Why would a designer create such seemingly bizare and different animal designs? I believe that every creature has it's purpose. Marrella, for example is being very much considered as a trilobite species. If you find pictures of it, you can see why. Here's a good source on it:
http://nmnhgoph.si.edu/paleo/shale/pmarella.htmWhether just a food source, or a creature designed to keep other creatures from overpopulating, maybe even a decomposer, sceintists don't seem to know concerning Hallucigenia, but it had to fit into the system in one way or another, hence it wouldn't exist if it didn't. I have read a couple of sources that say that it was originally constructed upside down, and is now being reconsidered as to how it walked. I can't at the moment recall where but I'm sure the search engine could look it up very quickly.
By the way, Electron, welcome to the sight! I look forward to hearing your input!