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One hypothesis—not sure if possible to prove—, is that anterior is in fact the comparative of *anterus in the same way that posterior is of posterus. Disclaimer: this is out of my comfort zone, and has to be taken as a not-very-informed hypothesis.

Posterus is the adj., postea the adverb. *Anterus would be the adj. corresponding to adv. antea.

About the existence of *anterus and the hypothesis that anterior is its regular comparative, I found two pieces of evidence: a XIX century lexicon by a Latin Professor at Oxford and a XV century book actually written in Latin by a prominent Flemish humanist (according to wp).

Now, L&S says anterior is late Latin and cites Sulpicius Severus using the word as early as the V century. Whether *anterus existed (at least in spoken) at that time, I haven't been able to found any evidence. But even if it did not, the -r- could have been derived (as an innovation) by analogy with postea > posterus > posterior

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