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Paleontology is the science of discovering and analyzing fossils to decipher the history of life on earth. Most often a fossil is the hard part of an animal like its bones, teeth or shell. Sometimes, the fossil is just the hard parts and sometimes it is the hard or soft parts hardened by minerals. Paleontologists invented a special calendar called a geological time chart to outline the history of the Earth. During most of the Mesozoic era-between 200 million and 60 million years ago-great animals called dinosaurs were the dominant land animals. Dinosaur fossils are often found when rock is mined, when nature exposes them by land movements and erosion, during scientific excavations and by amateur fossil-hunters, who stumble onto something curious. Skulls and teeth are more frequently found than other parts because of their solid and hard characteristics. Sometimes softer parts, such as ribs, bones or skin can be found if they have been protected from rapid decay and mineralization. To excavate a fossil, scientists establish coordinates over an excavation site. Using a variety of techniques from dynamiting and digging, covering the entire area in a plaster cast for transport to laboratories and cleaning with small picks and brushes, they preserve the dinosaur's fossils. Dating the fossils is an important aspect of the scientist's work. There are three ways paleontologists determine the age of a fossil. One is studying how deeply the fossil was buried. Another is using chronometric techniques (chrono=time, metric=measurement) which include Carbon-14 and Potassium-Argon dating. The third dating method looks at the Earth's magnetic field and its patterns in history. When these fields are preserved in rock, they leave a pattern that is unique for each span of geologic time.