Evolutionary change is never constant in pace, and "gradual" is a relative term for the evolution of organisms over tens of millions of years.
The idea that arthropods and annelids and onychophorans all evolved in the Cambrian is complicated by the observation of fossil tracks showing that something multilegged was crawling around in the Precambrian. Either it was one of those, or else there were more phyla that evolved earlier.
" Trace fossils of burrows and tracks are perhaps the best evidence for the existence of animals ancestral to modern forms in the Precambrian. Unlike the various fossils represented in the Vendian impressions, these animals would have possessed the key characteristic of most animals - mobility. Unfortunately there are no known fossils of these organisms themselves, so we can't know who made these tracks and burrows."
http://www.humboldt.edu/~natmus/lifeThr ... reCam.web/
Either way, it sort of spread out the "explosion" to something more like a deflagration. Not to say that the appearance of completely scleretized exoskeletons (which marks the beginning of the Cambrian) didn't result in a rapid evolutionary radiation. It did. But most of the modern body plans are missing from the Cambrian. No sharks, no octopi, no mammals, no birds, reptiles, or amphibians, no insects or spiders. No vascular plants or any land animals whatever.
All that evolved in subsequent ages.