Seashells, like people, exhibit right and left symmetries. (In people this is obvious as right and left handedness).
Before the great Permian extinction 90% of Nautalis fossils were "left handed". Now 90% of seashells exhibit a "right handed" symmetry, certain shells with a left based symmetry are highly prized by collectors. Why did this change? And what survival value, if any, does this provide?