From a 70 million to 80 million years process the rate of evolution (assuming it's true) accelerated by an order of magnitude. Single celled bacteria which has not changed at all suddenly exploded into a extremely wide variety of multi-cellular organisms that are billions of times their size and even trillions of times more complex. It does not make sense imagining bacteria cells evolving into plants and animals in such a short period (it does not even make sense in a long period). Also, isn't evolution about slow changes over time? (although I don't think such huge jumps of mutations are possible). According to the evolution tree, the bacteria cell remained unchanged for 3 billion years and then suddenley started to changing into millions of different species that are trillions of times as complex as they are.

Evolution depends on the organism's environment in reference to how quickly said organism evolves. Therefore, if a mutation helps a bacterium survive in it's environment, then it will multiply extremely quickly, thusly making room for even more mutations to occur, which creates a sort of landslid affect, or so I would assume. Evolution can start to make apparent changes within mere hundreds of years. The mystery skulls of Palau are an excelent example of quickly occurring mutations in the human race. It's been theorized that these humans have experienced dramatic evolutionary changes as early as several hundred years compared to the thousands it takes for the normal rate of evolution. This is because of the environment in which they live. The islands of Palau do not have large amounts of food, so large members of these humans died off very quickly, while the small members were left to reproduce because they needed less food to survive. The changes in the skull are the most dramatic. The teeth of these ancient people are about the same size as modern humans, but their skull is dramatically smaller. This is a result of extremely fast evolution. The teeth are one of the slowest parts of the body to change, so these peoples' teeth remained the same size as the original inhabitants, when their skulls continued to shrink at a rate far faster than their teeth were shrinking. This caused the teeth to be very large for their size in reference to the skulls. But, again, evolution does not occur at a steady rate; it fluctuates depending on the organism in question's environment.
http://abcnews.go.com/Technology/popup?id=4444687
http://thepiratebay.org/tor/4097857/%5BNGC...s_of_Palau.divx
http://www.reuters.com/article/pressReleas...008+PRN20080311
http://www.monstersandcritics.com/smallscr..._St._Paddys_day

NGC Video(can't find the whole thing, sadly, but I'm sure there will be reruns on the NatGeo Channel sometime soon:
http://channel.nationalgeographic.com/epis...Videos/05134_00

We are sort of pre-conditioned to think evolution is some sort of slow process overtime. However, it is becoming more accepted that evolution has fits and starts. It is really not such a slow process that we have been conditioned to believe. If you don't think you have been conditioned about what evolution is then ask someone two or three hundred years ago what evolution is and they will look at you with a big question mark on their face. Evolution is a new concept for us and we don't really understand as much as we would like to think we do.

http://www.youtube.com/watch?v=Sui4CadfhDM

Hmm, I think its important to add this since it appears no one has yet. But your description leading up to the Cambrian "explosion" is actually incorrect. Multicellular life actually existed before the start of the Cambrian. In fact we have some Ediacara fossils (multicellular ones) that were some 20 million years before the Cambrian. Its likely that multicellular life existed millions of years before that first fossil, and pseudo-multicellular life (for lack of a better term) millions of years before that even.

Pre-Cambrian multicellular life lacked hard parts, which is a big reason we don't see as many fossils as later in the Cambrian. The Cambrain had two big developments, The use of Chitin by living organisms (a polymer of modified carbohydrates) and the development of exoskeletons.

As others have pointed out as well, evolution is not always an extremely slow process. Generation time, mutation rates etc play an important role in time frames of evolution. Most importantly the speed at which the environment changes as well. As the Pre-Cambrian came to a close, the contents had drifted apart creating shallow warm seas. Our planet had just come out of (possibly) the worst glaciation events in our history. Temperature was rapidly rising. O₂ levels were rapidly rising. Coincided with the rise of the exoskeleton, this creates a recipe for evolutionary "explosions" (though if one considers 40+ million years rapid, I think they should try to ponder time scales of that nature).