> But Celtic *φateroni̯o- would yield **Aithrinne in Old Irish. Aithirne
could continue *φaternii̯o- < *ph₂terno-,
Is *ph₂terno- attested in any other IE language? de Vaan, Etymological
Dictionary of Latin and the other Italic Languages, Brill, 2008, p. 450,
suggests that paternus was a Latin development ("paternus was probably
formed after māternus."). IIRC, in Latin paternus and maternus, there was
early syncope of a vowel between -r- and -n-.
- Chris Gwinn