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Animal fossils are often bones that have been turned to rock. Sometimes part of the flesh is preserved, but most of the time only skeletons remain. Because of this, it is more difficult to find fossils of invertebrates. (Invertebrates are animals that don’t have bones.)
The Record of Fossils Animals: The first animals appeared 600 million years ago during the early Cambrian Period (Article: The Geologic Record of Animal Fossils)
Why are Animal Fossils Rare: It has been estimated that 99% of any plant or animal species that ever existed is extinct now and never left a trace in the fossil record. (Article: The Rarity of Animal Fossils and Formation of Animal Fossils)
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