The Collaborative International Dictionary of English v.0.48:

Autochthonal \Au*toch"tho*nal\, Authochthonic
\Au`thoch*thon"ic\, Autochthonous \Au*toch"tho*nous\, a.
   Aboriginal; indigenous; native.
   [1913 Webster]

WordNet (r) 3.0 (2006):

autochthonous
    adj 1: of rocks, deposits, etc.; found where they and their
           constituents were formed [ant: allochthonous]
    2: originating where it is found; "the autochthonal fauna of
       Australia includes the kangaroo"; "autochthonous rocks and
       people and folktales"; "endemic folkways"; "the Ainu are
       indigenous to the northernmost islands of Japan" [syn:
       autochthonal, autochthonic, autochthonous, endemic,
       indigenous]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "autochthonous":
   abecedarian, aboriginal, ancestral, antenatal, antepatriarchal,
   atavistic, beginning, budding, creative, elemental, elementary,
   embryonic, endemic, fetal, formative, foundational, fundamental,
   gestatory, homebred, homegrown, humanoid, in embryo,
   in its infancy, in the bud, inaugural, inceptive, inchoate,
   inchoative, incipient, incunabular, indigenous, infant, infantile,
   initial, initiative, initiatory, introductory, inventive, nascent,
   natal, native, native-born, original, parturient, patriarchal,
   postnatal, preadamite, preglacial, pregnant, prehistoric, prehuman,
   prenatal, primal, primary, prime, primeval, primitive, primogenial,
   primoprimitive, primordial, pristine, procreative, protohistoric,
   protohuman, rudimental, rudimentary, ur, vernacular