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The reason why skeletons of ancient sharks are hard to find in fossil form is because their skeletons are composed of cartilage instead of bones. Cartilage is a flexible and lightweight tissue that does not fossilize as easily as bones. When an animal with a bony skeleton dies, its bones have a better chance of being preserved over time compared to cartilage, which tends to decay more quickly.
Shark skeletons being made of cartilage make them less likely to leave behind fossilized remains. However, teeth and scales of ancient sharks are more commonly found in the fossil record since these structures have a higher chance of being preserved due to their composition.
In summary, the predominance of cartilage in ancient shark skeletons is the key reason why their complete skeletons are scarce in the fossil record.
