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You might try the Randolph Genealogy Forum for some more opinions on this connection. I myself have never seen anything more than speculation on finding a common ancestor, and no one seems to have been able to go beyond that. Randolph, or Ranulf, Ranulph,Ralph or any of its other forms was originally a given name, particularly before surnames were used; the fitz was added to it to show that one's father was named Ranulph (or whatever other form the name took), very much like the use of patronymics among the Russians, Scandinavians, etc. So Robert Fitzrandolph once meant Randolph's son Robert and differentiated him from all the other Roberts in the community, and Robert's son John could presumably be called John Fitz Robert and so on. The use of Fitz apparently came with the Normans in 1066. When surnames gradually were adopted and became fixed, at least one family group kept Fitz Randolph as their surname.