's existence) are dated, resulting in a time-range for the dinosaur's life. Thus, dinosaurs are dated with respect to volcanic eruptions.
4. Looking for index fossils - Certain common fossils are important in determining ancient biological history. These fossil are widely distributed
around the Earth but limited in time span. Examples of index fossils include brachiopods (which appeared in the Cambrian period), trilobites (which
probably originated in the pre-Cambrian or early Paleozoic and are common throughout the Paleozoic layer - about half of Paleozoic fossils are
trilobites), ammonites (from the Triassic and Jurassic periods, and went extinct during the K-T extinction), many nanofossils (microscopic fossils
from various eras which are widely distributed, abundant, and time-specific), etc.