I'm not entirely convinced that one could argue that cephalopods are undergoing an evolutionary explosion. After all, one could argue that the class has been somewhat reduced since the Cretaceous period following the disappearance of the ammonites and belemnites. Indeed, the nautiloids dominated the worlds oceans during the Ordovician period, covering quite a number of orders, but are now reduced to just five species. They certainly have not undergone any evolutionary explosion and have barely changed in over 100mya or so.
The problem in attempting to ascertain prehistoric cephalopod diversity is that fossils of squid and octopi are extremely rare, obviously soft-bodied creatures require very specific environmental conditions to fossilise, and we require a great deal of luck in being able to find the deposits. (The Solnhofen Jurassic squids are a notable example of when this does happen). Hence it is difficult to ascertain how diverse the coleoids were in the past to make comparison to the present. There were certainly giant squids in the late Cretaceous, and squid and octopi appear to have evolved in the Jurassic. (There is an extremely controversial 'squid' from way back in the Devonian called Eoteuthis, but whether or not this really was a squid and the interpretation of the soft bodied parts, if that is what the staining on the fossil is, has been the subject of much debate).
Personally I think the class cephalopoda has not shown any great
evolutionary leaps in the last 140 million years or so and has been somewhat reduced, largely due to a mass extinction and the explosive radiation of the fish in the early Tertiary world providing direct competition in many ecological niches. This is not to say that they are not a success, on the contrary the Teuthoidea seem to be very diverse group, but, unless fish disappeared from the oceans I cannot really see the cephalopods exploiting new niches and adapting and evolving new forms at an order level. (That's just my opinion, by the way)!
That’s the best thing about soft-bodied cephalopod fossils, there is so little known, you can theorise all you like!
Off to the pub now!