Incorporating steel into bones (and, presumably shells) would result in lifeforms that would be very difficult to kill. It may be the case that an ecosystem can

Incorporating steel into bones (and, presumably shells) would result in lifeforms that would be very difficult to kill. It may be the case that an ecosystem cannot establish itself if the members' defense mechanisms are too strong. It may be physically impossible to evolve muscles strong enough to crush a steel skull. (Or, at earlier stages, to crush a clam.) Is there not an evolutionary pressure that tempers the establishment of defense mechanisms (such as steel-plate covered creatures) that are too effective?