The Collaborative International Dictionary of English v.0.48:

Primogenitor \Pri`mo*gen"i*tor\, n. [LL., fr. L. primus first +
   genitor a begetter.]
   The first ancestor; a forefather.
   [1913 Webster]

WordNet (r) 3.0 (2006):

primogenitor
    n 1: an ancestor in the direct line [syn: progenitor,
         primogenitor]