Two thing to bear in mind.
1. The hard shells on trilobites must have evolved too, so we'd not expect the first appearance of that trait to be the complex formation we see in a trilobite, there must have been countless generations from the first trace of shelly material to something as sophisticated as a trilobite.
2. In many places were we find trilobites and other Cambrian beasts, we also find fossils of tiny delicate organisms like small jellyfish even embryos and so on, so we know that fossilization was taking place before the first appearance of these animals.
e.g.
A hilly 512 ha site in Yunnan province, Chengjiang’s fossils present the most complete record of an early Cambrian marine community with exceptionally preserved biota, displaying the anatomy of hard and ...
whc.unesco.org
This record found at this site (and there some others) flies in the face of the empirical expectations of Darwinism, there's no credible trace of the evolutionary record of Cambrian animals and no credible explanation as to why...