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Sciences - Geology > Paleontology
The "present is key to the past" is a simple but important statement in paleontology. Paleontologists look at present day organisms to help interpret fossils. "Paleo" means old and "onto" means life. Present day organisms help us to understand the life and environment of past organisms.
For example, a paleontologist finds a fossil skull of an organism with large, flat teeth. The skull is about 3 feet in diameter, with two large holes, where eyes would have been. Two broken objects come from the face, which were probably tusks. The paleontologist realizes that it probably belongs in the elephant family, because of the shape of the skull. However, the condition of the fossil with a brownish color, suggest that it is much older than a living elephant. It was found in rock layers that have been age dated to be of Pleistocene of age. The fossil may be a mastodon or a mammoth. Teeth were found and identified as belonging to a mastodon. 
Fossils are not always that easy to determine. From the time of the ancient Greeks until 1600's, fossils were thought to be oddities of nature. Shiny, button-shaped fossil teeth from fossil fish were thought to come from the heads of living toads. Clam fossils were hoof marks of recent sheep.
Fossils were also explained through myths and stories. Ammonites, a type of extinct sea animal, were believed to be coiled snakes turned to stone by a saint. The fossil tusk of the narwhal, a small whale, was for many years thought to be from the magical unicorn. Fossil oysters were nicknamed the Devil's Toenails.
When humans discovered there was a connection between fossils and now extinct species, it was a breakthrough in understanding the significance of fossils and geologic time.