Webster’s 1913 has no entry under this spelling.
Shades of meaning
Fernald has no entry of its own for autochthonic; he lists it under primeval.
Where he writes about it:
Autochthonic (Gr. autos, self, and chth[=o]n, earth) signifies sprung from the earth, especially from the soil of one's native land. — from primeval
Aboriginal, autochthonic, and primeval combine the meanings of ancient and original; aboriginal inhabitants, autochthonic races, primeval forests. — from primeval