I think it probably went like this: a new feature (hard shells) was discovered by evolution, which gave so much advantage that all sorts of varieties could coexist for a while (even sub-optimal ones, as they still had the advantage over old organisms). Eventually, some winners among the organisms emerge, the advantage disappears and the sub-optimal variations begin to die out.

I like to compare it to the evolution of computer games. When they became viable first (home computer revolution), an incredible amount of games was developed. Eventually doom arrived on the scene and every game became a 1st person shooter. (Simplified story, of course). Or the dot com boom maybe - for a while, money was thrown at everything internet related. After a while some winning concepts emerged, and most of the contenders went away again.