Answer: All have calcium made structures which restricts their degradation and decomposition.
Explanation:
Echinoids and snails have exoskeleton which is hard and made up of calcium carbonate which is the least degradable material even after death of the organisms and remains as traces and remains in the fossils. Also the shark tooth is made of dentin which is a hard calcified tissue which does not decompose very easily. It is denser and harder then the bones. It is surrounded by the enamel shell. The dentin and enamel prevent the degradation and decomposition of the shark tooth for long hence, it gets available as fossil.
