Still, gaps in the fossil record will keep paleontologists busy for decades. Most kinds of fossils are extremely rare. After all, to become a fossil, the species not only must exist, but individuals also must die in places where conditions are right for preservation. In other words, the skeleton must be buried in sediments with preservative properties before scavengers or weathering can destroy it. Then only a few of those places will undergo erosion or uplifting that exposes the long-buried remains.
Fossilisation requires rapid burial and cementing agents.
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