The Collaborative International Dictionary of English v.0.48:

Fronto- \Fron"to-\ [L. frons, frontis, the forehead.] (Anat.)
   A combining form signifying relating to the forehead or the
   frontal bone; as, fronto-parietal, relating to the frontal
   and the parietal bones; fronto-nasal, etc.
   [1913 Webster]