The Collaborative International Dictionary of English v.0.48:

throwback \throw"back\, n.
   1. the reappearance in an organism of characteristics of an
      earlier ancestral type; atavism.
      [PJC]

   2. an organisms having characteristics of an earlier
      ancestral type.
      [PJC]
      [1913 Webster]

WordNet (r) 3.0 (2006):

throwback
    adj 1: characteristic of an atavist [syn: atavistic,
           throwback(a)]
    n 1: an organism that has the characteristics of a more
         primitive type of that organism [syn: atavist,
         throwback]
    2: a reappearance of an earlier characteristic [syn: atavism,
       reversion, throwback]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "throwback":
   atavism, backset, backsliding, backward deviation, backward motion,
   backward step, check, comedown, descent, down, falling back, lapse,
   reaction, recession, recidivation, recidivism, recrudescence,
   recurrence, reentry, refluence, reflux, regress, regression,
   relapse, renewal, retroaction, retrocession, retroflexion,
   retrogradation, retrogression, retrusion, return, reversal,
   reverse, reverse of fortune, reversion, rollback, setback,
   severe check, sternway