Are there fossils of all extinct animals?

                   The fossil record has very large gaps in it, so there are periods where we know little about the life that once existed. Conditions may have changed long ago, making it unlikely that fossils were preserved. Also, changes on the Earth’s surface may have worn away rocks that contained fossils.

                 Fossil shells are quite common, but fossils of larger animals are very rare. It is even more unusual to find a complete skeleton of an animal such as a dinosaur, because its carcass would have been partly eaten by scavengers and the remains scattered. Some of the best fossils were formed in shallow lakes or mudflats, where the animal has been preserved because it was quickly covered with protective silt. Preservation of a delicate animal in fossil form requires ideal conditions, such as soft mud where there is no oxygen present, so scavengers cannot destroy the carcass.

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