The Collaborative International Dictionary of English v.0.48:

Progenitor \Pro*gen"i*tor\, n. [OF. progeniteur, L. progenitor,
   fr. progignere, progenitum, to bring forth, to beget; pro
   forth + gignere to beget. See Gender kind.]
   An ancestor in the direct line; a forefather.
   [1913 Webster]

         And reverence thee their great progenitor. --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "progenitor":
   ancestor, ancestors, ancestress, antecedent, antecedents,
   archetype, ascendant, ascendants, begetter, elders, fathers,
   forebear, forebears, forefather, forefathers, foregoer, forerunner,
   grandfathers, grandparent, grandparents, guide, origin, original,
   originator, parent, patriarchs, pattern, precursor, predecessor,
   predecessors, primogenitor, procreator, progenitors, progenitress,
   progenitrix, prototype, source