Explosions of new life forms followed by stasis are emphatically not what evolution expects to find, but it is what paleontologists do find.
What about shrimp? Their brains turn out to have memory and learning centers, which has triggered an evolution squabble.
Scientists debate whether ctenophores are the earliest animals to appear in the Cambrian explosion. If so, they arrived with multiple tissues, a nervous system, and a digestive system.