Modern Indo-Pacific snails are much better armored than were their ancestors of the Triassic (~250 mya). Modern snails would face few predators and would easily be able to prey on the Triassic fauna. The defense features of modern snails (thickened shells, spikes etc.) all come at an energetic cost. If the benefits were few (i.e., the features were not necessary to defend them from Triassic predators), then the balance of costs and benefits for maintaining these structures would shift, and natural selection would favor individuals with decreased armor.